Corrosion behavior of electrophoretically coated galvannealed-, galvanized-, and cold rolled steel sheet was investigated by salt spray test, cyclic corrosion test, 5% NaCl immersion test and heat cycle test. Galvannealed steel showed the best resistance against rust formation, followed by galvanized steel. Brown rust developed easily on cold rolled steel. Galvannealed steel also has the best resistance against blistering and galvanized steel the worst. This was confirmed by the direct observations of film exfoliation of specimens partly covered with clear lacquer in corrosive environments. According to the electrochemical experiments galvannealed steel, although its surface layer consists of Fe-Zn alloy, behaves like galvanized steel on the early stage of corrosion. Zinc corrosion products cover its surface under the film and suppress rust formation.
Scanning electro microscopic observations revealed that on galvanized steel Zn/phosphate interface under the film is attacked by OH- ions formed by cathodic reaction. This results in film exfoliation. On the other hand Fe-Zn/phosphate interface on galvannealed steel is not damaged easily by OH- ions because of the roughness of the Fe-Zn alloy surface, which brings about good film adherence.
