Investigation of the electrochemical corrosion behavior and passive film for Fe-Mn, Fe-Mn-Al, and Fe-Mn-Al-Cr alloys in aqueous solutions

The corrosion behavior of austenitic Fe-(17 wt% to 31 wt%) Mn-(1 wt% to 9 wt%) Al-(1 wt% to 7 wt%) Cr alloys in different aqueous solutions and the corrosion protection mechanism induced by adding Al or Al and Cr, were studied by anodic polarization and Auger electron spectroscopic/x-ray photoelectron spectroscopic (AES/XPS) analysis. Binary Fe-(17 wt% to 31 wt%) Mn-0.15 wt% C alloys passivated with difficultly in 1 mol/L sodium sulfate (Na2SO4) solution. Mn was greatly detrimental to the corrosion resistance of mild steel. Corrosion current density (I-corr) decreased, and the corrosion potential (E-corr) increased with increasing Al or Cr content in the Fe-Mn-based alloys, Polarization curves exhibited a stable passive region for Al or Cr concentrations > similar to 5 wt%. Appropriate combinations of Al and Cr added to Fe-Mn alloys obviously promoted electrochemical corrosion resistance of the alloys. The passivation characteristics in Na,SO, solution of Fe-25% Mn-5% Al, Fe-30% Mn-9.2% Al, and Fe-24% Mn-4% Al-5% Cr or Fe-31% Mn-2.7% Al-7.5% Cr were comparable to those of mild steel, Fe-9% Ni-0.13% C steel, and Fe-13% Cr-0.1% C stainless steel (SS), respectively. All the experimental Fe-Mn-based alloys and steels showed no passivation in 3.5 wt% sodium chloride (NaCL) solution. In rainwater, the Fe-24% Mn-4% Al-5% Cr alloy exhibited a broad passive region with a critical current density < 5 mu A/cm(2), and no active region was apparent, The passive film formed on the Fe-24% Mn-4% Al-5% Cr alloy in Na2SO4 solution consisted of three parts: bound water and hydroxides probably were present al the surface; the outer portion of film was made up of a mixture of Fe, Mn, Cr, and Al oxides; and a transition region existed, consisting of low valence oxides and metallic Fel Mn, Cr, and Al between the passive film and the matrix. The corrosion resistance probably was imparted by a barrier film of bound water, hydroxide, and oxides of Cr, Al, and Fe.