Phosphate chemical conversion coatings

Iron phosphate coating See chemical conversion coating. 

Iron Phosphate Coating See Chemical 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... 

Chemical conversion coatings are applied to previously deposited metal or basis material for increased corrosion protection, lubricity, preparation of the surface for additional coatings, or formulation of a special surface appearance. This operation includes chromating, phosphating, metal coloring, and passivating. 

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... 

Inorganic (microdiscontinuous), chrome flash, hard chromium, nickel-iron Chemical Conversion Coatings Zinc phosphate, manganese phosphate, iron phosphate, black oxide, anodic oxide (aluminum). 

The films contain soluble chromates that act as corrosion inhibitors that provide a modest improvement in corrosion resistance. As with phosphatizing, the film produced serves as an excellent coating base. Indeed, it can be difficult to make most polymeric coatings adhere to the nonferrous alloys without such a treatment. Epoxy primer, for example, which does not adhere well to bare aluminum, adheres very well to chemical conversion coatings. Recent concerns with the toxicity of the hexavalent chromium used in chromating have, however, limited its usage. 

Conversion coatings are adherent surface layers of low-solubility oxide, phosphate, or chromate compounds produced by the reaction of suitable reagents with the metallic surface. These coatings affect the appearance, electrochemical potential, electrical resistivity, surface hardness, absorption, and other surface properties of the material. They differ from anodic coatings in that conversion coatings are formed by a chemical oxidation-reduction reaction at the surface of... 

As with chemical etches, developing optimum conversion coatings requires assessment of the microstructure of the steel. Correlations have been found between the microstructure of the substrate material and the nature of the phosphate films formed. Aloru et al. demonstrated that the type of phosphate crystal formed varies with the orientation of the underlying steel crystal lattice [154]. Fig. 32 illustrates the different phosphate crystal morphologies that formed on two heat-treated surfaces. The fine flake structure formed on the tempered martensite surface promotes adhesion more effectively than the knobby protrusions formed on the cold-rolled steel. 

The processes are dealt with fully in Chapters 11, 14 and 15. Because many paint systems include an initial surface pretreatment, e.g. chromated aluminium or phosphated steel, BS4479 1990, Part 3 deals with conversion coatings and should be consulted by designers. Whatever the method of treatment, liquids must be able to drain quickly and freely from the surfaces. Crevices where liquids can become entrapped are best avoided. The surface configuration needs to be such that active solutions can be washed away, leaving the surface to be painted completely free from unreacted pretreatment solution. Failure to achieve the requisite level of freedom from the surplus chemicals causes paint failure, e.g. osmotic blistering. 

Chromate conversion coatings for aluminum are carried out in acidic solutions. These solutions usually contain one chromium salt, such as sodium chromate or chromic acid and a strong oxidizing agent such as hydrofluoric acid or nitric acid. The final film usually contains both products and reactants and water of hydration. Chromate films are formed by the chemical reaction of hexavalent chromium with a metal surface in the presence of accelerators such as cyanides, acetates, formates, sulfates, chlorides, fluorides, nitrates, phosphates, and sulfamates. 

The enhanced corrosion resistance of phosphated steel can be attributed to two phenomena the increased paint adherence and the chemical passivation of the metal surface by an insulating barrier film of phosphates. A difference between phosphate and chromate conversion coatings is that the later are thought to function as... 

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