Development of Alloy-Coated Steel having High Corrosion Resistance for Methanol fuel tanks of Fuel Cell Electric Vehicles

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Corrosion and electrochemical properties of Sn-8%Zn alloy-coated steel prepared by a melt-dipping method have been investigated in methanol containing H_2O, NaCl and HCOOH, which are known as contaminants in methanol fuel. The coated steel showed high corrosion resistance in methanol containing 0.1-30%H_2O and 0.1-30%H_2O+0.1%NaCl. The corrosion rates of the coated steel in these solutions were less than 0.66 μg・m^<-2>・s^<-1>. However, the coated steel showed slight corrosion in methanol containing 0.1-30%H_2O+0.1%HCOOH and its corrosion rates were 2.4 -3.5 μg・m^<-2>・s^<-1>. Anodic and cathodic polarization curves measured in methanol containing 0.1-30%H_2O+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 close to that of Sn. According to AES and XPS analyses, the surface film of the coated steel after an immersion corrosion test in methanol containing 0.1%H_2O+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 proceeded through the selective dissolution of Zn component in the coating layer in methanol containing H_2O and NaCl.