Effect of sulphide inclusions on the pitting-corrosion behaviour of high-Mn steels in chloride and alkaline solutions

The corrosion behaviour of the 27Mn-4Si-2Al-and 26Mn-3Si-3Al-type austenitic steels were evaluated in chloride 3.5 % NaCl and alkaline 0.1-M NaOH environments using potentiodynamic polarization tests.The type of non-metallic inclusions and their pitting-corrosion behaviour were investigated.In the chloride solution, both the steels exhibited a lower corrosion resistance in comparison to the alkaline solution.The high-Mn steels showed evidence of pitting and uniform corrosion, both in the chloride and alkaline solutions.SEM micrographs revealed that the corrosion pits are characterized by various shapes and an irregular distribution at the metallic matrix.Corrosion damage is more numerous in the chloride solution than in the alkaline solution.EDS analyses revealed that the corrosion pits nucleated on MnS inclusions or complex oxysulphides.The chemical composition of the steels (change in the Al and Si contents) does not affect the privileged areas of pit nucleation, whereas it influences the electrochemical behaviour of the steels in the chloride solution.