effects of alloying elements (cr, mn) on corrosion properties of the high-strength steel in 3.5% nacl solution

Effects of chromium and manganese as alloying elements on corrosion resistance of carbon steel were examined using evaluation of corrosion resistance in 60°C NaCl solution with a weight loss test, polarization test, analysis of rust with X-ray diffractometer, Raman spectroscopy, transmission electron microscopy, energy dispersive spectroscopy, and electron energy loss spectroscopy. The weight loss behavior conformed to a typical parabolic law, and the oxidation state of iron in rust was higher along the fast pathway but was disproportionate to the distance from the alloy/AR interface. It suggests that the corrosion process of the alloys was controlled by transport of oxygen to the rust layer. The improvements in corrosion resistance of 18Mn and 18Mn5Cr resulted from both the refinement of grain in adherent rust (AR) and the increase of the amounts of goethite in nonadherent rust (NAR) by chromium and manganese. Especially, the effectiveness of chromium on corrosion resistance was also related to the refinements of grain in AR and the amounts of goethite in NAR. The Tafel extrapolation method was inadequate to measure the instantaneous corrosion rate of steels with various alloying elements and immersion periods because of the difference in electrochemical reduction rates of rust, depending on its constituent.