Characteristics of a Chromate Conversion Coating

Conversion coating does not simply involve the deposition of a thin layer of, for example, hydrated metal oxide that "converts" a surface to a corrosion resistant state (Ref 3). As illustrated in Fig. 1, an Auger electron spectroscopy (AES) depth profile for a typical chromated layer on aluminum alloy 2024-T3, the film contains not only chromium and oxygen, but also part of the substrate (in this case aluminum) and the principal alloying element (copper). The profile was taken on the solid solution matrix, off any of the intermetallic second-phase particulates that typically form in the aluminum-copper alloy system. In the case of chromate coatings, the amorphous hydrated oxide mixture that is formed on the surface must involve oxidation of the substrate, followed by precipitation of the dissolved metal ions. Precipitation arises from an increase in pH at the surface due to reaction with excess hydroxyl ions produced as a result of the cathodic (i.e., reduction) half reaction. Either reduction of dissolved oxygen or reduction of water itself will cause such an increase in pH at the surface. A similar mechanism is postulated for the formation of crystalline phosphate conversion coatings on steel, in which two principal crystalline phases have been identified: hopeite, Zn3(PO4)24H2O, and phosphophillite, Zn2Fe(PO4)24H2O (Ref 5). The latter compound involves incorporation of the main element in the substrate (i.e., iron) through oxidation of the surface in the presence of acidic zinc phosphate.

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Fig. 1 Auger electron spectroscopy depth profile of polished aluminum alloy 2024-T3 solid solution matrix exposed to Alodine 1200S conversion coating solution for 5 min. Sputter rate approximately 300 A/min (vs. SiO2). Experiment details in Ref 7. Source: Ref 4

As evidenced by the above discussion, a conversion coating is simply a film that contains a mixture of the corrosion-resistant species and the major elements found in the substrate. The mixture promotes strong adhesion of the conversion coating to the substrate. This is unlike a film formed by physical vapor deposition, where there is a very clear interface between the film and the substrate and where essentially none of the substrate is distributed through the deposited film. Poor film adhesion can result at such an interface, and if water and/or oxygen reach the interface region, corrosion can readily occur and cause delamination of the film. Because no clear interface exists in conversion coating films, a certain amount of oxygen and water can be tolerated; the substrate concentration gradient is observed through the protective film.

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