First of a three-part series on applying zinc phosphate coatings to substrates other than steel...
This is the first of a three-part series on the application of zinc phosphate coatings to substrates other than steel. Part I will expose process control problems encountered during the production of zinc phosphate coatings on zinc-plated surfaces. On various parts, the zinc phosphate coating was to provide corrosion resistance, prepaint treatment or pretreatment to enhance adhesion. Part II will show the results of applying different types of zinc phosphate coatings on zinc-plated and galvanized surfaces. Part III will show the results of applying various types of zinc phosphate coatings on cadmium plated and aluminum surfaces.
Having some knowledge of light (paint base) zinc phosphate coatings on steel, I was asked to participate in a project that involved the application of these coatings on zinc-plated parts. One zinc-plated part required a zinc phosphate coating as a supplemental finish for corrosion protection. Another zinc-plated part required a phosphate coating to promote adhesive bonding of a rubber. A third part required paint adhesion to a zinc-plated part.
All the parts were failing for various reasons. The supplemental phosphate on zinc-plated parts exhibited white corrosion after only eight hrs of salt spray testing. The salt spray requirement on the hardware was 48 hrs exposure with no evidence of white corrosion. Parts with the rubber showed disbonding after simulation testing. The phosphated zinc surfaces failed adhesion tests that were performed after the epoxy paint had cured for one week.
The approach to solving the problems was to characterize the phosphate coating in each application by determining coating weight, crystal structure and coverage. Coating weight on the zinc surfaces was determined using a sodium dichromate/ammonium hydroxide stripping solution on the phosphated zinc surfaces. Crystal structure and coverage were determined using a scanning electron microscope at 500x magnification. Photos at this magnification were used to previously characterize zinc phosphate coatings on steel.
An investigation determined the ability of zinc-phosphate-coated zinc-plated parts to provide corrosion resistance (the formation of white corrosion product during salt spray testing). Werner Raush reported corrosion resistance to white corrosion of up to 48 hrs in the salt spray test 1 . Mr. Raush also stated that phosphating improves resistance to "white rust" formation on zinc surfaces during storage or transport under normal conditions. Initial testing of a phosphate coating showed white corrosion product after eight hours of five pct salt spray (ASTMB117) exposure. Incorporation of a bright dip prior to phosphating produced a smaller size crystalline structure that provided 24 hrs of salt spray resistance prior to formation of white corrosion. A third process provided a very fine crystalline zinc phosphate coating on zinc-plated parts. An investigation into the chemicals showed no grain refinement step in the phosphating operation. Contact with the chemical manufacturer revealed that the alkaline cleaner contained titanium salts.
Since the concentration of the titanium salts in the cleaner was less than one pct, and the titanium is not considered toxic or hazardous, titanium salts were not listed on the Material Safety Data Sheet (MSDS). The phosphating applicator had no way of knowing what was contributing to the fine grain structure on the product. Accelerated salt spray testing showed no white corrosion after 48 hrs exposure on these parts. This investigation essentially shows that a fine crystalline zinc phosphate coating with complete coverage can reduce the susceptibility of zinc-plated surfaces to the formation of white (zinc) corrosion.
An investigation determined zinc-phosphate coating processing parameters for adhesive bonding. The adhesive manufacturer recommended a calcium-modified zinc phosphate with a coating weight of 150300 mg per sq ft for optimum adhesive bonding on zinc surfaces. Examination of the calcium-modified zinc phosphate coating on zinc-plated parts revealed a large crystal structure of essentially zinc phosphate and coating weights in the 9001,000 mg per sq ft range. The phosphate coating applicator processed the parts for three min. Reducing the time to one and a half min did not reduce the coating weight appreciably. A titanium predip prior to phosphating reduced the coating weight to 600800 mg per sq ft and reduced the crystal size.
Testing followed formation of the phosphate coating. Acid-zinc-plated specimens were zinc phosphate coated in a calcium-modified solution for 5, 10, 20, 40 and 180 seconds. Next, acid-zinc plated specimens were treated with a titanium salt rinse/dip and phosphated in the same calcium-modified solution. The specimens were removed after 5, 10, 20, 40, and 180 seconds.
The titanium rinse resulted in a finer crystal structure and quicker coverage compared to the specimens not given the treatment. Coating weight at 180 seconds was essentially the same, which could be expected since the phosphating solution was the same. Two significant observations of the calcium-modified coating (no titanium predip) on zinc were that no nodular coating was formed, and the coating weights exceed the 150300 mg per sq ft, which is typical of calcium-modified zinc phosphate coating on steel.
When the adhesive manufacturer was asked to provide a sample, the nodular coating was received and had a coating weight of 200 mg per sq ft. Further testing showed the substrate to be steel, not zinc. Thus, the adhesive manufacturer did not know that it was impossible to meet its own specifications.
Specimens processed in an alkaline-zinc solution were given a titanium rinse and processed in a nickelfluoride (NiF) zinc phosphating solution. The final coating weight is approximately 40 pct lower than the calcium-modified coating.
This shows that other phosphating solutions could be used to achieve the lower coating weight recommended by the adhesive manufacturer. It is suspected that the adhesive manufacturer's experience was with calcium-modified phosphate on steel and they assumed that the calcium-modified solution would provide a similar coating on zinc.
A third investigation evaluated a phosphate coating on cyanide-zinc-plated parts for the purpose of applying an epoxy primer and urethane topcoat. Some time had elapsed (a few days to a week) prior to actually applying the phosphate coating. The zinc-plated parts were immersed directly into the phosphating solution as the parts were still perceived to be "clean" after zinc plating. A majority of the zinc surface had little or no phosphate coating. On another part, the only phosphate coating formed on a silicate inclusion (parts were sandblasted to remove rust prior to zinc plating) in the zinc plating. On still another area, where a fingerprint was obvious, a uniform fine-grain phosphate coating was observed with complete coverage. The fingerprint residue acted as a grain refiner. Some type of oxidation or passivation had occurred prior to phosphating, making the zinc surface relatively unresponsive to the phosphating solution. Since phosphate coatings form on cathode areas and the blast media would be a cathode with respect to zinc plating, the observation of a phosphate crystal could be expected on the particle of silica. Subsequently, an alkaline cleaner with a titanium grain refiner was used prior to phosphating. This coating provided an adequate paint base that successfully passed the paint adhesion test.
There is still additional knowledge needed to be somewhat proficient at knowing what to expect when various types of pretreatments, activators and phosphating solutions are used to produce zinc phosphate coatings on zinc-plated surfaces. For those thinking about trying this or are doing it for the first time, this article should present some indication of the degree of complexity that is involved in applying quality coatings that enhance the functional requirements of coating systems.
The federal specification for paint-based phosphate coatings, TTC490, states that phosphate coatings can be applied to substrates other than zinc-plated and steel parts2. Paint-based phosphate formulations can also be applied to galvanized steel, cadmium plating and aluminum. Phosphate solutions include nitrate, chlorate
and iron accelerated types, nickel- fluorides, calcium-modified, and manganese-modified (trimetal or polycrystalline) formulations applied by spray or immersion processes. Subsequent articles will show the results of using various phosphating formulations and processes and showing the resulting phosphate crystal structures.
The author would like to thank the Rock Island Arsenal Laboratory, especially Dr. Richard Perry.