Study on intergranular corrosion initiation behavior of Inconel alloy 600 in high temperature water, (1)

IGA/SCC has been experienced on the secondary side of steam generator (SG) tubes in PWR's. Various remedial actions, such as boric-acid injection, Na/Cl mol ratio control and high concentration N2H4 operation have been applied to power plants. The inhibitory effect of boric-acid on IGA/SCC was reported by many researchers. However, the inhibitory mechanism of boric-acid on IGA of Inconel alloy 600 for SG tube has not been clarified. This report describes the inhibitory effects of boric-acid on the surface corrosion film stability of alloy 600 which is closely related to IGA/SCC initiation and propagation behavior, by using a.c. impedance method. The experiments were carried out in 100 degC deaerated H3BO3, CH3COOH, HCl and NaOH aqueous solutions. The main test results are as follows: (1) It was shown that the corrosion potential of alloy 600 which is very important to construct corrosion mechanism of alloy 600 was controlled by dissolution reaction of nickel and hydrogen reductive reaction within a wide range of pH. (2) Form the results of investigation on the stability of surface film of alloy 600, it was revealed that the stable surface film was formed on alloy 600 in boric-acid and its corrosion resistance was most high. This result has a good relation to earlier analysis of the chemical compositions of the corrosion film. So it was very applicable to investigation on analysis of the corrosion film stability of alloy 600. (author)