Corrosion of Galvanised Iron and Aluminium Sheets under Wet Storage Conditions

Abstract

To develop methods of preventing the incidence or, at least, of reducing the severity of wet storage staining of galvanised iron and aluminium sheets, a more detailed study of this type of corrosion was undertaken. It appeared that renewal of the oxygen originally present in the moisture, condensed or entrapped between individual sheet surfaces in a closely packed bundle, is severely restricted, and that the available supply of dissolved oxygen, being consumed by corrosion during the normal cathodic reduction type of reaction, is gradually exhausted. During this process, the hydrogen reduction reaction to produce hydrogen gas develops, to become the major and eventually the only cathodic reaction once completely oxygen-free conditions prevail. The presence of impurities of low hydrogen overpotential, such as iron, in the surface of the galvanised coating either as iron/zinc alloys or as iron pArticless picked up during the manufacturing process, increases the rate of hydrogen evolution and the severity of wet storage staining. In the case of aluminium and two of its alloysthe mechanism of wet storage staining is substantially the same with distilled water except that the hydrogen evolution reaction ceases once a sufficiently thick protective layer of oxide has developed.