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.
Monday, May 18, 2009
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
Tuesday, May 5, 2009
Phosphate Conversion Coatings
Convert: To change into another form, substance or state. In the case of phosphate conversion coatings, the substrate metal participates in the coating reaction and becomes a component of the coating. Phosphate conversion coatings are formed from the surface of the base metal outward. Therefore, the thickness of the coating is dependent upon the porosity of the coating as it forms. Once the surface is sealed from the chemical solution, the reaction stops.
Phosphate conversion coatings are an integral part of most finishing operations, serving one or more of the following functions: increase corrosion resistance, absorb lubricants, enhance appearance; promote adhesion and provide wear resistance or facilitate cold forming.
Phosphating is a chemical conversion coating that transforms the surface of the basis metal into a non-metallic crystalline coating. The reaction occurs in an acidic solution containing phosphate ions. Due to the loss of hydrogen at the metal/solution interface, there is a localized rise in pH and subsequent precipitation of the coating.
Phosphate coatings can be categorized into three main types: zinc, manganese and iron. There are many proprietary formulations available for each, depending on the functional requirements of the part.
Figure 1. SEM of heavy zinc phosphate crystal (500x)
Heavy Zinc PhosphateHeavy zinc phosphate is usually chosen for its ability to retain rust preventive oils and waxes. The heavy coating (figure 1), in the range of 1,000–3,000 mg/sq ft, acts as an absorbent substrate, holding the rust preventive on the surface of the part. This can provide corrosion protection in excess of 400 hours of 5% neutral salt spray exposure, depending on the formulation and concentration of the chosen rust preventive. Figure 1 shows the crystal structure of a heavy zinc phosphate coating at 500× magnification.
Heavy zinc phosphate coatings are applied in an immersion process. Parts are run either on racks or in tumbling barrels. The bath is charged at 3–4% by volume (30–40 total acid points) and operates at 175-185°F. The bath is controlled with simple titrations that measure concentration (total acid), aggressiveness (free acid) and iron content.
Iron buildup is usually the limiting factor in the service life of the bath. When iron levels become higher than zinc, usually due to high metal throughput, coating quality and uniformity are compromised. At this time, a portion of the bath can be decanted, or the bath is charged fresh.
Figure 2. SEM of calcium-modified zinc phosphate crystal (500×)
Calcium-modified zinc phosphateCalcium-modified zinc phosphate is typically used as a base for paint or other organic coatings. The calcium co-deposits with zinc and acts as a built-in grain refiner to form a smooth microcrystalline structure (Figure 2). Coating weights are typically in the range of 150–500 mg/sq ft, which allows for enhanced adhesion properties without being as absorptive as a heavy zinc phosphate coating. In addition, the chemical-resistant nature of calcium-modified zinc phosphate coatings confines corrosion to a limited area, often referred to as creepage, if the applied topcoat is damaged.
Calcium-modified zinc phosphate can be applied by spray or immersion. The bath is often a two-component system, with the calcium-rich component added upon start up and infrequently thereafter. Operating temperature is 150°F for spray and 170–180°F for immersion.
Iron content in the bath may interfere with grain refinement and nonuniform coatings may result. Small but frequent additions of a strong oxidizer will precipitate the iron from the solution, resulting in extended bath life.
Cold-forming zinc phosphatesCold-forming zinc phosphates are used to facilitate drawing, cold heading, stamping or extruding of the basis metal. These phosphate coatings are designed to retain lubricants under severe conditions of heat and pressure during deformation. The use of the phosphate coating allows increased tool life, faster drawing speeds and more severe reductions of the basis metal.
Coating weights for cold forming zinc phosphates can range from 500–2,000 mg/sq ft. The phosphating solution in this application is operated iron-free to ensure a less abrasive zinc phosphate crystal, which will not scratch dies, score or gall. A strong oxidant, commonly nitrite or chlorate, is needed to drop the iron out of the bath in the form of sludge. In addition, a titanium-based grain refining pre-dip can be used prior to the phosphate to produce an even smoother, more uniform crystalline coating.
