Effects of Cr, Cu, Ni and Ca on the corrosion behavior of low carbon steel in synthetic tap water

Aqueous corrosion behaviors of the low-alloy steels with small amounts of Cr, Cu, Ni and Ca in synthetic tap water were studied by electrochemical corrosion tests (potentiodynamic polarization tests, electrochemical impedance spectroscopy (EIS)) and analytical techniques (XPS, EPMA). Potentiodynamic polarization test represented that all curves showed active corrosion behavior and corrosion potential was shifted to noble direction and corrosion rate tended to decrease as the result of alloying. Results of EIS measurement showed that the more the content of element, the larger the polarization resistance. It can be seen that these results were caused by the formation of a coherent protective film, which contained alloying elements. Actually, XPS results showed that Cr, Cu, and Ca formed protective metal oxide at the surface of rust layer under stagnant condition. The results of EPMA indicated that Cr, Cu, and Ca were distributed densely at inner layer of the rust layer, while Ni was distributed at the whole layer under flow condition. Consequently, alloying elements improved corrosion resistance of low carbon steel. It was reflected that corrosion rates of all specimens were quite low; especially, Cu-, Ni-, and Ca-containing specimens revealed the lowest corrosion rate due to the formation of insoluble rust layer at the initial stage of corrosion.