Electrocoating provides a decorative and protective finish. Common coating defects may adversely affect these properties, so it is important that electrocoat users develop basic, effective troubleshooting skills.
Troubleshooting process. It is important to understand troubleshooting in order to develop sound, workable solutions to problems. There are five steps to troubleshooting.
1. Define the problem. Defining the problem is the first step in the troubleshooting process. It characterizes the problem by answering several questions:
Isolation—Is the problem in the operation of the system or appearance related?Identification—What type of defect is occurring and does it occur on all parts? Location—Does the problem move around on the part or is it always in the same area? Timing—When did the problem start? Duration—Is it constant or sporadic?
Problems do not occur without something having changed. Problems are not solved unless something is changed.
2. Identify the root cause. Identifying possible root causes is step two in the troubleshooting process. It is necessary to understand common causes for the defects. There could be several bath variables and process areas to investigate. Commonly asked questions include the following.
Correlation—What are the common causes for this type of defect? Have there been any recent changes made to the line? Is there a correlation between tank parameter test data and the appearance of the problem?
Mechanical—Is the testing equipment functioning properly? Procedural—Have the testing procedures been followed?
3. Determine corrective action. Determining the appropriate corrective action is the next step in the troubleshooting process. This step is probably the most time consuming. Developing a logical plan that tests one variable at a time is crucial to identifying potential solutions. Answering the following questions should help develop a practical action plan.
Which variables can be tested on line quickly?
Which variables can be tested with little disruption to production?
Which variables can be tested on line in the paint lab?
Which variables need technical support from the suppliers?
4. Implement corrective action. Implementing corrective action is the success step in the troubleshooting process. As each variable is tested, it will either be eliminated or identified as a possible solution. There may be multiple solutions and in rare cases there may be no solution. In the cases where no solutions have been found, it is necessary to start the troubleshooting process again by redefining the problem.
5. Follow-up. Follow-up is the last step in the troubleshooting process. It involves determining what permanent changes are needed to prevent the problem from recurring. This troubleshooting model provides guidance for identifying, reacting to and solving problems and if properly followed and documented, it can provide a faster response for future problems.
Preventive action methods. Troubleshooting should be more than a reaction to problems. Preventive maintenance is often overlooked as a tool to guard against problems and defects. It is not possible to list every housekeeping item or maintenance program associated with an electrocoat system, but there are essential mechanical, chemical and substrate considerations.
Tank agitation. Agitation of a paint tank is necessary for paint suspension, filtration and removing excess heat generated from pumping and coating. Proper agitation is dependent on the header/eductor system and the circulation pump. Broken or misaligned eductors can cause appearance defects in production and dead zones in the paint tank. A malfunctioning pump can cause poor bath circulation, foam and appearance defects. Excessive tank agitation can cause parts to fall. The velocity of the paint in recirculation lines should normally fall between seven and ten fps. Semi-annual tank cleanings should be performed to check agitation, and weekly cleaning of the pump screens will prevent blockage.
Oven. The oven is critical to the final coating appearance and the desired performance properties. It is recommended that the oven buildup be monitored and cleaned out when necessary. Oven temperature recorders should be run semi-annually to ensure correct oven balance and dehydration zone temperatures.
Rectifier. The rectifier should be checked yearly for ripple. It should not exceed five pct under anticipated load conditions. The amperage and voltage displays should also be checked to ensure calibration accuracy. There should be no stray voltage to the paint tank during non-production hours.
Electrodes. The electrodes should be checked weekly for degradation, proper operation of supply and return flows and to ensure electric leads are connected. Periodic checks of each electrode's amperage draw can be used to monitor anode performance. It is important to maintain a 4:1 or lower ratio of coating surface to electrode (based on a two-minute immersion time).
Anolyte/catholyte. The anolyte/catholyte is needed to remove excess acid/base from the paint bath and should be checked for proper liquid level, proper functioning of the conductivity set-point indicator and probe, water supply availability and that the supply and return flows are operational.
Racking. Racking includes design, part loading and cleaning. There should be specially designed racks available for odd-shaped parts and spring-loaded hooks for small parts. It is recommended to have one contact point for part grounding and to handle them in a way as to eliminate liquid pooling, air pockets, falling off and contact with other parts. Maintaining clean racks and contact points will help prevent rack buildup and coating failures. The grounding system should be checked yearly for wear and good contact.
Rinsing. Ensure the rinse system is operating monthly without any plugged or misdirected nozzles. Make sure spraying is contained within the tank, and the recirculated rinse flow is balanced from stage to stage. Check for a buildup of paint solids and parts that have fallen in the tank. The headers should be cleaned periodically to remove any settled material from the piping and pressure maintained between five and ten psi.
Filtration. Filtration of the electrocoat tank includes both bag filters and ultrafilters. Bag filters should be changed when the pressure differential inlet to outlet is five psi. If oil absorbent media is being used, it should be changed frequently. The ultrafilter flux rate should be checked daily using the site gauges and cleaner per the manufacturer's recommendations. Most manufacturers recommend a cleaning at 70 pct of the stabilized flux rate, or membranes can be irreversibly fouled, shortening life span and making them more difficult to clean. The ultrafilter pump should be monitored by pressure gauges.
ELECTROCOATED golf car from Yamaha Motor Corp.
Cleaning process. Proper cleaning parameters will allow for removal of substrate contaminants such as stamping oils, surface dirt, fibers and weld smut. Whether an acid or alkaline cleaner is used, the concentration and the process time must be maintained and monitored daily. This includes any auxiliary cleaning steps such as shot blasting and pickling. A clean substrate is necessary for proper pretreating and coating. Dirty or contaminated substrate surfaces will cause final appearance and/or performance problems leading to rejected parts and rework.
Pretreatment process. Pretreatment functions as a conversion coating for improved paint adhesion and performance. It is crucial that this process be closely monitored and followed per the supplier's recommendations. The four basics to optimum pretreatment chemistry require monitoring of time, temperature, pressure and concentration. Routine testing for proper coating weights and crystal morphology should also be performed because the final appearance and performance of the electrocoated part is only as good as its preparation.
Water. High-quality water is essential for operation of an electrocoat system. Quality water is characterized by low conductivity, less than 10 micro mhos/cm for deionized or 20 micro mhos/cm for reverse osmosis, low silica levels and microbe free. It is recommended to have a minimum of two water sources, either two alternately functioning units or one unit plus water storage capability. Resin bed cleanings and regeneration procedures should be followed.
Substrate. Substrate quality is the first consideration in achieving a quality finish. There are many types of metals used in electrocoating ranging from aluminum to cold-rolled steel and from galvanized to heat treated metals. Using the first in, first out rule will keep substrates clean. Protective storage conditions will aid in the prevention of flash rusting and other surface defects.
Operating parameter effects. All systems, from cleaner and pretreatment through electrocoat, have specifications that recommend optimal ranges of operation. Understanding how each specification affects the appearance and performance of the coating will allow for corrective adjustments. Also, through accurate testing and charting, a historical picture of the system can be built and the occurrence of defects minimized.
Bath solids. This includes the pigments and non-volatile components of paint. Low bath solids cause lower film thickness, decreased throwing power, higher rupture voltage and higher ultrafilter flux rate. Low bath solids occur from normal excessive replenishment additions of paste.
Pigment to binder ratio. Low pigment to binder ratios cause higher gloss, decreased throwing power, less hiding and more cratering. Low pigment levels occur from excessive replenishment additions of resin and settling in the tank. High pigment to binder ratios cause lower gloss, increased throwing power, settling in the bath and rinses, and it makes the film more sensitive to water spotting. High pigment levels occur from excessive additions of paste.
Bath pH. High bath pH for cationic systems can cause tank settling, dirt, a decrease in ultrafilter permeate rates and sensitivity to streaking. High pH can occur from excessive anolyte purges, excessive replenishment and caustic contamination from carryover or deionized water. Low pH occurs from excessive acid levels and can cause redissolution. Potential causes of low pH are deficient anolyte purges, anolyte leakage in the paint tank, insufficient membrane surface, membranes surface plugging, excessive acid additions and acid contamination from carryover or poor quality deionized water.
Low bath pH for anionic systems also can cause tank settling, dirt, a decrease in ultrafilter permeate rates and sensitivity to streaking. Low bath pH occurs from excessive ultrafilter purge, excessive replenishment, and acid contamination from carryover or deionized water. High pH occurs from insufficient ultrafilter purges, excessive amine additions and caustic contamination from carryover or poor quality deionized water.
Bath conductivity. Low bath conductivity can cause poor throwing power, low film build and roughness. Low conductivity is caused by excessive ultrafilter purges and low bath solids. High bath conductivity can cause rupturing, high film build and roughness. High conductivity is caused by high bath solids and ionic contamination from carryover or poor quality deionized water.
Solvent. Low solvent levels can cause low film builds, higher rupture voltages, sensitivity to streaking (phosphate mapping), lower gloss and poor flow or orange peel. Low solvent levels are a result of inadequate solvent additions and excessive ultrafilter purges.
High solvent levels can cause high film builds, lower rupture voltages, higher gloss and poor throwing power. High solvent levels are a result of excessive solvent additions.
Common Electrocoat Defects. Coating defects are numerous and this section will not address every one, but it will provide some common defects related to the electrocoat tank, potential causes and their solutions.
Cratering. Craters are bowl-shaped depressions with material in the center and raised circular edges. They are usually caused by contamination of the bath, rinses or substrates with particulates or incompatible oils. These contaminants can be from the substrate forming process, greases or lubricants and other processes that allow airborne contaminants to enter the system. Craters may also be caused by post tank contamination of parts. This can come from chain oils, conveyor drips and blow out of contamination in the oven. Often it is difficult to identify the cause of cratering without close investigation of the line. A permanent solution to cratering must be to identify and eliminate the source. Short-term solutions are using oil absorbent media inside of bag filters, increasing the pigment to binder ratio and in some severe cases diluting the contaminant with fresh feed.
Rupturing is the bursting of the deposited film by an excessive generation of heat (anodic) and electrical sparking/gassing at the film/substrate interface (cathodic). Rupture defects are caused by excessive voltage, excessive ripple, high film build, electrode/counter electrode in close proximity and bath contamination. By racking the parts according to substrate type, size and weight, the voltage can be adjusted as necessary. Rupture can also be due to high bath temperature, solvent levels and bath solids. The cathode or anode should be a safe distance away from its counter electrode. Bath contamination by ionic species can be removed by ultrafiltering to drain and replacing with deionized water.
Roughness is indicated by patches on a cured film that exhibits an alternately non-uniform and smooth appearance. Patchy roughness can be due to ionic contamination, low solvent levels and substrate irregularities. Ionic contamination is typically brought into the bath through part carryover, poor water quality and anolyte/catholyte malfunctions. Adding solvents can also help smooth the overall coating appearance. Using clean, high-quality substrates and controlling pretreatment will minimize the non-uniformity of the cured electrocoat film.
Redissolution is where all or part of the electrocoat film washes off or dissolves. Redissolution can seriously limit the high transfer efficiency of an electrocoat system. It occurs in the paint bath or post rinses and can be caused by excess solubilizer, high solvent levels and line stoppages. Excess solubilizer and high solvent levels in the bath lead to aggressive permeate post rinses that dissolve the deposited coating during rinsing. This can be eliminated by maintaining proper bath pH and solvent levels. The amount of time the ware is in the bath and rinse stages during the line stoppages should be minimized.