Figure 3. SEM of manganese phosphate crystal (500×)
Manganese phosphateManganese phosphate (Figure 3) is most commonly chosen for its wear-resistant properties. The manganese phosphate coating not only prevents metal-to-metal contact between moving parts, such as cylinder liners, camshafts, piston rings and transmission gears, it also has excellent oil retentive properties for both lubricity and corrosion resistance.
A grain-refining pre-dip is often used prior to manganese phosphating to ensure a controlled microfinish. The manganese phosphate bath is charged at 10% by volume (12 total acid) and operated at 195-205°F.
The balance of the bath concentration (total acid) when compared with the bath activity (free acid) is critical to control crystal size and coating uniformity. A ratio of 5.5–6:5.1 is recommended and maintained with additions of manganese carbonate.
High iron content in the bath can be lowered by treatment with hydrogen peroxide.
Iron phosphateIron phosphate is used as a base for paint or powder coat to enhance adhesion. Unlike both zinc and manganese phosphates, in which the metal cation is found in the phosphating solution, the cation of the iron phosphate coating is contributed by the basis metal. The phosphating solution typically contains alkali metal phosphate and accelerators. Coating weights for iron phosphates range from 25-100 mg/sq ft.
Application is usually in a three- or five-stage spray washer, but there are some immersion processes. For three-stage washers, the iron phosphate has an incorporated detergent system to clean and phosphate in one step. In five-stage washers, the cleaning is done separately in stage one, while the phosphate is applied in stage three. A seal is applied (either chrome or non-chrome) in the final stage to minimize under-film corrosion.
Iron phosphates are easy to control with a simple titration to determine concentration, 1–3% by volume, and pH adjustment, 4.5–5.5 optimum. Operating temperatures are relatively low, 100–130°F.
Developments in the area of phosphate conversion coatings have been aimed at lowering operating temperatures, decreasing sludge volumes produced as a function of the surface area of parts being processes and adjusting formulations to meet more stringent requirements for coating weights and coating thicknesses.
These changes have been mainly to address environmental and economical concerns while at the same time not compromising performance characteristics or quality of resultant coatings.
Phosphate coatings are economical, easy to use and offer a variety of valuable properties to extend the service life and improve the performance characteristics of the parts being treated.
Phosphate conversion coatings are an integral part of most finishing operations, serving one or more of the following functions: increase corrosion resistance, absorb lubricants, enhance appearance; promote adhesion and provide wear resistance or facilitate cold forming.
Phosphating is a chemical conversion coating that transforms the surface of the basis metal into a non-metallic crystalline coating. The reaction occurs in an acidic solution containing phosphate ions. Due to the loss of hydrogen at the metal/solution interface, there is a localized rise in pH and subsequent precipitation of the coating.
Phosphate coatings can be categorized into three main types: zinc, manganese and iron. There are many proprietary formulations available for each, depending on the functional requirements of the part.
Figure 1. SEM of heavy zinc phosphate crystal (500x)
Heavy Zinc PhosphateHeavy zinc phosphate is usually chosen for its ability to retain rust preventive oils and waxes. The heavy coating (figure 1), in the range of 1,000–3,000 mg/sq ft, acts as an absorbent substrate, holding the rust preventive on the surface of the part. This can provide corrosion protection in excess of 400 hours of 5% neutral salt spray exposure, depending on the formulation and concentration of the chosen rust preventive. Figure 1 shows the crystal structure of a heavy zinc phosphate coating at 500× magnification.
Heavy zinc phosphate coatings are applied in an immersion process. Parts are run either on racks or in tumbling barrels. The bath is charged at 3–4% by volume (30–40 total acid points) and operates at 175-185°F. The bath is controlled with simple titrations that measure concentration (total acid), aggressiveness (free acid) and iron content.
Iron buildup is usually the limiting factor in the service life of the bath. When iron levels become higher than zinc, usually due to high metal throughput, coating quality and uniformity are compromised. At this time, a portion of the bath can be decanted, or the bath is charged fresh.
