The last of a three-part series characterizes zinc phosphate coatings on cadmium-plated and aluminum substrates...
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This final part of a three-part series characterizes various zinc phosphate coatings on cadmium-plated or aluminum substrates. Dip and spray processes with and without grain refinement were used to coat the substrates. Calcium-modified (immersion only), manganese-modified (dip and spray) and nickel-fluoride (spray only on aluminum and spray and immersion on cadmium plating) formulations were used.
Standard two-by-three-inch or four-by-12-inch panels of electrodeposited cadmium or aluminum were used in the study. Various phosphate coating chemical suppliers applied the coatings using the formulations mentioned in the opening paragraph. The phosphated panels were examined using a scanning electron microscope at 500x. Testing was also performed to determine the phosphate coating weight. X-Ray analysis on the SEM was also performed, and significant observations are included in the following text.
The calcium-modified coatings on cadmium appear to have a nodular structure. This could be misleading since the structure of the cadmium plating could also be somewhat nodular, as can be seen in Figures 1 and 2.
An x-ray analysis of Figure 3 shows minimal zinc, a small amount of phosphorus and no calcium in the coating. The coating could simply be an oxide/phosphate of cadmium similar to iron phosphate coatings on steel. The use of a grain refiner prior to the calcium-modified zinc phosphate treatment produced large crystals (with poor coverage) that was essentially zinc phosphate.
As noted previously on zinc, the grain refiner changes the surface activity causing zinc phosphate to form with no modifying effect added by the calcium. A five pct hydrochloric acid dip was used in a separate attempt to deoxidize the cadmium prior to treatment with and without a grain refiner. The only difference was a slight increase in coating weight. Crystal size and coverage remained the same as in Figures 3 and 4.
The only effect of the deoxidizing treatment was a minimal increase in the surface reactivity of the cadmium. The application of calcium-modified zinc phosphate coatings on 2024 aluminum with and without a grain refiner resulted in some etching but not coating. It appears that the natural oxide that forms on aluminum tends to re-form during the phosphating process or in the air after removal from the solution.
Calcium-modified zinc phosphating solutions are not capable of providing phosphate coatings on cadmium and aluminum surfaces that meet the coating weight and coverage requirements of TT-C-490 for immersion processes. It is interesting that Rausch1 states that there are no problems with the phosphate coating of cadmium. Freeman2 states that cadmium can be phosphated in the same types of baths that are used for zinc and aluminum.
The application of manganese-modified coatings to cadmium surfaces met with mixed results. Without a grain refiner, both spray and immersion coatings produced crystals that exhibited weird shapes and poor coverage. With a grain refiner, both spray and immersion coatings produced crystals that were uniform in size and coating weight and met specifications. However, neither process produced phosphate coatings that provided complete coverage. These coatings (with grain refiner) may provide a suitable paint base but would not prevent the formation of white corrosion products during salt spray testing of cadmium-plated hardware.
The application of manganese-modified coatings to 2036 aluminum surfaces also exhibited significant variation in crystal structure and coverage. Without a grain refiner, the immersion process produced a few large crystals with poor coverage.
The spray process provided crystals with a smaller size and complete coverage. With a grain refiner prior to phosphating, both processes provided fine-grain crystals and complete coverage. All four phosphate coated samples were given approximately 1.5 mils of MIL-C-53022 epoxy primer and exposed to the ASTM B117 salt spray for 1,000 hrs. Only the specimen with incomplete coverage (Figure 5) failed the salt spray test due to blistering.
It may be possible to substitute phosphate coatings for chromate coatings as a pretreatment prior to painting if crystal size and coverage are adequately maintained. Rausch states that only by maintaining the correct bath composition and operating parameters can a phosphate coating be applied to aluminum over an extended period.
Automotive manufacturers use the manganese-modified solution with grain refinement because the fine-grain size and uniform coverage on steel, zinc and aluminum provide maximum corrosion resistance over lower thicknesses of electro-deposited epoxy primers. The manganese-modified phosphate coatings provide numerous benefits that have been previously reported--3,4.
The application of nickel fluoride coatings on cadmium shows similar results to the manganese-modified coating when a grain refiner was not used. In both cases, the spray application provided a better coating than did the immersion process.
With a grain refiner, however, the nickel-fluoride solutions provided complete coverage and a fine-grain structure that not only would provide a good paint base, but could possibly provide resistance to white corrosion on cadmium. Unfortunately, specimens were not available to run the necessary salt spray tests. The application of nickel-fluoride coating by spray on 3003 aluminum resulted in large crystals with poor coverage without grain refinement prior to phosphating. When a grain refiner was used, more and finer crystals resulted, but coverage was still incomplete.
Processing of other specimens, 6061 and 7039 aluminum, showed that complete coverage could be accomplished using the process. It may be possible to use nickel-fluoride immersion phosphate coatings as a paint base pretreatment on aluminum instead of chromate conversion coatings.
Rausch reported a major improvement in resistance to underfilm corrosion on painted aluminum (AlMgSi) surfaces after phosphating with a nickel-fluoride crystalline coating. He also stated that fluoride additives prevent buildup of aluminum in phosphating solutions that can interfere with the phosphating process at concentrations above three mg/liter. Lorin5 reports the use of phosphating aluminum prior to cold deformation processes rather than for corrosion resistance.
The nickel fluoride solutions are especially suitable for cadmium surfaces and are also adaptable to aluminum surfaces. Obtaining the correct processing variables for various aluminum alloys is necessary for optimum performance.
This article concludes the series on phosphating non-ferrous surfaces. It is by no means complete. The nitrate, chlorate and iron accelerated phosphating solutions that are available were not included nor were various pretreatments like abrasive blasting. Optimizing the parameters for quality phosphate coatings is a continuing effort. The author started this series not knowing the answers to some questions on how to obtain a good phosphate coating on zinc-plated surfaces. And ended up getting a real education in the area of coatings on non-ferrous surfaces. That education is continuing.
Many thanks to McGean-Rohco, Inc. for applying the calcium-modified coatings; Oakite Products for applying the nickel-fluoride coatings; and PPG Chemfil Corporation for applying the manganese-modified coatings shown in this series. Thanks are also necessary to Dr. Richard Perry who put up with an array of specimens for SEM, x-ray and coating weight analysis. PF
1. Rausch, W., The Phosphating of Metals, Finishing Publications, Ltd., Middlesex, England, 1990, pg. 92, 174, 204.
2. Freeman, D.B., Phosphating and Metal Pre-Treatment, Industrial Press Inc., New York, New York, 1986, pg. 120.
3. Kent, G. Polycrystalline Conversion Coatings, Products Finishing magazine, September 1988, pg. 56-62.
4. Schrantz, J., Automotive Pretreatment: What's New?, Industrial Finishing, Nov. 1988, pg. 56-61.
5. Lorin, G., Phosphating of Metals, Finishing Publications Ltd., Middlesex, England, 1974, pg. 65.