Dirt. Dirt has three sources, process, environmental and oven. Process dirt develops within the bath or rinses from inadequate solubilizer levels, pump shear, altered circulation and improper filtration. In the early stages, dirt appears on a horizontal surface, but in severe cases it can affect all surfaces. Environmental dirt is caused by airborne particles that fall into the bath or settle on the ware. Electrocoat areas exposed to vehicular traffic, ventilation fans, and grinding/sanding operations are susceptible. Oven dirt is caused by condensation of electrocoat by-products that flake off when drying. On a cured part, oven dirt is more surface oriented, while process and environmental dirt is somewhat imbedded in the paint film.
Streaking. Streaking in an electrocoat film can be due to pretreatment, rinsing and racking. Pretreatment variations can cause differences in ware surface conductivity. This defect is usually a telegraphing or mapping of the pretreatment through the electrocoat film or an electrocoat film phenomenon. Rinsing issues include low solvent or solubilizer levels in the rinse stages and clogged or misaligned rinse nozzles. The greater the length of time from paint bath to post rinse can increase drying of the dragout, making it difficult to rinse. Dirty racks and improper racking also can be sources of drips or spots.
Pinholing/outgassing is a pattern of relatively small, random volcano-like holes in the electrocoat film that penetrates to the substrate. Pinholing is primarily seen on galvanized/zinc-coated substrates, but can be caused by poor metal quality and rectifier problems. Galvanized and other zinc-coated substrates may inherently have surface microvoids. These microvoids may allow for the gasses normally generated in the electrocoat process to be trapped under the electro-coating. During curing, the gasses blow out through the electrocoating, leaving a volcano-like hole. Poor metal quality, metal that cannot be pretreated evenly and voltage spikes from an unfiltered rectifier can cause rapid electrodeposition. This does not allow the normal gasses generated in the process to escape, therefore holes result.
Foaming/air entrapment. Foaming is typically caused by pump problems, poor tank circulation and improper part loading. Cavitating pumps allow for aeration of the electrocoat bath and poor tank circulation does not allow gas to dissipate. Odd-shaped ware entering the electrocoat bath at an angle or through surface foam can also be a reason for air entrapment.
Gloss variations can be caused by several factors, including pigment to binder ratios, solvent levels and solubilizer levels. Pretreatment variations cause gloss differences not only part to part, but also on one part. Cure time and temperature also affect the final gloss.
Color variations can be caused by iron contamination, improper cure and poor tank agitation. Iron contamination can cause a yellowing or browning of the coated film. Oven problems can discolor cured films. Poor tank agitation can cause pigment pooling that can cause a streaked or blotched discoloration on products.
Throwing power. Poor throwing power is usually related to low voltage, low bath solids, low conductivity, high solvent levels and insufficient deposition time. By increasing some or all of these variables and decreasing solvent and bath temperature, throwing power will increase. Throwing power also can be impacted by the addition of auxiliary electrodes close to areas where more film build is needed.
Thin coating. These coats may be caused by poor contact, a faulty rectifier, inadequate electrode surface, high part loading, low voltage and low bath temperature and inadequate deposition time. Clean hooks, proper electrical supply and maintaining the proper coating surface to electrode ratio is essential to proper film build. High part loading can cause an overall film-build decrease. Film build can be increased by increasing voltage, bath temperature and deposition time.
Orange peel is related to iron contamination and low solvent levels. Iron contamination can be caused by fallen parts, exposed mild steel and leaking anolytes. This type of contamination, although ionic, cannot be ultrafiltered from the bath. Coating out, adding fresh feed to dilute the contamination and eliminating the source are the recommended solutions. Increased solvent levels can improve the flow characteristics of the electrocoating, eliminating the orange peel.
The "Big Four" electrocoating troubleshooting areas include the troubleshooting process, establishing preventive maintenance schedules, controlling operating parameters and classifying common electrocoat defects. Troubleshooting electrocoating will help users develop effective skills needed to promote optimum electrocoating.
Monday, June 8, 2009
Understanding Vibratory Finishing Revisited
Vibratory finishing has not changed much during the years. Several years ago, Products Finishing published a series of articles by John Kittredge. These articles have good, pertinent information that is still valid for the vibratory finishing industry today. The series covered media, compound solution, equipment and parts.
Four elements are significant in vibratory finishing: parts, media, compound solutions and equipment. These link to form an interdependent network, or Tetrahedron of Interdependence. These elements depend on each other to such an extent that if one fails, the process fails.
MediaMedia is one of the supporting parts; however, it is also supported by the other three parts.
Functions. The chips or stones in vibratory finishing are called media. While there are many variations, from aggressive grades capable of removing large quantities of metal from parts in short cycles to media that cannot cut at all. Each type has the following functions:
1. Separate and cushion parts. Parts are mixed with media in a random distribution. The ratio of parts to media helps control the amount of contact the parts have among themselves. If insufficient media is between the parts, they will collide. Media cushions this impact so that irreparable damage does not occur. This is not important for forgings, but is a key consideration of preplate finishing of die castings.
2. Contact critical edges and surfaces. Certain media shapes may lead to a false sense of security. Plastic cones are popular and available in a variety of sizes. That pointed end can touch all areas of a part, but if the cone is large and not a blend of sizes, the pointed end does not do much work.
3. Provide proper degree of cut or luster. Abrasive media will cut. More and bigger abrasive will result in more cut. Non-abrasive media can only produce luster.
1. TETRAHEDRON OF INTERDEPENDENCE shows close relationship of equipment, media, compound solution and parts.
4. Resist lodging in parts. If media sticks in the parts, someone must remove it. If this is difficult, the cost of removal may exceed the cost of the process. To prevent lodging, manufacturers of media produce a great variety of shapes and sizes.
5. Permit separation from parts. Any time you mix parts and media, the two must eventually be separated. The efficiency of this operation depends on the equipment available, but also on the shape and size of the media and parts.
Abrasive media has distinct functions. It releases abrasive slowly into the system. It can cut and deburr edges. It can develop the required surface smoothness and will last as long as possible.
Media wears and exposes fresh grains. Media also wears against other media, which it is designed to do. This keeps media clean and ready to cut with fresh abrasive.
The rate of media wear varies with use. Media designed for rotary barrels will wear out too fast in higher energy equipment such as vibratory systems. Media designed for vibratory use wears out too fast in barrels and other centrifugal equipment.
Abrasive particle size controls the depth of cut. Bigger particles cut deeper and produce rougher surfaces. The hardness of the abrasive should be greater than the material being cut. This cutting action removes burrs and changes the surface contour.
Surface smoothness can be measured using a capacitive sensing probe. Units are read in microinches or micrometers, AA or arithmetic average. Developing good, smooth, low-micro-inch surfaces is easily done, but care must be exercised. Fast cutting products give rougher surfaces at their minimum capability.
The ideal media lasts forever, and this can happen with non-abrasive media such as steel, but not with abrasive media. Heavy-cutting grades wear faster.
Non-abrasive media peens or pound edges and surfaces. It also deburrs edges lightly while brightening surfaces. It also experiences little or no wear or loss.
2. TUB vibrator processes parts as shown here
Since non-abrasive media cannot cut and energy is imparted to it, it can only pound surfaces and edges. No scratch pattern develops; instead, a series of small, uniform dents is imparted to the surface of the work.
The small dents are followed by more and more pounding until the surface gets brighter. This occurs quickly with steel media on malleable metals, but may never develop on hard steels.
Media wears primarily by rubbing against other pieces of media until abrasive grains fall out or are cut away. With no abrasive, cutting stops and so does wear.
Media has an amazing influence on the vibratory finishing process. However, for the media to work, the solution system must behave, the equipment must perform and the parts must be uniform and above all, people must not make waves too high for the process to get through.
Compound SolutionThe compound solution is the least understood and most frequently abused part of the mass finishing process. It has the following functions:
1. Control pH, foam and water hardness. Many compounds are buffered to prevent big changes in pH from taking place. Mass finishing solutions vary from a pH 1 to 14. The most popular range is from 4 to 14.
Some foam is necessary in most applications, especially those involving cleaning. Foam holds the solution in place on the surface of the parts and media. Without foam, process cycles are greatly extended or cleaning just does not occur.
Too much foam kills the action in a vibratory machine. It cushions the mass so effectively that cutting can literally come to a stop. In rotary barrels or centrifugal barrels, however, foam merely moderates action.
In the U.S. water hardness varies from low to high, and public water supplies are often quite different from water drawn from a well in the same area. These great variations in hardness strain the solution since it tried to equalize the differences.
2. Wet surfaces. If the compound solution will not wet or spread on the surface to be cleaned, it will not clean well. Wetting is dependent on concentration.
3. Emulsify oily soils. Physical action improves the speed and stability of emulsification. Because vibratory finishers are excellent scrubbers, they clean well. Media continually works the soils, allowing sometimes-marginal cleaners to "get away with it."
4. Remove tarnish or scale. Both high- and low-pH compound solutions can chemically attack tarnish and scale. These oxides of the metal darken the surfaces and lower reflectivity. Highly alkaline solutions remove rust or scale from some steel. These alkaline solutions can produce light, bleached metal surfaces. Strongly acidic solutions vigorously attack rust and other metal oxides. Unfortunately, they also attack the base metal unless the solution is properly inhibited.
3. ROUND vibrator with no elevation on the chamber
5. Suspend soils. Soil removed or made during the process can redeposit on the parts, media and/or lining of the chamber. Excessively hard water contributes to the problem. Usually a special compound is needed to overcome this.
6. Control lubricity. For years, it was believed that the solution had to be slippery to burnish or brighten a part. This is often true in rotary barrels. In vibratory equipment, excessive slip brings everything to a halt. Traction is lost. Media stops rolling, and nothing happens to the parts. Lubricity should not be completely eliminated, since some lubricity does assist the cutting of the compounds.
7. Control part color. Coloring or darkening parts in vibratory finishing is caused by oxidation or dirt impregnation. Highly alkaline surfaces can bleach out surfaces and acidic compounds can give too clean a surface.
8. Prevent corrosion. The only way to stop corrosion is to include an inhibitor in the compound solution. Some inhibitors are effective on ferrous metals, while others are more effective on non-ferrous metals or those that will oxidize.
9. Provide cooling. If a vibratory process were to run dry, friction would generate high temperatures quickly. The solution cools the mass of media and parts. Low flow rates do not carry away heat as fast. Then more oxidation occurs. Elevated temperatures are seldom needed, even for severe cleaning problems.
Solution FeedThere are three major types of solution-feed systems: batch, recirculation and flow-through. Batch is the simplest system. It is used in barrels and vibrators with no drains. Recirculation systems mix a solution in the tank and pump it into the process, allowing it to drain back into the tank for reuse. This sounds efficient, however the high rate of soil production creates severe contamination problems. Flow-through systems pump fresh solution into the machine, allow it to act and then drain it. Compound flow-through means the parts see only clean media. The solution is always the same strength. There is no sludge tank to clean.
There are three methods of adding compound: manual, batch premix and automatic.
