Effect of alloying elements on the electrochemical polarization behavior and passive film of Fe–Mn base alloys in various aqueous solutions

Abstract

The corrosion properties of austenitic Fe–Mn, Fe–Mn–Al, Fe–Mn–Cr and Fe–Mn–Al–Cr alloys with compositions of 23–30 wt% Mn, 2.8–8.2 wt% Al and 4.9–6.9 wt% Cr in various aqueous solutions of pH −0.8 to 15.3 and the passivating mechanism induced by the presence of Al, Cr, or Al and Cr have been studied using electrochemical measurements and Auger electron spectroscopic/X-ray photoelectron spectroscopic analysis. Binary Fe–Mn alloys can be passivated only in 10–50% NaOH solutions, and alloying of binary Fe–Mn alloy with Al or Cr or combination of Al and Cr seems not so obviously beneficial to corrosion resistance in HNO 3 or Na 2SO 4 solutions. All of the experimental Fe–Mn based alloys and steels for comparison cannot passivate in either 10% HCl or 3.5% NaCl solution. The Fe–Mn based alloys containing Al or Cr or Al and Cr can passivate in 10–50% HNO 3 or 1 mol l −1 Na 2SO 4 solutions and rainwater. In general, Fe–Mn based alloys can passivate in oxidizing acid, neutral and basic solution, but cannot passivate in reducing acid or solution containing active Cl − ions. In the passive film formed on the surface of Fe–Mn base alloys in various aqueous solutions, bound water and hydroxides are present at the surface of the film, while mixed oxides of Al, Cr, Mn and Fe are located in the inner part. The resistance to corrosion is imparted by a barrier film of bound water, hydroxides and oxides of Al, Cr or Fe, while the Mn oxides in passive film reduce the corrosion resistance.