Figure 2. SEM of calcium-modified zinc phosphate crystal (500×)
Calcium-modified zinc phosphateCalcium-modified zinc phosphate is typically used as a base for paint or other organic coatings. The calcium co-deposits with zinc and acts as a built-in grain refiner to form a smooth microcrystalline structure (Figure 2). Coating weights are typically in the range of 150–500 mg/sq ft, which allows for enhanced adhesion properties without being as absorptive as a heavy zinc phosphate coating. In addition, the chemical-resistant nature of calcium-modified zinc phosphate coatings confines corrosion to a limited area, often referred to as creepage, if the applied topcoat is damaged.
Calcium-modified zinc phosphate can be applied by spray or immersion. The bath is often a two-component system, with the calcium-rich component added upon start up and infrequently thereafter. Operating temperature is 150°F for spray and 170–180°F for immersion.
Iron content in the bath may interfere with grain refinement and nonuniform coatings may result. Small but frequent additions of a strong oxidizer will precipitate the iron from the solution, resulting in extended bath life.
Cold-forming zinc phosphatesCold-forming zinc phosphates are used to facilitate drawing, cold heading, stamping or extruding of the basis metal. These phosphate coatings are designed to retain lubricants under severe conditions of heat and pressure during deformation. The use of the phosphate coating allows increased tool life, faster drawing speeds and more severe reductions of the basis metal.
Coating weights for cold forming zinc phosphates can range from 500–2,000 mg/sq ft. The phosphating solution in this application is operated iron-free to ensure a less abrasive zinc phosphate crystal, which will not scratch dies, score or gall. A strong oxidant, commonly nitrite or chlorate, is needed to drop the iron out of the bath in the form of sludge. In addition, a titanium-based grain refining pre-dip can be used prior to the phosphate to produce an even smoother, more uniform crystalline coating.
Figure 3. SEM of manganese phosphate crystal (500×)
Manganese phosphateManganese phosphate (Figure 3) is most commonly chosen for its wear-resistant properties. The manganese phosphate coating not only prevents metal-to-metal contact between moving parts, such as cylinder liners, camshafts, piston rings and transmission gears, it also has excellent oil retentive properties for both lubricity and corrosion resistance.
A grain-refining pre-dip is often used prior to manganese phosphating to ensure a controlled microfinish. The manganese phosphate bath is charged at 10% by volume (12 total acid) and operated at 195-205°F.
The balance of the bath concentration (total acid) when compared with the bath activity (free acid) is critical to control crystal size and coating uniformity. A ratio of 5.5–6:5.1 is recommended and maintained with additions of manganese carbonate.
High iron content in the bath can be lowered by treatment with hydrogen peroxide.
Iron phosphateIron phosphate is used as a base for paint or powder coat to enhance adhesion. Unlike both zinc and manganese phosphates, in which the metal cation is found in the phosphating solution, the cation of the iron phosphate coating is contributed by the basis metal. The phosphating solution typically contains alkali metal phosphate and accelerators. Coating weights for iron phosphates range from 25-100 mg/sq ft.
Application is usually in a three- or five-stage spray washer, but there are some immersion processes. For three-stage washers, the iron phosphate has an incorporated detergent system to clean and phosphate in one step. In five-stage washers, the cleaning is done separately in stage one, while the phosphate is applied in stage three. A seal is applied (either chrome or non-chrome) in the final stage to minimize under-film corrosion.
Iron phosphates are easy to control with a simple titration to determine concentration, 1–3% by volume, and pH adjustment, 4.5–5.5 optimum. Operating temperatures are relatively low, 100–130°F.
Developments in the area of phosphate conversion coatings have been aimed at lowering operating temperatures, decreasing sludge volumes produced as a function of the surface area of parts being processes and adjusting formulations to meet more stringent requirements for coating weights and coating thicknesses.
These changes have been mainly to address environmental and economical concerns while at the same time not compromising performance characteristics or quality of resultant coatings.
Phosphate coatings are economical, easy to use and offer a variety of valuable properties to extend the service life and improve the performance characteristics of the parts being treated.
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