With the manual procedures, you add compound and water manually using a measuring device. In the batch premix method the solution is held in a tank where it is mixed prior to being pumped into the machine. Automatic addition employs a compound-metering pump and rotameter for measuring flow rate of water.
EquipmentEquipment serves several functions.
1. Process parts. The equipment must do the deburring, cleaning, surface conditioning or burnishing.
2. Separation. Separation skills are high on the normal system capability list. The simplest separators are screens. For parts with projections that would cause them to hang up on screens, the tie rod deck is a good choice. It allows parts to slide down parallel rods while the media drops through. Or the inverse if the parts are smaller than the media.
3. Control mass action. Equipment must have adjustments so that more aggressive or gentler work can be done. Tub-type vibratory equipment has a variable-weight system to adjust energy input. Generally, there is a single shaft mounted directly underneath the tub. On each end is an eccentric weight that has some variability in its mass. More weight, more amplitude, faster cutting, rougher surfaces, greater media wear.
The rotational speed of the eccentric weights is another variable. Higher speed generally promotes faster cut, rough surfaces and greater media wear rates.
In round vibratory equipment, the eccentric weights are mounted on each end of a vertical shaft or motor in the center tube of the tub. The amount of weight on the top eccentric generally controls the speed of mass travel around the tub, while the amount on the bottom controls the rollover rate of the mass.
TABLE I - Media-to-Part Volumetric Rates
Media-to-PartRatio by Volume
Normal Commercial Application
0:1
No media. Part-on-part. Used for beating off burrs. No media for cutting.
1:1
Equal volumes of media and parts. Forgings, sand castings; to produce crude, very rough surfaces.
2:1
More gentle, more separation. But still allows relatively severe part-on-part damage.
3:1
About minimum for non-ferrous parts. Considerable part-on-part contact. Fair-to-good for ferrous metals.
4:1
Probably average for non-ferrous parts. Good for ferrous metals.
5:1
Good for non-ferrous metals. Minimal part-to-part contact.
6:1
Very good for non-ferrous parts. Common for preplate on zinc with plastic media.
8:1
For higher quality preplate finishes.
10:1 to 20:1
Even better. Used for very irregularly shaped, fragile parts.
Infinite
Absolutely no part-to-part contact. One part per machine or compartment or the part is fixtured.
PartsThe most important of the Tetrahedron of Interdependence is parts. Without the parts, there would be no process. And, depending on the requirements for the parts, the process is designed, equipment is chosen, media and compound solution are selected. Only when a new process is set up to use existing equipment is the procedure changed. Normally, the parts dictate all else.
There are seven significant purposes of vibratory finishing.
1. Clean. Cleaning can be done with little or no increase in cost, quickly and without heat. Typical parts cleaned commercially in vibratory units include machined engine components, stampings, copper plumbing fittings, brass forgings, plastics, ceramics, rubber and wood.
2. Deburr or radius. This is the best known use of vibratory finishing. All metals, ceramics and some plastics can be deburred. Deburring prevents people from cutting themselves and makes parts feed through automatic feeders more easily.
3. Improve surfaces. Rough surfaces need to be smoothed before they take a high-quality electroplate, paint or anodize. Surface roughness values below 10 microinches AA are common and with good preplate plastic media, values below five microinches are commercially achieved.
4. Brighten. Brightening or burnishing metal surfaces is a good, viable application of mass finishing. It is done easily to the more malleable metals using non-abrasive media. Aluminum, copper, brass and mild steels can be brightened in 10 to 30 min depending on finish quality required and starting surface quality. Harder metals such as stainless, some brasses and other steels take 30 min to several hours.
5. Inhibit. Sensitive metals, including powdered iron, cast iron, copper, brass, zinc and steels of all types will corrode unless protection is provided. Metal surfaces will look cleaner and brighter for longer if they are properly inhibited.
6. Dry. To prevent corrosion or discoloration or to improve parts handling, parts should be dried. Spin basket, hot-air conveyors and dryers filled with ground corncobs can be used.
7. Transfer. Mass finishing is capable of any degree of automation. The easiest transfer to use and one of the most effective is a simple conveyor belt. In-house handling of parts is also important.
Big, heavy parts can crush media, leading to premature media breakdown and higher costs. Crushed or whole media can stick or lodge in parts and be carried out of the vibrator.
One of the worst effects parts can have on media is to soil the media. Oils are especially bad for media if the compound solution cannot remove them rapidly enough. Oil stops cutting and parts emerge dirty or dark.
An important relationship between media and parts is the media-to-parts volumetric ratio. In a mass finishing process, the amount of contact between parts is controlled by the ratio of parts to media. Machine settings control the force of these contacts. This is a statistical phenomenon based on the probabilities involved. If you want smooth parts, keep media levels up. Don't overload the machine with parts.
Parts and people. People make parts. Or at least they control the machines that make the parts. When the quality of the parts is maintained, the vibratory finishing process is consistent. If part quality strays, expect the process to as well.
People make decisions. They make changes that affect the parts. The process cannot. People will muddle the process or keep it simple.
Smart people take every possible advantage of the mechanical process. They insist the process run the same everyday. People rely on the vibratory finishing process and they learn how to use it as an excellent quality control tool.
Four elements are significant in vibratory finishing: parts, media, compound solutions and equipment. These link to form an interdependent network, or Tetrahedron of Interdependence. These elements depend on each other to such an extent that if one fails, the process fails.
MediaMedia is one of the supporting parts; however, it is also supported by the other three parts.
Functions. The chips or stones in vibratory finishing are called media. While there are many variations, from aggressive grades capable of removing large quantities of metal from parts in short cycles to media that cannot cut at all. Each type has the following functions:
1. Separate and cushion parts. Parts are mixed with media in a random distribution. The ratio of parts to media helps control the amount of contact the parts have among themselves. If insufficient media is between the parts, they will collide. Media cushions this impact so that irreparable damage does not occur. This is not important for forgings, but is a key consideration of preplate finishing of die castings.
2. Contact critical edges and surfaces. Certain media shapes may lead to a false sense of security. Plastic cones are popular and available in a variety of sizes. That pointed end can touch all areas of a part, but if the cone is large and not a blend of sizes, the pointed end does not do much work.
3. Provide proper degree of cut or luster. Abrasive media will cut. More and bigger abrasive will result in more cut. Non-abrasive media can only produce luster.
1. TETRAHEDRON OF INTERDEPENDENCE shows close relationship of equipment, media, compound solution and parts.
4. Resist lodging in parts. If media sticks in the parts, someone must remove it. If this is difficult, the cost of removal may exceed the cost of the process. To prevent lodging, manufacturers of media produce a great variety of shapes and sizes.
5. Permit separation from parts. Any time you mix parts and media, the two must eventually be separated. The efficiency of this operation depends on the equipment available, but also on the shape and size of the media and parts.
Abrasive media has distinct functions. It releases abrasive slowly into the system. It can cut and deburr edges. It can develop the required surface smoothness and will last as long as possible.
Media wears and exposes fresh grains. Media also wears against other media, which it is designed to do. This keeps media clean and ready to cut with fresh abrasive.
The rate of media wear varies with use. Media designed for rotary barrels will wear out too fast in higher energy equipment such as vibratory systems. Media designed for vibratory use wears out too fast in barrels and other centrifugal equipment.
Abrasive particle size controls the depth of cut. Bigger particles cut deeper and produce rougher surfaces. The hardness of the abrasive should be greater than the material being cut. This cutting action removes burrs and changes the surface contour.
Surface smoothness can be measured using a capacitive sensing probe. Units are read in microinches or micrometers, AA or arithmetic average. Developing good, smooth, low-micro-inch surfaces is easily done, but care must be exercised. Fast cutting products give rougher surfaces at their minimum capability.
The ideal media lasts forever, and this can happen with non-abrasive media such as steel, but not with abrasive media. Heavy-cutting grades wear faster.
Non-abrasive media peens or pound edges and surfaces. It also deburrs edges lightly while brightening surfaces. It also experiences little or no wear or loss.
2. TUB vibrator processes parts as shown here
Since non-abrasive media cannot cut and energy is imparted to it, it can only pound surfaces and edges. No scratch pattern develops; instead, a series of small, uniform dents is imparted to the surface of the work.
The small dents are followed by more and more pounding until the surface gets brighter. This occurs quickly with steel media on malleable metals, but may never develop on hard steels.
Media wears primarily by rubbing against other pieces of media until abrasive grains fall out or are cut away. With no abrasive, cutting stops and so does wear.
Media has an amazing influence on the vibratory finishing process. However, for the media to work, the solution system must behave, the equipment must perform and the parts must be uniform and above all, people must not make waves too high for the process to get through.
Compound SolutionThe compound solution is the least understood and most frequently abused part of the mass finishing process. It has the following functions:
1. Control pH, foam and water hardness. Many compounds are buffered to prevent big changes in pH from taking place. Mass finishing solutions vary from a pH 1 to 14. The most popular range is from 4 to 14.
Some foam is necessary in most applications, especially those involving cleaning. Foam holds the solution in place on the surface of the parts and media. Without foam, process cycles are greatly extended or cleaning just does not occur.
Too much foam kills the action in a vibratory machine. It cushions the mass so effectively that cutting can literally come to a stop. In rotary barrels or centrifugal barrels, however, foam merely moderates action.
In the U.S. water hardness varies from low to high, and public water supplies are often quite different from water drawn from a well in the same area. These great variations in hardness strain the solution since it tried to equalize the differences.
2. Wet surfaces. If the compound solution will not wet or spread on the surface to be cleaned, it will not clean well. Wetting is dependent on concentration.
3. Emulsify oily soils. Physical action improves the speed and stability of emulsification. Because vibratory finishers are excellent scrubbers, they clean well. Media continually works the soils, allowing sometimes-marginal cleaners to "get away with it."
4. Remove tarnish or scale. Both high- and low-pH compound solutions can chemically attack tarnish and scale. These oxides of the metal darken the surfaces and lower reflectivity. Highly alkaline solutions remove rust or scale from some steel. These alkaline solutions can produce light, bleached metal surfaces. Strongly acidic solutions vigorously attack rust and other metal oxides. Unfortunately, they also attack the base metal unless the solution is properly inhibited.
3. ROUND vibrator with no elevation on the chamber
5. Suspend soils. Soil removed or made during the process can redeposit on the parts, media and/or lining of the chamber. Excessively hard water contributes to the problem. Usually a special compound is needed to overcome this.
6. Control lubricity. For years, it was believed that the solution had to be slippery to burnish or brighten a part. This is often true in rotary barrels. In vibratory equipment, excessive slip brings everything to a halt. Traction is lost. Media stops rolling, and nothing happens to the parts. Lubricity should not be completely eliminated, since some lubricity does assist the cutting of the compounds.
7. Control part color. Coloring or darkening parts in vibratory finishing is caused by oxidation or dirt impregnation. Highly alkaline surfaces can bleach out surfaces and acidic compounds can give too clean a surface.
8. Prevent corrosion. The only way to stop corrosion is to include an inhibitor in the compound solution. Some inhibitors are effective on ferrous metals, while others are more effective on non-ferrous metals or those that will oxidize.
9. Provide cooling. If a vibratory process were to run dry, friction would generate high temperatures quickly. The solution cools the mass of media and parts. Low flow rates do not carry away heat as fast. Then more oxidation occurs. Elevated temperatures are seldom needed, even for severe cleaning problems.
