Anodizing aluminum adds a thick layer of extra oxide to its surface, helping to improve its corrosion resistance and some of its mechanical properties, including its ultimate strength and yield strength, or the stress point at which deformation occurs. Some parts like pistons may last longer when anodized. Despite these advantages, however, anodized aluminum can sometimes become discolored. Several different culprits can be behind the discoloration.
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Weathering bloom is the first likely cause of discoloration. Ordinarily, the surface of the anodized aluminum is sealed with hot water during manufacturing. If the sealing is not adequate, the surface may later become etched, especially if brought in prolonged contact with an acidic environment. Another, similar problem is chemical attack, which occurs when acid or alkaline solutions remain in prolonged contact with the product. By the time the discoloration has become visible, it's largely irreversible and the part may need to be replaced.
Chalking is another possible cause of discoloration. If the product was anodized improperly -- if the temperature during anodizing was too high, for example -- the resulting oxide coat may be soft. When exposed to the air, this soft coating will begin to crack and its behavior will reflect changes in prevailing humidity. The cracked layer may eventually flake off, exposing the aluminum beneath and leaving it to become degraded in the same manner.
Welding aluminum after it's been anodized requires grinding away the surface coating, which leaves a mess and is not recommended. Anodizing after welding, however, poses its own special challenges. It's important to use the lowest amount of heat possible, because if nearby metal is disturbed by the heat, it can potentially become discolored following anodizing. Welders must also use the correct alloy welding wire; using the wrong kind of wire can create a gray or black coloring on the anodized product.
Iridescence or a purple hue on the aluminum coating is another, more unusual kind of discoloration problem. If the aluminum was sealed at high pH after anodizing, crystals of aluminum hydroxide or "sealing smut" may have formed; these crystals will create a faint pattern of unusual colors like the patterns on an oil drop. The colors are more visible if the product is black in color. Using a high bath temperature to anodize may also lead to iridescence in the product.