The objective of this study is to investigate the effect of strain hardening on the passive film repassivation kinetics. This helps understand the film breakdown and repair processes occurring at the crack tip of a ductile material. Straining electrode tests were carried out by applying load-unload-reload cycles at different strain rates on 304 SS and alloy 22 tensile specimens in 4 M NaCl solution at 60ºC at a constant potential of -250 mV (Ag/AgCl). Current transients were measured during straining and the slope of strain rate vs maximum current plot for 304 SS in mill-annealed condition was 0.65 and with strain hardening it decreased to 0.5. The slope of the strain rate vs maximum current plot for alloy 22 in mill-annealed condition was 0.73 and with strain hardening the slope increased marginally. This slope could be directly related to the slope of the current decay during formation of the passive film. Based on this analogy, the experimental results indicated that the strain hardening adversely affected the repassivation kinetics of 304 SS. Strain hardening of alloy 22 did not show any adverse effect on repassivation kinetics.
Stress corrosion cracking (SCC) of stainless alloys is often related to the kinetics of film rupture and reformation1-4. The crack growth rate during SCC is related to the anodic current density at the crack tip following Faraday?s law as
Equation 1
In the above equation, the crack tip current density is related to the dissolution charge density, Q, for one cycle of film rupture and repassivation as3
The exponent of current decay, n, basically describes the repassivation kinetics of the metal surface in the given environment.
During SCC of a ductile material, the crack tip undergoes a considerable amount of plastic deformation and strain hardening. The size of the plastic zone near the crack tip is proportional to the square of the ratio of stress intensity at the crack tip to the yield strength of the material6. According to the localized surface plasticity model proposed by Jones7, the anodic current density of the crack tip resulting from anodic dissolution could cause attenuation of the strain hardening, inducing localized surface plasticity. During a film rupture and repassivation cycle of SCC, the crack tip has different material conditions, viz., initially it has the bulk material property, then it strain hardens and further it shows enhanced plasticity at the surface. These changes in material condition could affect the film formation kinetics also. Generally the film formation properties are related to the chemistry of the material. The effect of cold working or strain hardening on the repassivation kinetics has not been documented in the literature for stainless alloys, especially for alloy 22.
The objective of this study is to investigate the effect of strain hardening on the film rupture behavior of stainless alloys such as AISI 304 stainless steel (304SS) and a Ni-Cr-Mo-W alloy, Alloy 22 using smooth tensile specimen and potential controlled straining electrode technique.
