Chromate treatments for steel.

This work examined the deposition of chromate films on steel to obtain a "conversion" coating which would increase the corrosion resistance of the steel surface, and provide an overall improvement in corrosion resistance when used as a pretreatment prior to the application of organic coatings. Non-electrolytic dip treatments in chromate solutions served only to complete the passivation of the surface; the invisible thin layer of chromate film deposited did not meet the requirements of a "conversion" coating. Electrolytic treatments, on the other hand, when applied at high current density from CrO[3] electrolytes, resulted in the deposition of a non-metallic film on a steel surface: these "chromate" films did meet the requirements of a "conversion" coating, as defined. One electrolytic process has been studied in detail. The parameters controlling deposition were examined. Although deposition of the chromate film showed some of the anticipated dependence on treatment time and current density, a far greater influence was exerted by the pretreatment of the steel, and it was subsequently shown that chloride ions or sulphate ions derived from an acid pretreatment or by their presence at low concentrations in the CrO[3] electrolyte greatly "catalysed" film deposition. A variety of techniques were used to study the composition of the chromate films. The properties indicated that the film was one of the poorly defined and poorly characterised mixed Cr(III), Cr(VI) oxides in a hydrated non-crystalline state. Radiotracer techniques permitted quantitative resolution of the chromium into components, viz. Cr(VI) 15%, Cr(III) (anodically oxidisible) 6%, Cr(III) (inert) 80%. Finally the properties of the electro-chromate treatment as a "conversion" pretreatment for steel were examined. The film provided some degree of protection to unpainted steel. With organic coatings a dramatic increase in coating adhesion was achieved accompanied by a corresponding increase in corrosion resistance of the painted steel