Abstract Three types of scales containing Al and Mg have been shown to form during the corrosion of Al in seawater in the pH range 6.3–8.5. The type most frequently occurring has a structure similar to that of the natural mineral hydrotalcite. It can be formed from the Al(OH) 3 produced by the corrosion of Al and Mg 2+ ions present above pH 6.3. The Al:Mg ratio in this compound increases with pH, resulting in a form of buffering action. In the presence of these scales anodic breakdown of the corrosion film takes place at a potential 100–200 mV more anodic than in their absence. The structures, mechanisms of formation, and effects of these scales in the operation of distillation plants with Al surfaces are discussed, as well as their effects on the operation and interpretation of laboratory corrosion tests.