In general, the difference in corrosion resistance is small for all common commercial aluminum alloys.
The explanation for the good corrosion resistance of aluminum lies in the natural oxide layer that forms spontaneously and is dense and insulating. In addition, the layer is stable (insoluble) over a wide pH range, the atmospheric pH range, pH 4 up to and including pH 9. The layer also has a low tendency to absorb SO2.
Under adverse conditions, aluminum may be subject to galvanic corrosion due to its electronegative nature, i.e. low position in the voltage series of dissimilar metals. Aluminum is non-precious with respect to steel, copper, nickel and passive with respect to stainless steel. Only magnesium and zinc are non-precious and can serve as sacrificial anodes, or cathodic protection of aluminum.
Galvanic corrosion occurs when aluminum is in contact with a noble metal or metal ions in solution. The contact can be via direct metallic contact or via an electrolyte.
The effective way to prevent galvanic corrosion is to consider it already at the design stage by
- Using uniform materials
- Avoid steel and copper alloys near aluminum
- Insulate against surrounding foreign metallic materials
- Use fasteners made of acid-proof stainless steel or aluminum
- Use anodized aluminum if possible
- Avoid residual water or moisture accumulation
- Design for runoff, drainage and ventilation
- Avoid contact with strong alkalis.
