Chromating Mechanism

Next to the alkali metals, the metals that chromate coatings must protect against corrosion (i.e., aluminum, magnesium, zinc, and cadmium) are the most active metals in the periodic table. In fact, from the large negative standard electrode reduction potentials for aluminum and magnesium dissolution, one would expect these metals to dissolve when immersed in water. This does not occur, of course; upon exposure to water or air, these metals immediately form a hydrated oxide film that is itself somewhat protective.

Chromate conversion treatments actually take advantage of this high surface activity. Through use of a strong oxidizing agent such as chromic acid, CrO3, a redox reaction occurs at acidic pH (pH ยป 2) where hexavalent chromium, either in the form of CrO22-7 or HCrO-, is reduced to trivalent chromium while aluminum is oxidized to trivalent aluminum:

Probably due to of the presence of fluoride ion (see next paragraph), another reduction reaction besides that involving chromic acid can occur. This reaction involves the reduction of either water, hydronium ion, or dissolved oxygen to form hydroxyl ions at the metal surface. This surface-localized increase in pH results in the precipitation of an amorphous mixture of hydrated aluminum plus chromium oxides.

As described in the previous section, the presence of fluoride ion is important for building films of significant thickness; without its presence, film growth is extremely slow. Fluoride presumably serves two roles. First, it solubilizes the aluminum oxide initially present on the surface and allows the redox and deposition reactions to proceed. Second, it solubilizes a portion of the growing film, which allows penetration of the electrolyte to the surface and ion transport from the surface into the growing film. Fluoride has been characterized as a unique monodentate ligand that enhances the dissolution rate of aluminum oxide (Ref 9).

The high corrosion resistance offered by chromate films is attributed to the presence of both hexavalent and trivalent chromium in the coating. Analyses of coatings by wet chemical methods (Ref 10) and with surface-sensitive techniques (Ref 7, 11, 12) have shown that both Cr(VI) and Cr(III) are present in the films. The trivalent chromium is believed to be present as an insoluble hydrated oxide, whereas the hexavalent chromium imparts a "self-healing" character to the film during oxidative (corrosive) attack by species such as chloride ion. The hexavalent chromium is reduced during corrosion to form an insoluble trivalent chromium species that terminates the oxidative attack.

The enhanced corrosion resistance attributed to accelerated chromate conversion coatings has been ascribed to the increase in coating weight that is due to incorporation of the accelerator into the growing coating (Ref 3, 13, 14). Recent studies on ferricyanide-accelerated chromate coatings deposited on a high-copper-containing aluminum alloy indicate that ferricyanide is distributed throughout the film only on the high-copper-containing intermetallic phases such as CuAl2 (Ref 7). Because these phases are known to accelerate greatly the corrosion rate of aluminum (Ref 15), the decrease in corrosion rate offered by the accelerator is believed to be due to the formation of copper ferricyanides on the intermetallic surface, which alters its activity with respect to the solid solution matrix. At adequate treatment times, corrosion resistance is also believed to be enhanced by the uniform surface composition of chromium and oxygen and the absence of aluminum and copper in the near-surface region.

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