Chromate treatment

Chromium Chromate. Chromium chromate treatment baths are acidic and made up from sources of hexavalent chromium and complex fluoride, fluorosiHcate, fluorozirconate, fluorotitanate, and siHcofluorides. Optional additional components added to accelerate coating rate are free fluoride, ferricyanide, and other metal salts such as barium nitrate. Conversion coating on aluminum precedes by the following reactions (2,3,17) ... 

Cooling water side Softened cooling water with chromate treatment... 

This type of corrosion can take place on any new surface of zinc and is best prevented by storing the metal in a dry, airy place until a protective layer has been formed. Zinc which has been properly aged in this way is safe against white-rust formation. Various methods are employed to prevent white rust. A chromate treatment is widely used for zinc-plated articles and for galvanised sheet, and occasionally for zinc die castings. Fatty substances, such as oils or lanolin, are sometimes used to protect larger items. 

Chromate treatments can be applied to a wide range of industrial metals. They are of two broad types (a) those which are complete in themselves and deposit substantial chromate films on the bare metal and (b) those which are used to seal or supplement protective coatings of other types, e.g. oxide and phosphate coatings. Types of treatment for various metals are summarised in Table 15.16. 

Chromates are very effective inhibitors of the corrosion of magnesium alloys by saline and other waters, and many treatments have been developed by means of which substantial hlms containing slightly soluble chromate are formed in the metal surface. Except on parts which are to be exposed only to a rural atmosphere, chromate treatment must be supplemented by paint, for which it provides a good base. 

The Dow No. 7 treatment, popular in the USA, also falls within this class. The process differs from other chromate treatments in that the activator, magnesium fluoride, is formed on the metal surface by immersion in 20% hydrofluoric acid solution, the parts then being immersed in a 10-15% alkali dichromate solution with or without sufficient alkaline earth fluoride to saturate it. A slow action occurs on the surface and the fluoride film is replaced by a chromate or mixed chromate/fluoride film. 

Alkaline chromate treatments for tin, e.g. the Protecta-Tin processes, have been developed by the Tin Research Institute. The solutions resemble the M.B.V. compositions for treating aluminium, but are more alkaline. Thin invisible films which resist staining by heat and sulphur-bearing compounds and give protection against humid atmospheres at pores are deposited. 

Surface Coatings of metals that passivate readily Surface treatments to facilitate formation of passive film Cr coatings on Fe Polishing stainless steel and removing Fe impurities by HNO3 chromate treatment of A1... 

Although chromate is the best aqueous corrosion inhibitor available, its use has been severely curtailed due to toxicity and environmental concerns ( ). One of the more successful non-chromate treatments involves the use of phosphate/phosphonate combinations. This treatment employs high levels of orthophosphate to promote passivation of the metal surfaces. Therefore, it is important to control calcium phosphate crystallization so that high levels of orthophosphate may be maintained in the system without fouling or impeding heat-transfer functions. 

Operating practices for chromate treatments are well established and it is recognized that the effectiveness of the program initially depends on ... 

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. 

The most effective treatments were those containing chromium. Of these, the chromium trioxide and the ammoniacal copper chromate treatments were the most effective. 

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