Effects of alloying elements (Cr, Mo, W, and Ni) on the repassivation kinetics of Fe-Cr stainless steels in deaerated $MgCl_2$ solution at 50 ℃ were examined using the rapid scratching electrode technique under a potentiostatic condition. 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, $i(t) = A \cdot t^{-\alpha}$, 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 to the thickness of about 2 nm 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 stress corrosion cracking (SCC) 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 SCC is the alloy.
Influences of alloying elements (Cr, Mo, W, and Ni) on the repassivation kinetics of Fe-Cr stainless steels 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. With an increase in Cr content ($X_{Cr}$), Mo content ($X_{Mo}$) and W content (XW) for Fe-Cr stainless steels, the value of cBV was linearly decreased and the results can be represented by the following experimental equations ;
$cBV(\times 10^{-3} C/cm^2) = 9.1-0.23X_{Cr}$ for Fe-18 ~ 29Cr,
$cBV(\times 10^{-3} C/cm^2) = 3.4-0.65X_{Mo}$ for Fe-25Cr-0 ~ 4Mo and $cBV(\times 10^{-3} C/cm^2) = 2.3-0.40X_W$ for Fe-29Cr-0 ~ 4W.
These results demonstrate that the increase in Cr, Mo and W contents of Fe-Cr alloys leads to the increase in the protectiveness of passive film even if it is thinner, and also to the increase in the SCC resistance of the alloys.
Addition of 2 wt % Ni to Fe-25Cr-2Mo increases the value of cBV thereby lowering the protectiveness of passive film.
In order to confirm the validity of the cBV as a parameter of predicting the SCC resistance of an alloy, effects of Ni content on repassivation kinetics of Fe-20Cr-0 ~ 80Ni alloys were also analyzed in terms of the cBV and compared with those on SCC resistance of the alloys measured under constant loading condition in boiling $MgCl_2$ solution. The cBV of Fe-20Cr-10Ni alloy exhibited the highest value and hence the highest susceptibility to SCC. This indicates that the passive film formed on Fe-20Cr-10Ni alloy during repassivation was found to be most defective or non-protective. With an increase or decrease of Ni content from 10 wt %, the cBV of alloy decreased, predicting an improvement of the SCC resistance. The prediction of the SCC resistance of Fe-20Cr-0 ~ 80Ni alloys in terms of the influences of Ni on the cBV was found to be in good agreements with the experimental results that are obtained from the constant loading SCC tests.
By examining the thickness of passive film formed on the scratched surface after complete repassivation, it was found that the thickness of the film of Fe-Cr alloys linearly increased with increasing the value of cBV, irrespective of type of alloying elements (Cr, Mo, and W). The experimental equation between the film thickness and the cBV value for the alloys was drawn as follow ;
$cBV(× 10^{-4} C/㎠) = -2.92 + 9.43ㆍt_{ss}$.
The equation demonstrates again that the lower the value of cBV for an alloy, the thinner and more protective is the passive film formed during repassivation, and that an alloy with the lower value of cBV has a higher ability of repassivation with more protective and thinner film, and hence exhibits higher SCC resistance.