Solution FeedThere are three major types of solution-feed systems: batch, recirculation and flow-through. Batch is the simplest system. It is used in barrels and vibrators with no drains. Recirculation systems mix a solution in the tank and pump it into the process, allowing it to drain back into the tank for reuse. This sounds efficient, however the high rate of soil production creates severe contamination problems. Flow-through systems pump fresh solution into the machine, allow it to act and then drain it. Compound flow-through means the parts see only clean media. The solution is always the same strength. There is no sludge tank to clean.
There are three methods of adding compound: manual, batch premix and automatic.
With the manual procedures, you add compound and water manually using a measuring device. In the batch premix method the solution is held in a tank where it is mixed prior to being pumped into the machine. Automatic addition employs a compound-metering pump and rotameter for measuring flow rate of water.
EquipmentEquipment serves several functions.
1. Process parts. The equipment must do the deburring, cleaning, surface conditioning or burnishing.
2. Separation. Separation skills are high on the normal system capability list. The simplest separators are screens. For parts with projections that would cause them to hang up on screens, the tie rod deck is a good choice. It allows parts to slide down parallel rods while the media drops through. Or the inverse if the parts are smaller than the media.
3. Control mass action. Equipment must have adjustments so that more aggressive or gentler work can be done. Tub-type vibratory equipment has a variable-weight system to adjust energy input. Generally, there is a single shaft mounted directly underneath the tub. On each end is an eccentric weight that has some variability in its mass. More weight, more amplitude, faster cutting, rougher surfaces, greater media wear.
The rotational speed of the eccentric weights is another variable. Higher speed generally promotes faster cut, rough surfaces and greater media wear rates.
In round vibratory equipment, the eccentric weights are mounted on each end of a vertical shaft or motor in the center tube of the tub. The amount of weight on the top eccentric generally controls the speed of mass travel around the tub, while the amount on the bottom controls the rollover rate of the mass.
TABLE I - Media-to-Part Volumetric Rates
Media-to-PartRatio by Volume
Normal Commercial Application
0:1
No media. Part-on-part. Used for beating off burrs. No media for cutting.
1:1
Equal volumes of media and parts. Forgings, sand castings; to produce crude, very rough surfaces.
2:1
More gentle, more separation. But still allows relatively severe part-on-part damage.
3:1
About minimum for non-ferrous parts. Considerable part-on-part contact. Fair-to-good for ferrous metals.
4:1
Probably average for non-ferrous parts. Good for ferrous metals.
5:1
Good for non-ferrous metals. Minimal part-to-part contact.
6:1
Very good for non-ferrous parts. Common for preplate on zinc with plastic media.
8:1
For higher quality preplate finishes.
10:1 to 20:1
Even better. Used for very irregularly shaped, fragile parts.
Infinite
Absolutely no part-to-part contact. One part per machine or compartment or the part is fixtured.
PartsThe most important of the Tetrahedron of Interdependence is parts. Without the parts, there would be no process. And, depending on the requirements for the parts, the process is designed, equipment is chosen, media and compound solution are selected. Only when a new process is set up to use existing equipment is the procedure changed. Normally, the parts dictate all else.
There are seven significant purposes of vibratory finishing.
1. Clean. Cleaning can be done with little or no increase in cost, quickly and without heat. Typical parts cleaned commercially in vibratory units include machined engine components, stampings, copper plumbing fittings, brass forgings, plastics, ceramics, rubber and wood.
2. Deburr or radius. This is the best known use of vibratory finishing. All metals, ceramics and some plastics can be deburred. Deburring prevents people from cutting themselves and makes parts feed through automatic feeders more easily.
3. Improve surfaces. Rough surfaces need to be smoothed before they take a high-quality electroplate, paint or anodize. Surface roughness values below 10 microinches AA are common and with good preplate plastic media, values below five microinches are commercially achieved.
4. Brighten. Brightening or burnishing metal surfaces is a good, viable application of mass finishing. It is done easily to the more malleable metals using non-abrasive media. Aluminum, copper, brass and mild steels can be brightened in 10 to 30 min depending on finish quality required and starting surface quality. Harder metals such as stainless, some brasses and other steels take 30 min to several hours.
5. Inhibit. Sensitive metals, including powdered iron, cast iron, copper, brass, zinc and steels of all types will corrode unless protection is provided. Metal surfaces will look cleaner and brighter for longer if they are properly inhibited.
6. Dry. To prevent corrosion or discoloration or to improve parts handling, parts should be dried. Spin basket, hot-air conveyors and dryers filled with ground corncobs can be used.
7. Transfer. Mass finishing is capable of any degree of automation. The easiest transfer to use and one of the most effective is a simple conveyor belt. In-house handling of parts is also important.
Big, heavy parts can crush media, leading to premature media breakdown and higher costs. Crushed or whole media can stick or lodge in parts and be carried out of the vibrator.
One of the worst effects parts can have on media is to soil the media. Oils are especially bad for media if the compound solution cannot remove them rapidly enough. Oil stops cutting and parts emerge dirty or dark.
An important relationship between media and parts is the media-to-parts volumetric ratio. In a mass finishing process, the amount of contact between parts is controlled by the ratio of parts to media. Machine settings control the force of these contacts. This is a statistical phenomenon based on the probabilities involved. If you want smooth parts, keep media levels up. Don't overload the machine with parts.
Parts and people. People make parts. Or at least they control the machines that make the parts. When the quality of the parts is maintained, the vibratory finishing process is consistent. If part quality strays, expect the process to as well.
People make decisions. They make changes that affect the parts. The process cannot. People will muddle the process or keep it simple.
Smart people take every possible advantage of the mechanical process. They insist the process run the same everyday. People rely on the vibratory finishing process and they learn how to use it as an excellent quality control tool.
Monday, May 18, 2009
Small Crystals, Big Benefits
Aircraft landing gear, hydraulic actuators, gas turbine engines, helicopter dynamic components and propeller hubs all make use of hex chrome coatings. Recently, however, an electrodeposited nanocrystalline cobalt-phosphorus alloy, developed with funding from U.S. and Canadian defense partners, has come onto the scene. It has properties that are in many ways better than chrome, overcomes its environmental limitations and can offer improved performance and reduced life-cycle costs. Here’s the scoop on this new technology.
The BackgroundElectroplated engineering hard chromium (EHC) coatings 0.00025–0.010 inches thick are used extensively for imparting wear and erosion resistance to components in aerospace applications. Hard chrome deposits from hex chrome (Cr6+) baths are used in a variety of aircraft components, in both manufacturing and repair/overhaul operations.
In landing gear, for example, outer cylinder IDs are often chrome plated for wear and corrosion resistance. Internal chrome plating is most prevalent on landing gear components and hydraulic actuators. Electroplating lends itself to such applications, which would be difficult or impossible to coat using many of the line-of-sight (LOS) potential replacement processes developed so far.
Unfortunately, hex chrome’s toxicity has reduced its use significantly. OSHA, for example, recently reduced the permissible exposure limit for hex chrome and its compounds from 52–5 µg/m3 as an 8-hr time-weighted average. The rule also includes provisions for employee protection, such as preferred methods for controlling exposure, respiratory protection, protective work clothing and equipment, hygiene areas and practices, medical surveillance, hazard communication and record keeping.
In addition to the health risks associated with hex chrome, there are other process and performance drawbacks associated with use of EHC coatings. EHC plating processes generally have a relatively low electrolytic efficiency, resulting in low deposition rates compared to other plated metals and alloys. Moreover, the intrinsic brittleness of EHC deposits invariably leads to micro- or macro-cracked deposits. These cracks do not compromise wear and erosion resistance, but they are wholly unsuitable for applications where corrosion resistance is required. In these applications, an electrodeposited underlayer of a more ductile and corrosion-resistant material—usually nickel—must be applied.
Landing gear cylinders are a prime example of components with geometries that do not lend themselves well to thermal spray and similar LOS processes. Photo Courtesy The Boeing Company
As a result of these health and safety restrictions and process/performance drawbacks, there is tremendous pressure in the electroplating industry to find a more environmentally benign alternative to hard chrome. Technologies considered as alternatives include thermal spray, plasma vapor deposition, and other chrome-free materials applied by electrolytic or electroless plating techniques.
Over the last 10 years, tungsten carbide-cobalt (WC-Co) and similar materials applied using high-velocity oxygen-fuel (HVOF) thermal spray have undergone extensive demonstration/validation testing as part of the U.S. Department of Defense Hard Chrome Alternatives Team (HCAT) program. These materials have generally been accepted as suitable alternatives for hard chrome within the North American aerospace industry and for other low-volume, high-added-value LOS coating applications. For coating applications requiring non-line-of-sight deposition (NLOS) and/or high-volume, low-value-added production, however, it’s generally believed that only electroplating technologies will be suitable and/or cost-effective.
Most of the electroplated coating alternatives investigated so far have been based on nickel alloys, including both electroless and electrolytic materials. Because nickel is listed by the EPA as a priority pollutant and is considered to be one of the 14 most toxic heavy metals, coatings containing nickel represent a short-term solution at best. Therefore, a non-nickel-based electroplating technology would be a practical, environmentally acceptable alternative for NLOS coating applications.
Micrographs of as-deposited surface (left) and cross-section through a 0.013-inch thick Nanovate CR coating on a 1-inch diam pipe. Of interest are small grain size and lack of pores and microcracks.
Enter NanotechNanotechnology is a relatively new field that deals with the design of extremely small structures having critical length dimensions on the order of a few nanometers. Nanostructured materials—materials with an ultra-fine average grain size usually less than 100 nm—were initially introduced as interfacial materials about 20 years ago. The main characteristic of these materials is an enhanced volume fraction of the interface component (the volume fraction of atoms associated with grain boundaries and triple junctions). This becomes significant when average grain size decreases below 100 nm. Having such a large fraction of atoms located at the interfacial defect structure causes changes in many mechanical, physical and chemical properties of nanocrystalline materials.
The first systematic studies on the synthesis of electrodeposited nanocrystalline materials attempted to optimize certain properties by deliberately controlling the volume fractions of grain boundaries and triple junctions in the materials. Since then, many nanocrystalline metals and alloys have been produced by electrodeposition, including pure nickel, cobalt, palladium and copper; binary alloys such as nickel-iron, nickel-phosphorus, zinc-nickel, palladium-iron and cobalt-tungsten; and ternary alloys such as a nickel-iron-chromium material.
Another such material is Nanovate CR, an electrodeposited nanocrystalline cobalt-phosphorus alloy developed and demonstrated by our company with funding from U.S. and Canadian defense partners. The electrodeposition process can be used in both LOS and NLOS applications, and the material can be viewed as part of an overall strategy to replace currently used EHC processes while significantly improving performance and reducing life-cycle costs.
As shown in Table 1 (below), the Nanovate CR process offers significant improvements over EHC. Like EHC, the material is produced by electrodeposition. It therefore represents a drop-in alternative technology that is fully compatible with the current hard chrome electroplating infrastructure and is well-suited for application to both LOS and NLOS surfaces. Unlike EHC, the process uses no constituents on U.S. EPA or other lists of hazardous materials, nor does it generate hazardous emissions or by-products.
