Pitting Corrosion of the Aluminum Ends of Tin-Free Steel Cans

Bimetal cans, in which the body is made of tin-free steel (TFS) with easy-open ends of aluminum alloy, may be used to hold ion-supplementing drinks, juice, and the like. In such cases, the rest potential of the TFS in the contents of the can may be more noble than the pitting potential of the aluminum alloy, resulting in galvanic corrosion of the Al-alloy ends. Accordingly, studies were carried out on two methods of preventing such corrosion: (1) development of Al alloys having a higher (more noble) pitting potential, and (2) a technique for cladding with a sacrificially anodic material. Furthermore, in order to develop an aluminum alloy for the can ends that is resistant to pitting corrosion (i.e., has a more noble pitting potential), the Cu²⁺ ion method for rapidly measuring the pitting potential of the aluminum alloy has been developed and is being used to investigate the effects of various additive elements. This method has made it possible to develop an Al-Si-Cu-Mn alloy in which pitting corrosion did not occur even when Cl⁻ ion concentration in the contents of the can reached 1000 ppm. These characteristics make safe its use as a container for ion-supplementing drinks. When the Cl⁻ ion concentration in the contents of the can exceeds 1000 ppm, it effectively provides cladding of a sacrificially anodic material on the aluminum end.