Localized corrosion behavior of lean duplex stainless steel in chloride containing synthetic tap water

The localized corrosion properties of lean duplex stainless steel (LDSS) and type 316L were compared by means of electrochemical testing and X-ray photoelectron spectroscopy (XPS) analysis. Electrochemical testing revealed that the pitting corrosion resistance of LDSS was superior to that of type 316L, but the crevice corrosion resistance of 316L was better than that of LDSS. In potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) analysis, as the pH decreased, the breakdown potential of type 316L did not change largely. However, the breakdown potential of LDSS sharply decreased at pH 4, and was lower than that of 316L at pH 2. Regardless of the pH, the passive film compositions of 316L and LDSS were the same, except for Mo compounds. Thus, the difference in the passive film stability of the two steels as the pH decreased was attributed to the difference in Mo content. Although pitting and crevice corrosion are similar in many respects, their initiation mechanisms differ. The resistance to these two types of corrosion can change depending on the concentration of Mo oxide in the passive film.