Influence of Metallic Powder Contents on Corrosion Resistance of Galvanized Steel with Metal Powder-containing Organic Coatings for Automobile Fuel Tanks

摘要

Pb- and Cr(VI)-free galvanized steel sheets for fuel tanks are coated with epoxy-resin films (thickness: approx. 3 μm) containing particulate Ni powder and flaky Al powder to provide a combination of weld-ability and degraded gasoline resistance (sour gasoline resistance). The corrosion behavior of galvanized steel specimens coated with epoxy resin containing different amounts of the two types of metal powders was investigated in a solution containing acetic acid, formic acid and NaCl at pH 3.2 and 40℃ to elucidate the mechanism of corrosion protection by the coatings. The oxygen gas permeability and water vapor permeability of the coatings were also examined. The results indicated that the addition of particulate Ni powder promoted galvanic corrosion between the Ni and the Zn coating. Voids generated around the embedded Ni powder particles also appeared to accelerate the penetration of the corrosive solution through the coating. On the other hand, the addition of the flaky Al powder improved corrosion resistance. This improved corrosion resistance is associated with the suppression of direct contact between the Ni powder and the Zn coating and also with increased barrier properties, which could be confirmed from oxygen gas and water vapor permeation measurements.