Corrosion and its mechanism of Al-10% Si alloy-coated steel in methanol containing H₂O, NaCl, and HCOOH

Corrosion behavior and mechanism of Al-10% Si alloy-coated steel produced by a melt-dipping method have been investigated in methanol containing H₂O, NaCl, and HCOOH as contaminants. Immersion corrosion tests were performed in deaerated or aerated methanol with and without 0.1-30% H₂O, 0.1% NaCl, and 0.1% HCOOH, and ex situ surface analysis and observation were made using an electron probe microanalyzer, X-ray photoelectron spectroscopy, optical microscopy, and scanning electron microscopy. The corrosion rate of the coated steel was negligibly small in methanol containing 0.1-30% H₂O. However, in methanol containing 0.1-30% H₂O with 0.1% NaCl or 0.1% HCOOH, the corrosion rate increased sharply at 0.1% H₂O. From the surface analysis and observation after the tests, the corrosion in methanol containing 0.1% H₂O with 0.1% NaCl proceeds on Al components in dendrites and eutectic structures of the coating layer. Al components corrode forming methoxide while the Si components in eutectic structures remain as metallic Si. In methanol containing 1-30% H₂O, the surface of the coating layer was covered by oxide films composed of SiO₂-Al₂O₃-Al(OH)₃, which prevents corrosion.