Effects of PBO on the repassivation kinetics of alloy 690 were examined using the rapid scratching electrode technique under a potentiostatic condition to elucidate the influences of PBO on SCC resistance of the alloy. The repassivation kinetics of the alloy was analyzed in terms of the current density flowing from the scratch, k(t), as a function of the charge density that has flowed from the scratch, q(t). Repassivation on the scratched surface of the alloys occurred in two kinetically different processes; passive film initially nucleated and grew according to the place exchange model in which log i(t) is linearly proportional to q(t) with a slope of 1/K, and thereafter grew according to the high field ion conduction model in which log i(t) is linearly proportional to 1/q(t) with a slope of cBV, which is a useful parameter to predict SCC resistance of an alloy. It was found that the value of 1/K, determined from the log i(t) vs. q(t) plot was directly proportional to the activation energy for the place exchange process of a metal-oxygen pair, and could be used as a quantitative measure of the repassivation rate as well as the protectiveness of passive film at the initial stage of repassivation. An alloy/environment system with a higher value of 1/K showed a faster repassivation rate with formation of a more protective film.
In order to confirm the validity of the 1/K and cBV as parameters of predicting the SCC resistance of Alloy 690, influences of aging time(0∼50 hr) on the repassivation kinetics of Alloy 690 were evaluated in terms of the value of 1/K and cBV. Alloy 690 aged at 700℃ for 1 hour exhibited the lowest value of 1/K and the highest value of cBV. On the other hand the alloys aged at 700℃ for 10∼15 hours showed the highest value of 1/K and the lowest value of cBV. The effects of aging time on the walue of 1/K and cBV are well corresponded to the effects of aging time on the susceptibility to SCC of the alloy in high temerature water. These results represent that 1/K and cBV are useful parameters in predicting the SCC
susceptibility of Alloy 690.
Influences of PBO on the repassivation kinetics of Alloy 690 were evaluated in terms of the value of 1/K and cBV. With addition of PBO to pH 4 and 10 solutions, the value of 1/K was decreased and cBV was increased, reflecting decrease in SCC resistance of the alloy with a formation of less protective passive film. The change in the value of 1/K and cBV was more significant in pH 10 than in pH 4. Addition of PBO to pH 10 solution containing $Cl^-$ ions also decreased 1/K and increased cBV.