Table 1: Comparison of Nanovate CR and EHC Processes
Nanovate CR
EHC
Deposition Method
Electrodeposition
Electrodeposition
Applicable Part Geometries
LOS and NLOS
LOS and NLOS
Efficiency, %
85–95%
15–35
Deposition Rate, iph
0.002–0.008
0.0005–0.001
Appearance
Free of pits, pores or cracks
Microcracked
Microstructure
Nanocrystalline (avg. grain size = 5–15nm)
-
Emission Analysis
Below OSHA limits
Cr6+
Use of the nanotechnology process also results in significant reductions in energy consumption and increases in throughput. Overall plating efficiency is approximately 90%, compared to less than 35% for EHC. Further, Nanovate CR has a deposition rate ranging from 0.002–0.008 iph, depending on current density, versus the 0.0005–0.001 iph deposition rate typically seen with ENC processes.
PropertiesVisually, nanocrystalline cobalt-phosphorus coatings are uniformly smooth and shiny, similar to EHC. Microscopically, deposits are fully dense structures free from pits, pores and microcracks.
Metallurgically, the material exhibits a hexagonal close-packed (HCP) crystal structure, the equilibrium structure typically found in conventional cobalt at room temperature. Unlike conventional cobalt, however, the material has an average grain size in the range of 5–15 nm. Testing shows that an average grain size in this range results in an optimal combination of strength and ductility. Table 2 (below) compares the properties of nanocrystalline cobalt alloy and EHC.
Table 2: Comparison of Nanovate CR and EHC Properties
Nanovate CR
EHC
Hardness, HVN
530–600 (as deposited)
Min 600
600–680 (heat treated)
–
Wear Volume Loss, mm3/Nm
6–7 × 10-6
9–11 × 10-6
Coefficient of Friction
0.4–0.5
0.7
Pin Wear
Mild
Severe
Corrosion Resistance*
8
2
Hydrogen Embrittlement
Pass with bake
Pass with bake
*ASTM B 537 protection rating after 1,000 hr salt-spray exposure per ASTM B 117
Nanocrystalline alloys such as Nanovate CR display significant increases in hardness and strength relative to their coarser-grained, conventional counterparts. Through a solid solution hardening mechanism, microhardness values typically range from 530–600 VHN.
Effect of annealing at various temperatures and times on Vickers microhardness of nanocrystalline cobalt-phosphorus deposit. A short heat treatment can substantially increase microhardness.
A further increase in hardness can be obtained by annealing the as-deposited material. A short heat treatment process results in microhardness increases of more than 150 VHN.
Nanovate CR also has improved wear and lubricity relative to EHC. The material exhibited less wear loss than EHC in pin-on-disk sliding wear testing. Wear loss of the mating material—in this case, an alumina ball—was also less severe, indicating that nanocrystalline cobalt-phosphorus has a lower coefficient of friction than EHC.
Corrosion resistance in salt-spray testing is also improved. In a comparison of Nanovate CR and other hardfacing materials after 1,000 hr of exposure in a salt-spray environment per ASTM B 117, the material’s ASTM B 537 protection rating decreased to only 8, compared with a rating of less 2 for EHC. Further, the nanocrystalline deposit was 50% thinner than the EHC and HVOF coatings used in the test.
Another important consideration in aerospace plating is the potential for hydrogen embrittlement in high-strength steel components. The high plating efficiency of the Nanovate CR process leads to significantly less hydrogen generation at the cathode compared with EHC processes, thus minimizing likelihood of hydrogen uptake and subsequent embrittlement of susceptible materials. Hydrogen embrittlement tests conducted in accordance with ASTM F 519 indicate that standard hydrogen embrittlement relief baking procedures for EHC can be applied to nanocrystalline deposits to fully eliminate any risk of embrittlement.
ASTM B 537 protection rating of Nanovate CR and EHC coatings as a function of salt-spray exposure time.
Adhesion of nanocrystalline cobalt-phosphorus deposits has been evaluated on a number of aerospace substrate materials. In bend tests conducted in accordance with ASTM B 571, deposits showed no signs of peeling or delamination at low (10×) magnification. In testing in accordance with ASTM B 553, samples coated with Nanovate CR were exposed to thermal cycling involving submerging the samples into liquids nitrogen for one minute followed by submersion in hot (90 ºC) water for one minute. After 30 thermal cycles no delamination occurred and the displacement of the coating relative to the underlying substrate was substantially zero.
The BackgroundElectroplated engineering hard chromium (EHC) coatings 0.00025–0.010 inches thick are used extensively for imparting wear and erosion resistance to components in aerospace applications. Hard chrome deposits from hex chrome (Cr6+) baths are used in a variety of aircraft components, in both manufacturing and repair/overhaul operations.
In landing gear, for example, outer cylinder IDs are often chrome plated for wear and corrosion resistance. Internal chrome plating is most prevalent on landing gear components and hydraulic actuators. Electroplating lends itself to such applications, which would be difficult or impossible to coat using many of the line-of-sight (LOS) potential replacement processes developed so far.
Unfortunately, hex chrome’s toxicity has reduced its use significantly. OSHA, for example, recently reduced the permissible exposure limit for hex chrome and its compounds from 52–5 µg/m3 as an 8-hr time-weighted average. The rule also includes provisions for employee protection, such as preferred methods for controlling exposure, respiratory protection, protective work clothing and equipment, hygiene areas and practices, medical surveillance, hazard communication and record keeping.
In addition to the health risks associated with hex chrome, there are other process and performance drawbacks associated with use of EHC coatings. EHC plating processes generally have a relatively low electrolytic efficiency, resulting in low deposition rates compared to other plated metals and alloys. Moreover, the intrinsic brittleness of EHC deposits invariably leads to micro- or macro-cracked deposits. These cracks do not compromise wear and erosion resistance, but they are wholly unsuitable for applications where corrosion resistance is required. In these applications, an electrodeposited underlayer of a more ductile and corrosion-resistant material—usually nickel—must be applied.
Landing gear cylinders are a prime example of components with geometries that do not lend themselves well to thermal spray and similar LOS processes. Photo Courtesy The Boeing Company
As a result of these health and safety restrictions and process/performance drawbacks, there is tremendous pressure in the electroplating industry to find a more environmentally benign alternative to hard chrome. Technologies considered as alternatives include thermal spray, plasma vapor deposition, and other chrome-free materials applied by electrolytic or electroless plating techniques.
Over the last 10 years, tungsten carbide-cobalt (WC-Co) and similar materials applied using high-velocity oxygen-fuel (HVOF) thermal spray have undergone extensive demonstration/validation testing as part of the U.S. Department of Defense Hard Chrome Alternatives Team (HCAT) program. These materials have generally been accepted as suitable alternatives for hard chrome within the North American aerospace industry and for other low-volume, high-added-value LOS coating applications. For coating applications requiring non-line-of-sight deposition (NLOS) and/or high-volume, low-value-added production, however, it’s generally believed that only electroplating technologies will be suitable and/or cost-effective.
Most of the electroplated coating alternatives investigated so far have been based on nickel alloys, including both electroless and electrolytic materials. Because nickel is listed by the EPA as a priority pollutant and is considered to be one of the 14 most toxic heavy metals, coatings containing nickel represent a short-term solution at best. Therefore, a non-nickel-based electroplating technology would be a practical, environmentally acceptable alternative for NLOS coating applications.
Micrographs of as-deposited surface (left) and cross-section through a 0.013-inch thick Nanovate CR coating on a 1-inch diam pipe. Of interest are small grain size and lack of pores and microcracks.
Enter NanotechNanotechnology is a relatively new field that deals with the design of extremely small structures having critical length dimensions on the order of a few nanometers. Nanostructured materials—materials with an ultra-fine average grain size usually less than 100 nm—were initially introduced as interfacial materials about 20 years ago. The main characteristic of these materials is an enhanced volume fraction of the interface component (the volume fraction of atoms associated with grain boundaries and triple junctions). This becomes significant when average grain size decreases below 100 nm. Having such a large fraction of atoms located at the interfacial defect structure causes changes in many mechanical, physical and chemical properties of nanocrystalline materials.
The first systematic studies on the synthesis of electrodeposited nanocrystalline materials attempted to optimize certain properties by deliberately controlling the volume fractions of grain boundaries and triple junctions in the materials. Since then, many nanocrystalline metals and alloys have been produced by electrodeposition, including pure nickel, cobalt, palladium and copper; binary alloys such as nickel-iron, nickel-phosphorus, zinc-nickel, palladium-iron and cobalt-tungsten; and ternary alloys such as a nickel-iron-chromium material.
Another such material is Nanovate CR, an electrodeposited nanocrystalline cobalt-phosphorus alloy developed and demonstrated by our company with funding from U.S. and Canadian defense partners. The electrodeposition process can be used in both LOS and NLOS applications, and the material can be viewed as part of an overall strategy to replace currently used EHC processes while significantly improving performance and reducing life-cycle costs.
As shown in Table 1 (below), the Nanovate CR process offers significant improvements over EHC. Like EHC, the material is produced by electrodeposition. It therefore represents a drop-in alternative technology that is fully compatible with the current hard chrome electroplating infrastructure and is well-suited for application to both LOS and NLOS surfaces. Unlike EHC, the process uses no constituents on U.S. EPA or other lists of hazardous materials, nor does it generate hazardous emissions or by-products.
Table 1: Comparison of Nanovate CR and EHC Processes
Nanovate CR
EHC
Deposition Method
Electrodeposition
Electrodeposition
Applicable Part Geometries
LOS and NLOS
LOS and NLOS
Efficiency, %
85–95%
15–35
Deposition Rate, iph
0.002–0.008
0.0005–0.001
Appearance
Free of pits, pores or cracks
Microcracked
Microstructure
Nanocrystalline (avg. grain size = 5–15nm)
-
Emission Analysis
Below OSHA limits
Cr6+
Use of the nanotechnology process also results in significant reductions in energy consumption and increases in throughput. Overall plating efficiency is approximately 90%, compared to less than 35% for EHC. Further, Nanovate CR has a deposition rate ranging from 0.002–0.008 iph, depending on current density, versus the 0.0005–0.001 iph deposition rate typically seen with ENC processes.
PropertiesVisually, nanocrystalline cobalt-phosphorus coatings are uniformly smooth and shiny, similar to EHC. Microscopically, deposits are fully dense structures free from pits, pores and microcracks.
Metallurgically, the material exhibits a hexagonal close-packed (HCP) crystal structure, the equilibrium structure typically found in conventional cobalt at room temperature. Unlike conventional cobalt, however, the material has an average grain size in the range of 5–15 nm. Testing shows that an average grain size in this range results in an optimal combination of strength and ductility. Table 2 (below) compares the properties of nanocrystalline cobalt alloy and EHC.
Table 2: Comparison of Nanovate CR and EHC Properties
Nanovate CR
EHC
Hardness, HVN
530–600 (as deposited)
Min 600
600–680 (heat treated)
–
Wear Volume Loss, mm3/Nm
6–7 × 10-6
9–11 × 10-6
Coefficient of Friction
0.4–0.5
0.7
Pin Wear
Mild
Severe
Corrosion Resistance*
8
2
Hydrogen Embrittlement
Pass with bake
Pass with bake
*ASTM B 537 protection rating after 1,000 hr salt-spray exposure per ASTM B 117
Nanocrystalline alloys such as Nanovate CR display significant increases in hardness and strength relative to their coarser-grained, conventional counterparts. Through a solid solution hardening mechanism, microhardness values typically range from 530–600 VHN.
