Of Fe-mn Based Alloy Electrochemical "endogenous" Study Of Surface Modification

The aim of the present investigation is to explore a simple surface modified process by using electrochemical methods, which are not required to transport certain elements or compounds to surface of alloys. In terms of this idea, the effects of anodic passivation aging, critical transpassivation and alternating voltage (A. C.) modulated passivation on the stability, constitution and corrosion resistance of passive film formed on Fe-Mn base alloying with Al, Cr and Si in 50% HNO3 solution or 1mol l-1 Na2SO4 solution were studied by way of combined electrochemical examination and Auger electron spectroscopic (AES)/X-ray photoelectron spectroscopic (XPS) analysis. (1) Anodic passivation aging: With increasing the aging time or the passivating potential in anodic passive region, the stability, improvement of constitute and corrosion resistance of passive film obviously increased. For an example of Fe-24Mn-4Al-5Cr alloy, with increasing anodic passivation time in 50% HNO3 solution from 15 min to 5 h, the activation time(Ta) of passive film in 1 mol l-1 Na2SO4+0.5 mol l-1 H2SO4 solution extended from about 300 s to above 12500 s, and the corrosion resistance of Fe-24Mn-4Al-5Cr alloy characterized by polarization curve in 1 mol l-1 Na2SO4 solution is superior to that of Fe-13Cr-0.1C SS. The modified efficiency of passive film for anodic aging in 1 mol l-1 Na2SO4 solution is inferior to that in strong oxidizing 50% HNO3 solution. The effect of anodic aging on the property and constitute of passive film mainly depend on the aging time, anodic potential and oxidizing of electrolyte. The higher the anodic potential, the longer the aging time and the stronger the oxidizing of electrolyte, the better the property and constitute of passive film aged. (2) Passivating at critical transpassivation potential: The passivating of Fe-24Mn-4Al-5Cr alloy in 1 mol l-1 Na2SO4 solution was performed under the optimal parameters of critical potential 1100 mV and passivation time 30 min. The potential decline time for passive film in 1 mol l-1 Na2SO4+0.5 mol l-1 H2SO4 solution is about 5400 s. (3) Alternatingvoltage(A.C.) modulated passivation: The effect of AC- modulation passivation on the corrosion resistance of Fe-24Mn-4Al-5Cr alloy in 1 mol l-1 Na2SO4 solution was studied by AC square wave of optimal parameters with duration of 300 ms and magnitude of 380 mv and the modulated passivation time of 10 min. The AC square wave superimposed onto the DC potential of 620 mv in the medium of anodic passivation region. The results indicate that AV modulation increases the potential decline (Td) of passive film in 1 mol l-1 Na2SO4+0.5 mol l-1 H2SO4 solution from about 2800 s(anodic aging at 620 mv for 5h) to about 4000 s. The results of depth profiles of passive film using AES/XPS analysis show that the anodic passivation aging, critical transpassivation treatment and AC-modulated passivation enhance the enrichment of Al and Cr, and the depletion of Fe and Mn in passive film. The increase of corrosion resistance is attributed to Al2O3 and Cr2O3 enrichment and depleting of Fe and Mn depletion in the passive film and thickening the effective barrier layer of oxides. In passive film of Fe-24Mn-4Al-5Cr alloy modified by transpassivation, the valence state of Cr3+-oxand Mn3+-ox present relatively higher binding energy. Finally, the electrochemistry and the formation of passive film of the three passivating modification has been discussed in brief.