Electrochemically Induced Surface Annealing of Stable Martensite and Its Influence on Corrosion Resistance
Authors: Li Zhilin , Li Li , Tang Aiyue , Wang Feng 
Published: February 2020

The phenomenon of electrochemically induced surface annealing (EISA) described the phase transformation from austenite to martensite after the treatment of a series of cathodic/anodic electrochemical pulses in an aqueous solution of sodium nitrite. It could promote the pitting resistance of steels. But all the EISA occured in metastable-austensite stainless steel. Most martensite in industry is caused by heat treatments which was much more stable, so EISA treatment for the stable martensite is important for its widely application.

The 17-7PH type stainless steel was applied whose composition (wt. %) was C≤0.09, Si≤1.0, Cr 16-18, Ni 6.5-7.75, Mn≤1.0, Al 0.75-1.5, S≤0.030, P≤0.035. Originally, the samples were solution treated and aged. The content of martensite was tested by XRD and estimated through the relative diffraction strength of its {110} crystal plane. The EISA treatmen was carried out in an 8M aqueous solution of NaNO2 at 80±1°C in a three-electrode device. A series of anodic/cathodic electrochemical pulses were applied at the output voltages of -1.44 V (v.s. SCE) for 280 s, and +0.30 V (v.s. SCE) for 60s, alternately. Potentiodynamic scanning was carried out in 3.5% NaCl solution. The corrosion morphology was observed at 400 times of magnification. The corroded samples were mechanically polished, and the pitting numbers were counted at various thicknesses, until no pits could be observed.

Figure (A) shows the XRD spectra of a sample before and after the anodic/cathodic electrochemical pulses of 6h. After the treatment, the relative diffraction strength of martensite {110} crystal plane decreased，which indicates the decrease of martensite content.  I
b and I
a were used to represent the relative diffraction strength of martensite {110} crystal plane before and after EISA treatment, respectively. Define R=(Ib-Ia)/Ib to represent the decrease of martensite content after the electrochemical pulse treatment. It shows that R increased with the increase of the treatment time, so the martensite content decreased more when the treatment time increased. Figure (B) shows the hardness almost kept the origin values after the pulse treatment. The constant hardness should only originated from the special martensite phase transformation because the precipitation hardening of the steel could not happen at the treatment temperature of 80°C. The martensite content decreased while the hardness almost kept constan. Such phenomenon is the character of EISA phase transformation. Therefore, the EISA phase transformation did occur during the pulse treatment.

The samples were anodic treated at +0.3V (v.s. SCE) for 6h and cathodic treated at -1.44V (v.s. SCE) for 6h,  respectively . The XRD spectra showed that the R was 2.63% after the anodic treatment and -11.33% after the cathodic treatment. So both the anodic treatment and the cathodic treatment could not cause the martensite transformation, the only cause of the martensite content decrease should be the anodic/cathodic alternative treatment. So EISA phase transformation did occur in the steels other than metastable-austenite stainless steels.

The corrosion morphology of the samples before and after the 6h EISA treatment was observed after the 1.0 V anodic charge at 50°C for 5 min in 3.5% NaCl aqueous solution. The average of pit number on the surface of three untreated samples was 2108 cm-2, whereas that of three EISA treated samples was only 1116 cm-2. Figure (C) shows the deepest pit of untreated samples was almost 180 μm, but that of EISA treated ones was only 140 μm.  The pit number of the EISA treated samples was smaller than that of the untreated samples at all depth. Therefore, the EISA treatment promoted the pitting resistance of the steel.

Figure (D) Shows that the pitting breakdown potential increased much from 0.087V to 0.523V. So EISA treatment could promote the pitting resistance of the steel.

On the whole, the EISA phase transformation was first observed in steels other than metastable austenite stainless steel. The EISA treatment promoted the pitting resistance. So the corrosion protection method of EISA can be extended to different kind of steels with relative stable martensite obtained in heat treatment.

Figure caption: (A) XRD test results of a sample before and after EISA treatment of 6h; (B) Hardness test results before and after electrochemical pulse treatment for different time; (C) Pit numbers at various distances to the surface of samples; (D) Dynamic potential scanning results of untreated and 6h EISA treated samples.

Figure 1