Effects of ageing time at 700 ℃ on the repassivation kinetics of Alloy 600 and Alloy 690 in deaerated $MgCl_2$ solution at 80 ℃ and in standard EPR solution at room temperature were examined using the rapid scratching electrode technique under a potentiostatic condition. And also the influences of an applied potential and solution temperature on the repassivation kinetics of thermally treated Alloy 690 in $MgCl_2$ solution were examined. The repassivation kinetics of the alloys was analyzed in terms of the current density flowing from the scratch, i(t), as a function of the charge density that has flowed from the scratch, q(t). Current transient upon scratching for the alloys followed the well known experimental equation, $i(t) = A ㆍt^{-α$, where log i(t) is linearly proportional to log t with slope of -α, a parameter of repassivation rate. Repassivation on the scratched surface of the alloys occurred in two kinetically different processes ; passive film initially grew according to the place exchange model in which i(t) is linearly proportional to q(t), and then according to the high field ion conduction model in which i(t) is linearly proportional to the 1/q(t) with the slope of cBV, where c is a constant for the alloy, B is a constant related with the activation energy barrier for ion movement and V is the potential difference across the passive film. It was demonstrated that the cBV, indeed, was more effective and consistent in characterizing the repassivation kinetics of an alloy than the α, repassivation parameter, and that the cBV is closely associated not only with the protectiveness of passive film but also with the susceptibility to intergranular stress corrosion cracking(IGSCC) for alloy/environment systems. The lower the value of cBV for an alloy/environment system, the more protective and thinner is the passive film formed on the alloy and hence the higher resistant to IGSCC is the alloy.
Influences of ageing time, applied potential and solution temperature on the repassivation kinetics of alloys were evaluated in terms of the value of cBV, determined from the slope of the linear region of log i(t) vs. 1/q(t) plots. Both Alloy 600 and 690 aged for 1 hr at 700 ℃, showing the highest susceptibility to IGSCC, exhibited the highest value of cBV. On the other hand the alloys aged for 10~15 hrs at 700 ℃, showing the lowest susceptibility to IGSCC, exhibited the lowest value of cBV. This results represents that the scratch test is applicable to predict the IGSCC susceptibility of steam generator tube materials(Alloy 600 and 690). The cBV value of the alloys aged for 10~15 hrs at 700 ℃ is below about $6×10^{-4}C/㎠$, therefore this is a proper limiting value to decide whether a given sample is resistant enough to IGSCC. For thermally treated Alloy 690, with the increase of an applied potential and solution temperature, the value of cBV and the passive film thickness were increased. This results shows that the higher the value of cBV for an alloy, the thicker and less protective is the passive film formed during repassivation and hence an alloy with the higher value of cBV exhibits lower IGSCC resistance.
