Pitting Corrosion Behavior of Cr–Mn Austenitic Stainless Steel with Addition of Molybdate and Tungstate Under Stagnant and Flow Condition in NaCl Solution

Molybdate and tungstate inhibitors were introduced in stagnant and flowing conditions for determining pitting corrosion resistance of Cr–Mn SS in chloride-containing media. Corrosion behavior was investigated using potentiodynamic and electrochemical impedance spectroscopy measurements in different NaCl concentrations (1, 2% and 3 wt.%). It was found that increase in NaCl concentration increases the corrosion rate. The addition of 0.01 M inhibitors decreased the corrosion rate in Cr–Mn SS. Better corrosion resistance was found with the addition of molybdate in stagnant conditions. However, the opposite behavior was observed in flowing conditions, but there was an increase in E pit with addition of molybdate. Additionally, the mechanism of the corrosion attack developed on the material surface after polarization was analyzed by scanning electron microscopy and energy-dispersive spectroscopy (EDS mapping and point scan). The inhibition effect of the additions was due to a more stable passive film against Cl ions. Mn in the steel may cause opposite effect (initiation of the pits on steel), mainly due to the presence of MnS inclusions which acted as pitting initiators.