Research On The Mechanism Of Electrochemically Induced Surface Annealing In Stainless Steel And Improvement Of The Technique

Electrochemically induced surface annealing (EISA) is a new kind of phase transformation phenomenon in stainless steel which can improve the pitting resistance of the steel. Since the discovery of the phenomenon, studies on the mechanism have been carried on, but no conclusions were confirmed. The simplex medium of sodium nitrite obstructs its actual application and the exhaust liquor is harmful to the environment. The reaction processes of the hydrogen during EISA treatment were observed by cathodic charging. The hydrogen ions were reduced to atoms in the cathodic part of the electric pulse and then re-oxidized back to ions during the anodic part, partly confirming the hypothesis of the mechanism of the EISA proposed by Burstein et al. Influences of the anodic/cathodic intervals, pH of the medium, cathodic potential on the EISA were studied, and the EISA phase transformation within 3 hours was successfully induced by applying a long-cathode/short-anode electric pulse, improving the efficiency of the treatment. Meanwhile, changes of the microstructure and pitting resistance of the steel after EISA treatment were researched with XRD, cyclic voltammetry test, dynamic potential scanning and metallographic observation. Results show that EISA phase transformation could occur in the medium other than sodium nitrite, such as sodium carbonate. The nitrogen is not an essential condition for the EISA phase transformation. Pitting corrosion sensitivity of the steel after EISA treatment in the sodium carbonate solution was reduced in the same way as it is in the sodium nitrite solution. However, electric pulse treatment in sodium sulfite, sodium acetate, sodium silicate and sodium hydroxide would cause preferential corrosion of the martensite in the steel.