Chromate zinc phosphate

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... 

Chemical inhibitors, when added in small amounts, reduce corrosion by affecting cathodic and/or anodic processes. A wide variety of treatments may be used, including soluble hydroxides, chromates, phosphates, silicates, carbonates, zinc salts, molybdates, nitrates, and magnesium salts. The exact amount of inhibitor to be used, once again, depends on system parameters such as temperature, flow, water chemistry, and metal composition. For these reasons, experts in water treatment acknowledge that treatment should be fine tuned for a given system. 

Some pigments exert an inhibitive effect on the corrosion of metal. The mechanisms are complex and not always fully understood. Inhibitive pigments include red lead, zinc phosphate and zinc chromate. 

The purity of the zinc is unimportant, within wide limits, in determining its life, which is roughly proportional to thickness under any given set of exposure conditions. In the more heavily polluted industrial areas the best results are obtained if zinc is protected by painting, and nowadays there are many suitable primers and painting schemes which can be used to give an extremely useful and long service life under atmospheric corrosion conditions. Primers in common use are calcium plumbate, metallic lead, zinc phosphate and etch primers based on polyvinyl butyral. The latter have proved particularly useful in marine environments, especially under zinc chromate primers . 

It is usual to define primers in terms of the principal inhibiting pigment e.g. zinc phosphate, zinc dust or zinc chromate, and the topcoats in terms of the binder, e.g. alkyd, chlorinated rubber, etc. This practice can be confusing, however, and lead to the selection of incompatible coatings. 

The reactions which take place when the mixed etch primer is applied to a metal are complex. Part of the phosphoric acid reacts with the zinc tetroxychromate pigment to form chromic acid, zinc phospliates and zinc chromates of lower basicity. The phosphoric acid also attacks the metal surface and forms on it a thin chromate-sealed phosphate film. Chromic acid is reduced by the alcohols in the presence of phosphoric acid to form chromium phosphate and aldehydes. It is believed that part of the chromium phosphate then reacts with the resin to form an insoluble complex. Excess zinc tetroxy chromate, and perhaps some more soluble less basic zinc chromes, remain to function as normal chromate pigments, i.e. to impart chromate to water penetrating the film during exposure. Although the primer film is hard... 

Environment Increase redox potential of solution Addition of anodic inhibitors Passivation of stainless steel by additions of O2, HNO3 or other oxidising species to a reducing acid Additions of chromates, nitrates, benzoates, etc. to neutral solutions in contact with Fe inhibitive primers for metals, e.g. red lead, zinc chromate, zinc phosphate... 

Molybdenum-based anticorrosive pigments offer a nontoxic alternative to the zinc chromate pigments [5.120], They all have a neutral color (white) but the pure compounds are very expensive. To produce economically competitive pigments molybdate and phosphate pigments are combined, or molybdate compounds are applied to inorganic fillers (e.g., zinc oxide, alkaline-earth carbonates, or talc) [5.75], [5.121]—[5.123]. 

A less expensive replacement for zinc chromate is obtained by combining the zinc salt of 5-nitroisophthalic acid with zinc phosphate pigments (Section 5.2.2.5) [5.65], [5.67], [5.181]. A concentration of 0.5-2.0% based on the liquid coating is recommended. 

Many of the finishes applied to other types of metal products can also be applied to zinc die castings, although some differences in formulation as well as occasional differences in method of application may be desirable. The types of finishes applicable to zinc die castings include mechanical finishes (buffed, polished, brushed, and tumbled) electrodeposited finishes (copper, nickel, chromium, brass, silver, and black nickel) chemical finishes (chromate, phosphate, molybdate and black nickel) and organic finishes (enamel, lacquer, paint and varnish, and plastic finishes). Electrodeposited coatings of virtually any metal capable of electrodeposition can be applied to zinc die castings. 

In the 1970s various PBS/AM compounds, such as 2-phosphonobutane-1,2,3,4-tetracarboxylic acid, were introduced. These were marketed as scale and corrosion inhibitors and were often formulated with zinc and phosphates to provide, along with the lignosulfonate/zinc/phosphate programs, an alternative to chromate treatments. 

PCA 4 (Belclene 500) is primarily a building-block corrosion inhibitor. It can be used to synergize with chromate, zinc, nitrite, phosphate, and silicate in order to lower the concentration of these traditional corrosion inhibitors in the cooling system. 

Pigmentation can range from "inert types, such as barium sulphate, magnesium and calcium silicates together with a primary coloured pigment such as iron oxide, to anticorrosive pigments such as red lead, zinc chromate, zinc or calcium phosphate, etc. Others, which may be included to... 

The European Community classifies lead-containing anticorrosive pigments like Red Lead as well as zinc chromate, zinc potassium chromate, zinc tetraoxichromate, strontium chromate and, since 2000, also zinc phosphate as N, Dangerous for the environment (dead fish / tree) with the risk (R) and safety phrases (S) [5.112] ... 

Zinc chromate and phosphate for corrosion protection Galvanizing... 

In the late 1950 s, chromate-phosphate systems incorporating zinc as another cathodic inhibitor were introduced, followed by zinc-chromate inhibitors, without phosphate. Using chromate-zinc, or polyphosphate-chromate-zinc inhibitors it was possible to cut working concentrations still further. It was necessary to control pH to make this inhibitor system function effectively. At increased pH (above pH 7.5)tendency for zinc loss by precipitation increases. Furthermore, pH rise may cause heat exchangers to become fouled by zinc hydroxide slimes or zinc phosphate. 

Electrochemistry Anticorrosive (in can, in film, flash corrosion).Neutralising and Buffering agents (pH). Chromates phosphates amines zinc thiocarboxylic acid salts basic sulphonates. Amino hydroxy compounds. 

To obtain a coating with good corrosion protection, anticorrosion pigments have to be used (e.g., red lead, zinc chromate, zinc phosphate) and/or the paint must act as a barrier against the aggressive media. 

Industrially, phosphate layers are the most important and phosphating is used to treat steel, aluminum, and zinc. Chromating produces layers containing trivalent or hexavalent chromium compounds and is mainly used with aluminum and zinc. 

Epoxy primers cured with amine adducts or polyamides, pigmented with zinc, zinc chromate and potassium chromate, strontium chromate, or, more recently zinc phosphate and calcium phosphate... 

Painting zinc and aluminium makes special demands on the pre-treatment of the substrate. Chromate treatment provides a good basis for paint, and it is still in widespread use in spite of the toxicity of chromate. Phosphate treatment or washpriming can also be used. Alternatives for aluminium are light blast cleaning or anodizing. 

Chem. Descrip. Zinc phosphate dihydrate CAS 7779-90-0 EINECS/ELINCS 231-143-9 Uses Corrosion inhibitor, pigment for coatings Features Replacement for zinc chromate, lead oxide, strontium chromate, barium chromate, etc. 

Features Highly effective heavy metal-free replacement for toxic anticorrosive pigments such as red lead, zinc chromate, strontium chromate alternative for heavy metal-contg. zinc phosphate-based pigments Toxicology Nontoxic Sibrid TC [Gelest]... 

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