Corrosion and electrochemical properties of Sn-8% Zn alloy-coated steel in methanol containing H₂O, NaCl, and HCOOH

Corrosion and electrochemical properties of Sn-8% Zn alloy-coated steel prepared by a melt-dipping method have been investigated in methanol containing H₂O, NaCl, and HCOOH, which are known as contaminants in methanol fuel. The coated steel showed high corrosion resistance in methanol containing 0.1-30% H₂O and 0.1-30% H₂O + 0.1% NaCl. However, the coated steel showed slight corrosion in methanol containing 0.1-30% H₂O + 0.1% HCOOH. Polarization curves measured in methanol containing 0.1-30% H₂O + 0.1% NaCl showed that the corrosion potential of the coated steel was close to that of Zn, while the pitting potential of the coated steel was closed to that of Sn. Polarization curves of the coated steel measured in methanol containing 0.1-30% H₂O + 0.1% HCOOH showed that both anodic and cathodic currents monotonously increased from corrosion potentials. This behavior was very similar to that of Zn. According to Auger electron spectroscopy and X-ray photoelectron spectroscopy analyses, the surface film of the coated steel after an immersion corrosion test in methanol containing 0.1% H₂O + 0.1% NaCl was composed of a large amount of oxide and hydroxide of Zn and a small amount of those of Sn. It was presumed that the corrosion of the coated steel in methanol containing H₂O, NaCl, and HCOOH proceeded through the selective dissolution of the Zn component in the coating layer.