Effect of annealing at various temperatures and times on Vickers microhardness of nanocrystalline cobalt-phosphorus deposit. A short heat treatment can substantially increase microhardness.
A further increase in hardness can be obtained by annealing the as-deposited material. A short heat treatment process results in microhardness increases of more than 150 VHN.
Nanovate CR also has improved wear and lubricity relative to EHC. The material exhibited less wear loss than EHC in pin-on-disk sliding wear testing. Wear loss of the mating material—in this case, an alumina ball—was also less severe, indicating that nanocrystalline cobalt-phosphorus has a lower coefficient of friction than EHC.
Corrosion resistance in salt-spray testing is also improved. In a comparison of Nanovate CR and other hardfacing materials after 1,000 hr of exposure in a salt-spray environment per ASTM B 117, the material’s ASTM B 537 protection rating decreased to only 8, compared with a rating of less 2 for EHC. Further, the nanocrystalline deposit was 50% thinner than the EHC and HVOF coatings used in the test.
Another important consideration in aerospace plating is the potential for hydrogen embrittlement in high-strength steel components. The high plating efficiency of the Nanovate CR process leads to significantly less hydrogen generation at the cathode compared with EHC processes, thus minimizing likelihood of hydrogen uptake and subsequent embrittlement of susceptible materials. Hydrogen embrittlement tests conducted in accordance with ASTM F 519 indicate that standard hydrogen embrittlement relief baking procedures for EHC can be applied to nanocrystalline deposits to fully eliminate any risk of embrittlement.
ASTM B 537 protection rating of Nanovate CR and EHC coatings as a function of salt-spray exposure time.
Adhesion of nanocrystalline cobalt-phosphorus deposits has been evaluated on a number of aerospace substrate materials. In bend tests conducted in accordance with ASTM B 571, deposits showed no signs of peeling or delamination at low (10×) magnification. In testing in accordance with ASTM B 553, samples coated with Nanovate CR were exposed to thermal cycling involving submerging the samples into liquids nitrogen for one minute followed by submersion in hot (90 ºC) water for one minute. After 30 thermal cycles no delamination occurred and the displacement of the coating relative to the underlying substrate was substantially zero.
Sunday, May 17, 2009
Notes From the Field
Electroless nickel has a unique range of properties, including consistency of deposit thickness, excellent corrosion protection, high and controllable hardness and wear resistance, controlled magnetic properties and others. EN processes form a deposit by the chemical reduction of nickel, which, depending on the reducing agent used, gives either a nickel-phosphorus or nickel-boron alloy.
This reaction must be managed to ensure that the reduction only takes place on the work and not on the tanks and equipment used for plating. To ensure this, EN baths require additives commonly known as stabilizers. Most stabilizers also give improved brightness to the EN nickel deposit.
Lead and cadmium have been the industry standards to provide both brightness and stability for the full range of electroless nickel processes over the last 30–40 years, are very well understood and result in extremely reliable baths. Use of these metals, however, is increasingly controlled, resulting in a search for replacement stabilizers. Although the main directives (RoHS, ELV and WEEE) allow limited use of lead and cadmium, these limits have severely curtailed lead use and effectively prohibited use of cadmium. One response to this is development of baths that are purely lead-stabilized; however, this gives semi-bright deposits and—depending on the size of additions made—may result in lead content that exceeds RoHS limits at some point in the bath life.
Another factor is that legislation on environmental issues will only become more intense. One effect of this is that EN users are demanding complete removal of lead from EN plating solutions. This has resulted in many new EN processes formulated to be lead- and cadmium-free. It is MacDermid’s experience that cadmium- and lead-free processes now make up more than 30% of the EN in Europe and the Americas.
Search for ReplacementsThe original cadmium- and lead-free processes often used bismuth—one of the original metallic stabilizers used in electroless nickel—as the main stabilizer. Over the next 30 years bismuth was replaced with more reliable, better-performing lead and cadmium. Reverting to bismuth stabilizing technology is therefore in some ways a step backwards. Use of bismuth can also be related to some recent issues with EN processes such as reduced shelf life, poor activation of copper substrates, and high stress in some plating solutions, so the move back to it as a main stabilizer has not been an unmitigated success.
Bismuth is one of many materials appearing on various watch lists, but there are no specific health and safety or environmental issues with its use in EN processes in the U.S. or Europe. However, our company made the decision to research EN processes containing no metal stabilizers, partly to make allowance for future legislation and also due to an element of dissatisfaction with existing cadmium- and lead-free processes.
Many products are now available as non-metallically stabilized chemistry, but this article will focus on our company’s NiKlad ELV 835, an EN bath containing 4–7% phosphorus. This phosphorus concentration results in a deposit with an excellent balance between hardness and corrosion resistance. Results reported here are from actual customers, mainly job shop platers, but results also include some in-house plating operations, where demands on the plating solution are often lower.
Bath OperationOrganically stabilized EN solution has bath life comparable to other EN technologies. In the laboratory, users achieve eight metal turnovers (MTOs); at customer facilities with drag-out, life can be extended to 10 or more MTOs.
Organically stabilized EN baths have a relatively consistent, high plating rate throughout bath life. Conventional EN solutions tend to slow as they age.
The bath can operate in both polypropylene and stainless steel tanks. For stainless tanks, this has included both nitric acid passivated and anodically protected installations. Stainless steel tanks are normally cleaned and passivated when a bath is dumped, which is also the case with organically stabilized bath. There’s no perceived advantage or disadvantage in moving away from metallically stabilized chemistry, although organic stabilizers are not plated out by the anodic protection equipment, making the bath more consistent.
With organically stabilized EN bath, the same chemistry is suitable for both polypropylene and stainless steel operations. Used in polypropylene tanks, the new chemistry gives slightly different performance than conventional . Anecdotal evidence suggests it is more stable than conventional systems, especially when used in old, scratched and damaged polypropylene tanks. One customer found that the tank plated up daily with existing ELV technology; with the new bath it can be used for three days before cleaning.
Organically stabilized baths have a high plating rate, which lasts throughout the bath life. Conventional solutions tend to slow as they age, partly due to build-up of contaminants such as orthophosphate, sulfate, ammonia or sodium, but also due to the need with metallic stabilizers for the metal concentration to slowly rise as the bath ages.
The high speed of organically stabilized baths can have some unplanned side effects. One customer using the bath found that when plating at more than 1mil/hr, operators could not keep pace with the solution. They slowed the bath down to 0.8mils/hr by reducing bath temperature. This also had the effect of reducing energy costs, and they were still obtaining a higher plating rate than their previous solution.
One of the major advantages seen by platers operating the organically stabilized baths is the lack of sensitivity to additions compared with metallically stabilized solutions. One plater operating the bath at 3g/L of nickel allowed nickel concentration to fall to 1.2g/L nickel. Rather than making several small additions to bring the bath back to strength, they added one large addition. In a metallically stabilized solution, this would have stopped plating or at best given skip plating. The organically stabilized bath continued to plate as normal.
This is an advantage when using the bath as a low-metal operation, because it means that auto-dosing is not needed to operate at nickel concentration below 6g/L. The mechanism that allows this is not clear but must be related to the strength of the stabilizer and the fact that normal transport mechanisms are not as critical as with the metallic atoms.
Ability to tolerate large additions is also important for platers operating with low (<0.1ft2/gal) bath loadings. The normal reduction in plating rate due to constant low loading is not an issue with organically stabilized chemistry.
Conversely, using much higher bath loadings (>1.5ft2/gal) is also acceptable. Despite this wider window of operation and control, organically stabilized EN baths, like all plating baths, perform best when operated within tight parameters, ideally between 90–105%.
SubstratesSome of the new RoHS-compliant EN chemistries have suffered from poor activation when plating some steel substrates (for example, materials with a high lead content), copper and brass (especially when parts are activated in plating solution) and aluminum. This is believed to be due to higher concentration of non-lead stabilizers in the solutions. Use of organically stabilized solutions has resulted in fewer issues. An advantage of the organically stabilized bath is its life when plating aluminum. A conventional EN solution used without a strike bath normally will not plate more than four MTOs on aluminum due to the increased risk of adhesion failure. One customer using the new chemistry to plate mainly on aluminum found they could still continue to five MTOs without a strike, even though the part is machined (not cast) and is hot solder dipped after plating—a tough adhesion test. The reasons for this are not certain, but there may be two factors at work: 1)A possible increased tolerance to metallic contamination; and 2)Low stress in the bath as it ages.
Additive StabilityTo ensure solubility of the lead and bismuth used as stabilizers, it’s common for metal-stabilized EN baths to contain strong complexors, normally EDTA or derivatives of EDTA. The result is that spent baths are more difficult to waste treat than solutions not containing these compounds. Because new solutions use no metals other than nickel, these materials are not used in the bath.
Even using very strong complexors, the shelf life of many ELV-compliant chemistries is shorter than conventional lead-stabilized systems, as the metals can slowly precipitate. This can cause inconsistent performance and a short shelf life. Bismuth is even worse; it can be difficult to prevent its precipitation without moving to a four-component system.
Deposit CompositionAs-plated deposits from organically stabilized baths look slightly different than deposits from conventional and metallically stabilized systems—they are “whiter” and less yellow. This is due to lack of metallic stabilizer being co-deposited, which can be as high as 0.15% if cadmium and lead are used and 0.1% for bismuth.
Wear resistance of organically stabilized EN deposits is slightly lower than that of conventional mid-phosphorus deposits, while micro-hardness is slightly higher.
With organically stabilized EN, the only materials co-deposited with the phosphorus and nickel are carbon and sulfur with the low- and medium-phosphorus systems, and carbon with the high-phosphorus bath. In a comparison of 4–6% phosphorus deposits plated using a lead-cadmium and an organically stabilized bath, the conventional chemistry co-deposited <200 ppm carbon and 173 ppm sulfur when new and <400 ppm carbon and 414 ppm sulfur after six MTOs. A similar deposit from an organically stabilized bath co-deposited <400 ppm carbon and <50 ppm sulfur when new; after six MTOs carbon and sulfur contents in the deposit were <200 ppm and 561 ppm, respectively.
The organically stabilized bath also shows a low/medium level of phosphorus throughout its life, with the level falling slightly as the bath ages.
Deposit PropertiesDeposits from organically stabilized EN baths are harder than those from a conventional medium-phosphorus bath. This is more related to phosphorus content than the stabilizers used in the bath.
Wear resistance measured using a Taber wear test is reduced in line with the extra hardness seen, while corrosion resistance of the deposit tested using electrochemical means was as good as conventional systems with slightly higher phosphorus content. Neutral salt-spray testing was then carried out to confirm results. Performance when the bath was new was very good, but it decreased slightly as the solution aged. This is seen as a result of increased porosity, and in practice, user parts tested side by side with metallically stabilized materials were found to give the same hours salt-spray resistance.
