Hydrogen And Its Function In Stainless Steels During Electrochemically Induced Annealing

Electrochemically Induced Annealing (EIA) is a new phase transformation behavior discovered recently in metastable austenite stainless steels. Although it is found that EIA treatment can promote the corrosion resistance of stainless steels, the mechanism of EIA has no convincing explanation. The methods of collecting gas though discharging oil, XRD, hardness analysis and so on are employed in this thesis to research the mechanism. The results verify that there is indeed the process of hydrogen diffusing into the steel and accumulating. Following phenomena are also discovered. Elimination of hydrogen from the steel after EIA treatment has little effect on the martensite phase transformation and hardness, so it will not decrease the effect of EIA. During the heating after the EIA treatment, hydrogen can diffuse to the surface and the interior of the steel simultaneously, but the effected depth is limited in the superficial layer within 100μm from the surface, so it will not cause hydrogen brittleness. These results can be a foundation of the application of EIA treatment to the corrosion prevention of stainless steel.With microhardness distribution method, based on one dimension unsteady diffusion model, the diffusion coefficient of hydrogen during EIA treatment is calculated out being D=0.43×10^-13 m²s^-1 at 80℃. That result not only accords with the theoretical deduction, but also accords with the diffusion coefficient measurement results of hydrogen obtained with other methods. Therefore the method can be used to test the hydrogen diffusion coefficient in EIA treatment. Influence factors, such as original pH of treating medium, electric pulse width, are also studied in this thesis. The results are as follows: the original pH of treating medium has little effect on EIA when its value is between 8 and 13. When the cathode stage is fixed at 280 seconds and the anode stage changes from 10 seconds to 350 seconds variably, EIA can reduce the martensites amount in the surface of stainless steel. The decrease of the martensites amount with the anode stage less than 20 seconds maybe cause by precedable corrosion, but the primary reason of the reduce of the martensites amount with the anode stage in more than 60 seconds is EIA.