Abstract
Corrosion of steel in fresh water is a serious problem in circulative cooling systems, water supply systems and water storage tanks, therefore many researchers have reported the corrosion of steel in fresh water.1 The initial corrosion behavior of steel has been discussed on the basis of oxide films, and the films are generally destroyed by chloride ions.2 On the other hand, Kato et al. investigated the corrosion behaviour of mild steel by focusing on the effects of metal cations, and they found that some cations show corrosion inhibiting ability.3 However, there have been a few study in which the quantitative effects of metal cations on corrosion of steel in fresh water, and the effect of metal cations in solution to the oxide films has not been clarified. The authors focused on “hardness of cations, X” to explain the different effect of metal cations on oxide films of mild steel. X is an indicator of the basis of HSAB concept,4 and the value means that tendency of the incorporation between the metal cations and oxygen atoms in the passive films. Therefore, it is expected that the effect of metal cations in solution to the oxide film and the corrosion of mild steel would be explained by using X. The effects of metal cations on the oxide films and corrosion of mild steel in model fresh water were investigated by immersion tests with surface analysis of X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). Five different salt solutions, NaCl (Nasol), MgCl2 (Mgsol), ZnCl2 (Znsol), AlCl3 (Alsol), and ZrCl2O (Zrsol), were used as model fresh water, and the concentration of chloride ions in each solutions was the same. Analysis of XPS showed that metal cations that have large hardness of cations, X, were incorporated with passive films, and then the hydroxides of metal cations were formed on the mild steel. The EIS and immersion results showed that the initial corrosion resistance of mild steel increased with increase in X except for the specimens immersed in Mgsol and Zrsol. Reference 1. R. E. Melchers, Corros. Sci., 48 ,4174 (2006). 2. H. J. Engell, Eletrochimica Acta, 22, 987 (1977). 3. M. Kato, Boshoku Gijutsu, 31, 27 (1982). 4. M. Misono, E. Ochiai, Y. Saito, Y. Yoneda, J. Inorg. Nucl. Chem., 29, 2658 (1967).
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