Deposits from organically stabilized baths show no signs of high tensile stress throughout bath life. The bath starts off slightly tensile and slowly becomes less, sometimes even crossing over to the compressive side. This is a function of managing phosphorus content of the bath and maintaining the speed of the solution throughout its life. It is also a factor in assessing the ability of the bath to continue to plate quality deposits as it ages. In conventional EN systems, the growth of tensile stress as the bath ages has been a limiting factor in bath life.
Although it has little effect on performance of the deposit, brightness is still a major contributor to perception of quality—it has happened, but we have rarely come across an application where the customer wants a duller deposit. The organically stabilized chemistry can produce bright deposits, but it does not yet match metallically stabilized processes in all installations. Users have said that the gloss and brightness of the deposit do not change as much as conventional systems and remain quite consistent for the life of the solution.
This reaction must be managed to ensure that the reduction only takes place on the work and not on the tanks and equipment used for plating. To ensure this, EN baths require additives commonly known as stabilizers. Most stabilizers also give improved brightness to the EN nickel deposit.
Lead and cadmium have been the industry standards to provide both brightness and stability for the full range of electroless nickel processes over the last 30–40 years, are very well understood and result in extremely reliable baths. Use of these metals, however, is increasingly controlled, resulting in a search for replacement stabilizers. Although the main directives (RoHS, ELV and WEEE) allow limited use of lead and cadmium, these limits have severely curtailed lead use and effectively prohibited use of cadmium. One response to this is development of baths that are purely lead-stabilized; however, this gives semi-bright deposits and—depending on the size of additions made—may result in lead content that exceeds RoHS limits at some point in the bath life.
Another factor is that legislation on environmental issues will only become more intense. One effect of this is that EN users are demanding complete removal of lead from EN plating solutions. This has resulted in many new EN processes formulated to be lead- and cadmium-free. It is MacDermid’s experience that cadmium- and lead-free processes now make up more than 30% of the EN in Europe and the Americas.
Search for ReplacementsThe original cadmium- and lead-free processes often used bismuth—one of the original metallic stabilizers used in electroless nickel—as the main stabilizer. Over the next 30 years bismuth was replaced with more reliable, better-performing lead and cadmium. Reverting to bismuth stabilizing technology is therefore in some ways a step backwards. Use of bismuth can also be related to some recent issues with EN processes such as reduced shelf life, poor activation of copper substrates, and high stress in some plating solutions, so the move back to it as a main stabilizer has not been an unmitigated success.
Bismuth is one of many materials appearing on various watch lists, but there are no specific health and safety or environmental issues with its use in EN processes in the U.S. or Europe. However, our company made the decision to research EN processes containing no metal stabilizers, partly to make allowance for future legislation and also due to an element of dissatisfaction with existing cadmium- and lead-free processes.
Many products are now available as non-metallically stabilized chemistry, but this article will focus on our company’s NiKlad ELV 835, an EN bath containing 4–7% phosphorus. This phosphorus concentration results in a deposit with an excellent balance between hardness and corrosion resistance. Results reported here are from actual customers, mainly job shop platers, but results also include some in-house plating operations, where demands on the plating solution are often lower.
Bath OperationOrganically stabilized EN solution has bath life comparable to other EN technologies. In the laboratory, users achieve eight metal turnovers (MTOs); at customer facilities with drag-out, life can be extended to 10 or more MTOs.
Organically stabilized EN baths have a relatively consistent, high plating rate throughout bath life. Conventional EN solutions tend to slow as they age.
The bath can operate in both polypropylene and stainless steel tanks. For stainless tanks, this has included both nitric acid passivated and anodically protected installations. Stainless steel tanks are normally cleaned and passivated when a bath is dumped, which is also the case with organically stabilized bath. There’s no perceived advantage or disadvantage in moving away from metallically stabilized chemistry, although organic stabilizers are not plated out by the anodic protection equipment, making the bath more consistent.
With organically stabilized EN bath, the same chemistry is suitable for both polypropylene and stainless steel operations. Used in polypropylene tanks, the new chemistry gives slightly different performance than conventional . Anecdotal evidence suggests it is more stable than conventional systems, especially when used in old, scratched and damaged polypropylene tanks. One customer found that the tank plated up daily with existing ELV technology; with the new bath it can be used for three days before cleaning.
Organically stabilized baths have a high plating rate, which lasts throughout the bath life. Conventional solutions tend to slow as they age, partly due to build-up of contaminants such as orthophosphate, sulfate, ammonia or sodium, but also due to the need with metallic stabilizers for the metal concentration to slowly rise as the bath ages.
The high speed of organically stabilized baths can have some unplanned side effects. One customer using the bath found that when plating at more than 1mil/hr, operators could not keep pace with the solution. They slowed the bath down to 0.8mils/hr by reducing bath temperature. This also had the effect of reducing energy costs, and they were still obtaining a higher plating rate than their previous solution.
One of the major advantages seen by platers operating the organically stabilized baths is the lack of sensitivity to additions compared with metallically stabilized solutions. One plater operating the bath at 3g/L of nickel allowed nickel concentration to fall to 1.2g/L nickel. Rather than making several small additions to bring the bath back to strength, they added one large addition. In a metallically stabilized solution, this would have stopped plating or at best given skip plating. The organically stabilized bath continued to plate as normal.
This is an advantage when using the bath as a low-metal operation, because it means that auto-dosing is not needed to operate at nickel concentration below 6g/L. The mechanism that allows this is not clear but must be related to the strength of the stabilizer and the fact that normal transport mechanisms are not as critical as with the metallic atoms.
Ability to tolerate large additions is also important for platers operating with low (<0.1ft2/gal) bath loadings. The normal reduction in plating rate due to constant low loading is not an issue with organically stabilized chemistry.
Conversely, using much higher bath loadings (>1.5ft2/gal) is also acceptable. Despite this wider window of operation and control, organically stabilized EN baths, like all plating baths, perform best when operated within tight parameters, ideally between 90–105%.
SubstratesSome of the new RoHS-compliant EN chemistries have suffered from poor activation when plating some steel substrates (for example, materials with a high lead content), copper and brass (especially when parts are activated in plating solution) and aluminum. This is believed to be due to higher concentration of non-lead stabilizers in the solutions. Use of organically stabilized solutions has resulted in fewer issues. An advantage of the organically stabilized bath is its life when plating aluminum. A conventional EN solution used without a strike bath normally will not plate more than four MTOs on aluminum due to the increased risk of adhesion failure. One customer using the new chemistry to plate mainly on aluminum found they could still continue to five MTOs without a strike, even though the part is machined (not cast) and is hot solder dipped after plating—a tough adhesion test. The reasons for this are not certain, but there may be two factors at work: 1)A possible increased tolerance to metallic contamination; and 2)Low stress in the bath as it ages.
Additive StabilityTo ensure solubility of the lead and bismuth used as stabilizers, it’s common for metal-stabilized EN baths to contain strong complexors, normally EDTA or derivatives of EDTA. The result is that spent baths are more difficult to waste treat than solutions not containing these compounds. Because new solutions use no metals other than nickel, these materials are not used in the bath.
Even using very strong complexors, the shelf life of many ELV-compliant chemistries is shorter than conventional lead-stabilized systems, as the metals can slowly precipitate. This can cause inconsistent performance and a short shelf life. Bismuth is even worse; it can be difficult to prevent its precipitation without moving to a four-component system.
Deposit CompositionAs-plated deposits from organically stabilized baths look slightly different than deposits from conventional and metallically stabilized systems—they are “whiter” and less yellow. This is due to lack of metallic stabilizer being co-deposited, which can be as high as 0.15% if cadmium and lead are used and 0.1% for bismuth.
Wear resistance of organically stabilized EN deposits is slightly lower than that of conventional mid-phosphorus deposits, while micro-hardness is slightly higher.
With organically stabilized EN, the only materials co-deposited with the phosphorus and nickel are carbon and sulfur with the low- and medium-phosphorus systems, and carbon with the high-phosphorus bath. In a comparison of 4–6% phosphorus deposits plated using a lead-cadmium and an organically stabilized bath, the conventional chemistry co-deposited <200 ppm carbon and 173 ppm sulfur when new and <400 ppm carbon and 414 ppm sulfur after six MTOs. A similar deposit from an organically stabilized bath co-deposited <400 ppm carbon and <50 ppm sulfur when new; after six MTOs carbon and sulfur contents in the deposit were <200 ppm and 561 ppm, respectively.
The organically stabilized bath also shows a low/medium level of phosphorus throughout its life, with the level falling slightly as the bath ages.
Deposit PropertiesDeposits from organically stabilized EN baths are harder than those from a conventional medium-phosphorus bath. This is more related to phosphorus content than the stabilizers used in the bath.
Wear resistance measured using a Taber wear test is reduced in line with the extra hardness seen, while corrosion resistance of the deposit tested using electrochemical means was as good as conventional systems with slightly higher phosphorus content. Neutral salt-spray testing was then carried out to confirm results. Performance when the bath was new was very good, but it decreased slightly as the solution aged. This is seen as a result of increased porosity, and in practice, user parts tested side by side with metallically stabilized materials were found to give the same hours salt-spray resistance.
Deposits from organically stabilized baths show no signs of high tensile stress throughout bath life. The bath starts off slightly tensile and slowly becomes less, sometimes even crossing over to the compressive side. This is a function of managing phosphorus content of the bath and maintaining the speed of the solution throughout its life. It is also a factor in assessing the ability of the bath to continue to plate quality deposits as it ages. In conventional EN systems, the growth of tensile stress as the bath ages has been a limiting factor in bath life.
Although it has little effect on performance of the deposit, brightness is still a major contributor to perception of quality—it has happened, but we have rarely come across an application where the customer wants a duller deposit. The organically stabilized chemistry can produce bright deposits, but it does not yet match metallically stabilized processes in all installations. Users have said that the gloss and brightness of the deposit do not change as much as conventional systems and remain quite consistent for the life of the solution.
Tuesday, May 12, 2009
Reducing Hexavalent Chromium Emissions
The plating industry is one of the most regulated industries in the U.S. Based on past practices, strict regulatory compliance was necessary and warranted. These past practices have cast a dark shadow across the entire industry and have caused regulations to become more stringent.
Consequently, the plating industry must be as forward thinking as possible in terms of meeting regulatory demands and standards set by the EPA and other regulating bodies. Two areas that the plating industry must focus on are the allowable discharge standards for air and water. It is no secret that at some point zero discharge is not going to be a buzz word for the future, but a desired standard.
Techmetal, Inc., Dayton, Ohio, a chromium plating shop with numerous tanks, operations (manual and automatic) and applications, wanted to make certain it would be able to meet the possible zero discharge limit. However, Techmetals found that virtually every conventional pollution control device could exceed certain limits as a result of equipment failure.
Techmetal reviewed many professional articles on EPA standards for chromium emissions, testing data and development of the MACT standard. After its research, Techmetals still had three concerns: Moving parts will break down, causing allowable discharge limits to be exceeded; The technology used to conduct MACT testing still emits chromium particles; and Some states and localities have concentration-based and/or risk-based rules that are more restrictive than the MACT standard. Because of these concerns, the chromium plater searched for a device that could produce zero chromium emissions.
Schematic diagram of the EED.
Conventional ventilation. Techmetals uses hexavalent chromium, which extends the life and enhances the performance of a variety of manufactured parts. Chromium coated parts offer excellent corrosion resistance and hardness and a low coefficient of friction, characteristics deemed invaluable by the defense and aircraft industries.
