Zinc phosphate conversion coatings

Fig. 31. Electron micrographs that compare crystal size of (top) a grain-refined microcrystalline coating and (bottom) a conventional zinc phosphate conversion coating [54].

This more involved multi-step process can be used for both iron phosphate and zinc phosphate conversion coating processes. 

On zinc phosphate conversion coatings, the "Mannich" derivatives of poly-4-vinylphenol have demonstrated performance equivalent to chromic chromate systems in salt spray, humidity, and physical testing. In addition. Table III illustrates results observed with automotive body paint systems evaluated by the "scab" or "cycle" test which causes failure more typical of actual end use conditions than do salt spray evaluations. Again, results equivalent to chromic-chromate post-treatments were obtained. In addition, the humidity resistance and adhesion tests were essentially equivalent to the chromium controls. 

FIGURE 23. Electron micrographs of (a) grain-refined microcrystalline conversion coating and (b) conventional zinc phosphate conversion coating. (From Reference 114.)... 

Boron nitride is the preferred tem-peratvire-resistant lubricant because of its higher lubricity, as well as ease of application and removal. Other lubricants used for hot forming have a graphite or molybdenum disulfide base. Zinc phosphate conversion coatings are sometimes first produced on the work metal stuface to aid in the retention of lubricants during severe forming. 

Wedge test results for ASTM A606 steel for grit-blasted and conventional and microcrystalline zinc phosphate conversion coatings. Failure of the microcrystalline zinc phosphate was 100% cohesive that for the grit blast and conventional zinc phosphate was 100% adhesive. Data are from Trawinski et al. (1984)... 

In the development of a reactive non-chrome post-treatment, a variety of phenolic resins were synthesized and commercial phenolic resins evaluated. It was found that phenol-formaldehyde resins, creso1-forma1dehyd e condensates, ortho-novo 1 ak resins, and phenol-formaldehyde emulsions gave positive results when employed as post-treatments over zinc and iron phosphate conversion coatings. The above materials all possessed drawbacks. The materials in general have poor water solubility at low concentrations used in post-treatment applications and had to be dried and baked in place in order to obtain good performance. The best results were obtained with poly-4-vinylphenol and derivatives thereof as shown in the following structure (8,9,10)... 

The phosphate conversion coatings are formed by metal dissolution and phosphate precipitation. In the case of zinc phosphate film formation, iron reacts at the anodic sites through the following steps ... 

Prior to dipping, the parts are first cleaned and pre-treated with a phosphate conversion coat to prepare the part for electrocoating. Iron and zinc phosphate are commonly used. 

Zinc phosphate coatings (0.5 4.5 g/nf) are pale grey, smooth and fine-grained and excellent for paint adhesion. They are compatible with almost all paints and are generally superior to iron phosphate conversion coatings on steel. They are particularly desirable on zinc surfaces to which direct paint adhesion is otherwise poor. 

Nowadays, the use of the reflection electron microscope (REM) or, recently, the tunnel electron microscope, as well as secondary ion mass spectrometry (SIMS), AES, electron-dispersive X-ray spectrometry, impedance spectroscopy, and so on, are yielding substantial increases in the knowledge of corrosion reactions in coatings and at their interface with metal or other substrates. As far as zinc or zinc-coated surfaces are concerned, problems of interfacial and intercoat adhesion, differential diffusion phenomena and electrolytic cell behavior on the substrate, and interreactions of zinc with conversion coatings (chromates, phosphates, silanes, silanols, etc.) have been analyzed, leading toward spectacular improvements in, for example, paint adhesion, absorption of conversion coatings and, in general, the protective action inside films as well as on their substrates. 

A chromate conversion coating over the zinc coating prevents wet storage stain while phosphate conversion coatings provide a good base (on a new sheet) for paints. Weathered coatings are often painted after 10-30 years for longer... 

Phosphate conversion coatings are well-known and widely used for applications on steel, zinc and aluminum. 

A primer On metal, the purposes of a primer are to enhance corrosion protection and to give excellent adhesion. The primer will contain anticorrosive pigments, such as strontium chromate or zinc phosphate, which will slowly release ions that can repair damage or faults in the underlying conversion coating. 

Conversion coating Conversion coatings are chemical solutions which react with the metal surface to create a corrosion-resistant layer onto which the coating can bond. For mild steel iron phosphate is used to attain good adhesion, but it does not give the underfilm corrosion resistance which can be obtained using zinc phosphate. Zinc coatings can be treated with either zinc phosphate or chromate. Aluminium is usually treated with chromate... 

With regards to the conversion coated substrate, Wittel (25) observed that at temperatures greater than 140°C, tetrahydrate zinc phosphates lose part of their water of hydration. It is likely that the water of hydration liberated in the phosphate recrystallization process has a negative effect on the adhesion of the polymer matrix to the B40 panel. 

In order to obtain maximum corrosion protection for painted metal articles, the metal parts are pretreated with an inorganic conversion coating prior to the painting operation. These zinc or iron phosphate coatings greatly increase both paint adhesion and corrosion protection. Traditionally, a chromic acid post-treatment has been applied to these phosphatized metal surfaces to further enhance corrosion protection. 

The electrolytic deposition of a coating that is known as E-coat provides an excellent corrosion protection as evidenced by automotive coating. Today nearly all automobiles are corrosion protected by applying the cathodic E-coat, in which the steel body of a car is used as the cathode of the electrolytic deposition of a primer coat, on the surface of zinc phosphated steel. It is quite logical to consider that if an E-coat is applied to a chromate conversion-coated aluminum alloy surface, a significant improvement of the corrosion protection of aluminum alloys could be realized because such an attempt represents the combination of the two best components, i.e., chromate conversion coating and E-coat. We could find the best example that demonstrates the need of SAIE in such attempts. 

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