| Corrosion protection of light metal alloys, including aluminum and its alloys, often involves the application of a chemical coating, such as a chromate. Chromates appear to be active coatings, which can repair themselves at defect sites. However, concerns over the environmental hazards that chromates present demand replacement by more environmentally friendly materials. Currently there are no entirely suitable replacements for the chromate coatings. More importantly, it is difficult to design such a replacement, as the mechanism by which chromates reduce corrosion is unclear. The research conducted provides information regarding the role chromates have in reducing corrosion of some aluminum alloys.; It was found that the controlling reaction for corrosion is the oxygen cathodic reaction. This reaction is oxygen in water being reduced to form hydroxyl ions. If the rate of the oxygen reaction can be reduced, then aluminum corrosion will decrease. This appears to be one of the roles chromate has, in that it reduces the reaction rate of the cathodic reaction on the surface. It is therefore suggested that chromate conversion coatings are successful on many different surfaces because they decrease the cathodic reaction rate. It is apparent then that the cathodic current density is a good measure of the corrosion resistance of these alloys and can be used to quantify the effectiveness of coatings designed to replace chromates. |
