Zinc coatings chromate passivation

Full chromate passivation (Section 15.3) improves the corrosion resistance of both zinc and cadmium towards all environments and is applied for a wide range of applications. Clear and olive-coloured chromate coatings can also be applied for certain purposes. The highest degrees of corrosion protection... 

Post-Treatments. Although many post-treatments have been used over plated metals, chromate conversion coatings remain as the most popular. Chromates are used to improve corrosion resistance, provide good paint and adhesive base properties, or to produce brighter or colored finishes. Formulations are usually proprietary, and variations are marketed for use on zinc, zinc alloys, cadmium, copper and copper alloys, and silver (157). Chromates are also used on aluminum and magnesium alloys (158,159). More recently, chromate passivation has been used to extend salt spray resistance of autocatalytic nickel plated parts. 

Clean zinc-coated steel is a suitable base for paint or adhesive systems, but the first treatment may be different from those used on uncoated steel. Chemical conversion coatings (chromate, phosphate, or oxide types) and primers have been specially formulated for first treatment of zinc-coated steel. Some passivated surfaces (e.g., chromated) are not suitable for phosphating... 

The white storage stains on zinc and aluminum coatings can be prevented via application of a chromate passivation treatment, which forms a hydrated chromium oxide film on the surface. 

Electrolytic zinc or zinc-nickel alloy (10-14% Ni) coating followed by chromate passivation and an organic topcoat... 

The wash primer is a special type of vinyl coating. This material contains a poly(vinyl butyral) resin, zinc chromate, and phosphoric acid in an alcohol-water solvent. The coating is so thin it is HteraUy washed onto a freshly blasted steel surface, where it passivates the metal surface by converting it to a thin iron phosphate-chromate coating. The alcohol solvent makes it possible to apply the coating over damp surfaces. The coating forms the first coat of... 

Only a few passivators are adequate for filling pol3mier primers [12]. For instance, red lead Pb304 with a structure of lead plumbate, which isolates PbO ions, is considered to be the best filler for passivating primers. The solubility of zinc chromate in water is 2 10 mole/1, which is sufficient for the creation of a protective concentration of CrO ions (>10 mole/1) on steel substrate surfaces under polymer coatings. 

The most common coating inhibitors are zinc chromate and plumbous orthoplumbate (red lead), which passivate steel by providing chromate and plumbate ions, respectively, as well as the zinc and lead cathodic inhibitors. These inhibitors are not effective against attack by seawater or brines because the high chloride concentration prevents passivation of steel. 

Where the corrosion resistance of a coating depends upon its passivity, it is common to follow plating with a conversion coating process to strengthen the passive film. Zinc, cadmium and tin in particular are treated with chromate solutions which thicken their protective oxides and also incorporate in it complex chromates (see Section 1S.3). There are many proprietary processes, especially for zinc and cadmium. Simple immersion processes are used for all three coatings, while electrolytic passivation is us on tinplate lines. Chromate immersion processes are known to benefit copper, brass and silver electrodeposits, and electrolytic chromate treatments improve the performance of nickel and chromium coatings, but they are not used to the extent common for the three first named. 

Zinc-plated coatings are usually passivated to prevent wet storage staining, an unsightly form of mild corrosion that occurs where zinc surfaces encounter heavy condensation. Historically, a simple chromate dip has been widely used, but other treatments are being developed. Phosphates are often favored when the dipped surface is to be painted. 

While it is possible to bond to a freshly abraded or cleaned metal surface, chemical treatments are preferred for rendering the metal surface inactive to corrosion over time. For low carbon steel, phosphatising is the recommended pre-bond surface preparation treatment. Stainless steel should be passivated or acid etched, while titanium is usually treated with a hydrofluoric acid pickle. Almninium or magnesium are best treated with a chromate conversion coating. Zinc and cadmium are generally prepared mechanically but a phosphate or chromic acid treatment may be used. Brass and copper may be treated with an ammonium persulphate etch or an acid-ferric chloride etch. 

Zinc or cadmiunMleclroplated steel articles are frequently passivated with a chromate coating in order to enhance corrosion resstance, improve solder-ability and provide a pleasing e.g. iridescent) finish. 

Zinc tetroxychromate, or basic zinc chromate, is commonly used in the manufacture of two-package polyvinyl butyryl (PVB) wash primers. These consist of phosphoric acid and zinc tetroxychromate dispersed in a solution of PVB in alcohol. These etch primers, as they are known, are used to passivate steel, galvanized steel, and aluminium surfaces, improving the adhesion of subsequent coatings. They tend to be low in solids and are applied at fairly low film thicknesses [23]. 

The classical zinc-corrosion inhibitor has been mercuric or mercurous chloride, which forms an amalgam with the zinc. Cadmium and lead, which reside in the zinc alloy, also provide zinc anode corrosion protection. Other materials like potassium chromate or dichromate, used successfully in the past, form oxide films on the zinc and protect via passivation. Surface-active organic compounds, which coat the zinc, usually from solution, improve the wetting characteristic of the surface unifying the potential. Inhibitors are usually introduced into the cell via the electrolyte or as part of the coating on the paper separator. Zinc cans could be pretreated however, this is ordinarily not practical. 

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