The housings of some of our instrument are made from anodized aluminum. In our experience it is very common to see color change when anodized housings are moored in seawater. We even see some discoloration during the brief time instruments undergo calibration and testing.
There may be several causes of discoloration:
- Zinc from the protective anodes tends to deposit on the surface, causing the color to lighten toward gray.
- Some seawater components, for example, carbonates, may precipitate onto the surfaces.
- The anodized coating does not completely cover the aluminum: at microscopic scale the coating has the appearance of a dry lake bed ? there are patches of anodizing surrounded by cracks. These cracks allow water to reach bare aluminum and cause local oxidation that is light in color. Fortunately, once a thin oxide coating forms on aluminum, further corrosion tends to be inhibited. Unless you see severe pitting, there is usually no danger to the safety of the housing.