Unfortunately, hexavalent chromium is highly toxic and a known carcinogen, especially when carried through the air as a vapor. Transport of chromium particles occurs during plating because a number of by-products are generated, such as hydrogen and oxygen gases, water vapor and chromic acid mist. Hydrogen and oxygen gases are generated due to the inefficiency of electrolytic reaction during chromium plating. Chromium plating is about 14 to 18 pct efficient when compared to acid zinc plating, which is 93 to 96 pct efficient. Chromic acid mist is generated by the bursting action of the hydrogen and oxygen gases escaping at the surface of the solution and air interface.
With conventional ventilation systems, the chromic acid mist is carried away by air moving across the tank via a blower or push-pull device. In addition, since the tank is open to the atmosphere, stray air currents add to the mobility of these particles. Typically, a push-pull system is designed to move these particles away from the plating tank to a scrubber where the air is cleaned of chromic acid and discharged.
Conventional ventilation systems, such as packed-bed scrubbers, composite mesh pads and fiber bed mist eliminators, are commonly installed by plating shops in order to comply with air emission standards and regulations set by the EPA. These systems lower the amount of hexavalent chromium released into the atmosphere. However, Techmetals was looking for something that eliminated emissions. It decided to use an emission elimination device (EED) developed by Responsible Alternatives, Inc., Dayton, Ohio. An EED is a specially engineered hood system that contains hexavalent chromium mist without interfering with the normal chromium plating operation.
Emission Elimination Device.
Techmetal placed an adapter on top of the tank walls with openings for bussings, utilities and other electrical conduits. A hinged cover was connected to the adapter ring. A sealing gasket material was applied between the tank and adapter and the hood and adapter. A vacuum pump process was connected to the cover to evacuate any chemical mists or fumes that might remain in suspension after the plating process had ceased and the cover remained closed. Gases from the vacuum system were exhausted through a filtering system near the plating tank.
When this tank cover system is used over a chromium tank, chromic acid mist particles simply rise and fall back into the solution because of gravity and the absence of forced air. The hydrogen and oxygen gases generated during the chromium plating process escape through the system's membrane, water condenses on the inside walls of the enclosure and the condensate trickles back into the plating solution. In setups such as Techmetals, where chromium mist extends to the height of the buss bars, the water droplets continuously clean the bussing.
Emission standards for platers. Data was collected from two chromium plating facilities using this system (Table I). These results are not from stacks or scrubbers, because the system eliminates them. The data reflects testing around the process tank with workers wearing testing equipment. Exposure times will vary, but even when compared to OSHA standards, the system provides results that are 10 to 30 times below the regulatory standards.
TABLE I—Air Sampling Results for Chromium at Two Plating Shops California Plating Shop Chromium Conc.(mg/m3) Permissable Exposure Limits(mg/m3) OSHA CalOSHA NIOSHA ACFGIH 0.0003 at filter 0.1 0.05 0.001/0.025 0.05 Ohio Plating Shop Person No. 1 0.0007 at filter ACGIH TLV 0.05 Person No. 2 0.0004 at filter ACGIH TLV 0.05
Energy cost savings. Techmetals conducted its own energy savings study using kilowatt/hour rate structures developed by the local utility company. Table II displays the on-site results when three of the plating shop's tanks were fitted with a cover system. Savings will vary depending on the geographic location, percentage of tanks with the system and whether stack testing was eliminated or reduced.
TABLE II—Operating Cost for a Three-Tank System Emission Elimination Device System (EED) Conventional Ventilation Three-Tank System Original Equipment andInstallation Cost $130,000 $107,000 Annual Operating Cost $6,866 $69,167 Total First Year Cost $136,866 $176,167
Testing the EED. Even though Techmetals eliminated the need for testing, the EPA still required that "stack" testing be done. As a result, Techmetals' staff had to develop its own procedure and application for testing in order to receive the EPA's approval.
The procedure took several steps. For initial compliance testing, Techmetals' staff performed a smoke testing procedure based on total tank surface area. The smoke test verified the design and placement of all seals and that the system's membrane was working. The next step was to initiate a continuance/compliance monitoring program, which required logs of daily, weekly and monthly inspections. Then Techmetals developed a recommended replacement program for all critical equipment, membranes, seals and tie-downs. Finally, Techmetals' reporting requirements were determined by the local air regulatory authority.
Techmetals had several concerns about using the system. Since the system conserves energy, additional cooling of the solution may be required. However, this was not a problem at Techmetals since it had adequate cooling capabilities. Another concern was that air agitation had to be eliminated. Techmetals used air for solution agitation and mobility to enhance plating conditions. It has now installed the appropriate pumps and hardware to perform solution agitation without air. The system also conserves water. Therefore, Techmetals needed a system designed to evaporate water in the rinse tank. Techmetals' final concern was correctly designing the system.
Despite these concerns, the plating shop has achieved a capital cost pay back of nine months per unit based on energy savings, while establishing a zero emissions standard for chromium.
source .pfonline.com/articles/129705.html
Consequently, the plating industry must be as forward thinking as possible in terms of meeting regulatory demands and standards set by the EPA and other regulating bodies. Two areas that the plating industry must focus on are the allowable discharge standards for air and water. It is no secret that at some point zero discharge is not going to be a buzz word for the future, but a desired standard.
Techmetal, Inc., Dayton, Ohio, a chromium plating shop with numerous tanks, operations (manual and automatic) and applications, wanted to make certain it would be able to meet the possible zero discharge limit. However, Techmetals found that virtually every conventional pollution control device could exceed certain limits as a result of equipment failure.
Techmetal reviewed many professional articles on EPA standards for chromium emissions, testing data and development of the MACT standard. After its research, Techmetals still had three concerns: Moving parts will break down, causing allowable discharge limits to be exceeded; The technology used to conduct MACT testing still emits chromium particles; and Some states and localities have concentration-based and/or risk-based rules that are more restrictive than the MACT standard. Because of these concerns, the chromium plater searched for a device that could produce zero chromium emissions.
Schematic diagram of the EED.
Conventional ventilation. Techmetals uses hexavalent chromium, which extends the life and enhances the performance of a variety of manufactured parts. Chromium coated parts offer excellent corrosion resistance and hardness and a low coefficient of friction, characteristics deemed invaluable by the defense and aircraft industries.
Unfortunately, hexavalent chromium is highly toxic and a known carcinogen, especially when carried through the air as a vapor. Transport of chromium particles occurs during plating because a number of by-products are generated, such as hydrogen and oxygen gases, water vapor and chromic acid mist. Hydrogen and oxygen gases are generated due to the inefficiency of electrolytic reaction during chromium plating. Chromium plating is about 14 to 18 pct efficient when compared to acid zinc plating, which is 93 to 96 pct efficient. Chromic acid mist is generated by the bursting action of the hydrogen and oxygen gases escaping at the surface of the solution and air interface.
With conventional ventilation systems, the chromic acid mist is carried away by air moving across the tank via a blower or push-pull device. In addition, since the tank is open to the atmosphere, stray air currents add to the mobility of these particles. Typically, a push-pull system is designed to move these particles away from the plating tank to a scrubber where the air is cleaned of chromic acid and discharged.
Conventional ventilation systems, such as packed-bed scrubbers, composite mesh pads and fiber bed mist eliminators, are commonly installed by plating shops in order to comply with air emission standards and regulations set by the EPA. These systems lower the amount of hexavalent chromium released into the atmosphere. However, Techmetals was looking for something that eliminated emissions. It decided to use an emission elimination device (EED) developed by Responsible Alternatives, Inc., Dayton, Ohio. An EED is a specially engineered hood system that contains hexavalent chromium mist without interfering with the normal chromium plating operation.
Emission Elimination Device.
Techmetal placed an adapter on top of the tank walls with openings for bussings, utilities and other electrical conduits. A hinged cover was connected to the adapter ring. A sealing gasket material was applied between the tank and adapter and the hood and adapter. A vacuum pump process was connected to the cover to evacuate any chemical mists or fumes that might remain in suspension after the plating process had ceased and the cover remained closed. Gases from the vacuum system were exhausted through a filtering system near the plating tank.
When this tank cover system is used over a chromium tank, chromic acid mist particles simply rise and fall back into the solution because of gravity and the absence of forced air. The hydrogen and oxygen gases generated during the chromium plating process escape through the system's membrane, water condenses on the inside walls of the enclosure and the condensate trickles back into the plating solution. In setups such as Techmetals, where chromium mist extends to the height of the buss bars, the water droplets continuously clean the bussing.
Emission standards for platers. Data was collected from two chromium plating facilities using this system (Table I). These results are not from stacks or scrubbers, because the system eliminates them. The data reflects testing around the process tank with workers wearing testing equipment. Exposure times will vary, but even when compared to OSHA standards, the system provides results that are 10 to 30 times below the regulatory standards.
TABLE I—Air Sampling Results for Chromium at Two Plating Shops California Plating Shop Chromium Conc.(mg/m3) Permissable Exposure Limits(mg/m3) OSHA CalOSHA NIOSHA ACFGIH 0.0003 at filter 0.1 0.05 0.001/0.025 0.05 Ohio Plating Shop Person No. 1 0.0007 at filter ACGIH TLV 0.05 Person No. 2 0.0004 at filter ACGIH TLV 0.05
Energy cost savings. Techmetals conducted its own energy savings study using kilowatt/hour rate structures developed by the local utility company. Table II displays the on-site results when three of the plating shop's tanks were fitted with a cover system. Savings will vary depending on the geographic location, percentage of tanks with the system and whether stack testing was eliminated or reduced.
TABLE II—Operating Cost for a Three-Tank System Emission Elimination Device System (EED) Conventional Ventilation Three-Tank System Original Equipment andInstallation Cost $130,000 $107,000 Annual Operating Cost $6,866 $69,167 Total First Year Cost $136,866 $176,167
Testing the EED. Even though Techmetals eliminated the need for testing, the EPA still required that "stack" testing be done. As a result, Techmetals' staff had to develop its own procedure and application for testing in order to receive the EPA's approval.
The procedure took several steps. For initial compliance testing, Techmetals' staff performed a smoke testing procedure based on total tank surface area. The smoke test verified the design and placement of all seals and that the system's membrane was working. The next step was to initiate a continuance/compliance monitoring program, which required logs of daily, weekly and monthly inspections. Then Techmetals developed a recommended replacement program for all critical equipment, membranes, seals and tie-downs. Finally, Techmetals' reporting requirements were determined by the local air regulatory authority.
Techmetals had several concerns about using the system. Since the system conserves energy, additional cooling of the solution may be required. However, this was not a problem at Techmetals since it had adequate cooling capabilities. Another concern was that air agitation had to be eliminated. Techmetals used air for solution agitation and mobility to enhance plating conditions. It has now installed the appropriate pumps and hardware to perform solution agitation without air. The system also conserves water. Therefore, Techmetals needed a system designed to evaporate water in the rinse tank. Techmetals' final concern was correctly designing the system.
Despite these concerns, the plating shop has achieved a capital cost pay back of nine months per unit based on energy savings, while establishing a zero emissions standard for chromium.
source .pfonline.com/articles/129705.html
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