Effects of Acetic Acid on the Initiation of Pitting Corrosion and Stress Corrosion Cracking on Low-Alloy Steel in pH Buffer Solutions

Abstract

Applying organic chemicals as an oxygen scavenger or a corrosion inhibitor to the water treatment of steam-water circuits of power plants possibly results in the generation of organic acids, such as acetic acid. This necessitates assessing the effects of the generated organic acids on the corrosion of the steam-water circuits prior to applying the organic chemicals. In this research, the effects of acetic acid on pitting corrosion and stress corrosion cracking (SCC) on low-pressure turbine materials of low-alloy steel were examined by electrochemical measurements and U-bend tests. Buffer solutions of various pHs from 4.5 to 10 were used as test solutions to investigate the effects of acidification by acetic acid on corrosion. Electrochemical measurements indicated that the steel was passivated at pHs above 8 and was the most susceptible to pitting corrosion at pH 8. In the U-bend test, SCC was initiated readily at a pH of around 8. Because SCC occurred at sites of pitting corrosion, pitting corrosion was suggested to be a precursor of SCC. Acetate ions did not trigger pitting corrosion or SCC on the steel. Rather, the addition of acetate ions to a Cl−-containing solution mitigated the initiation of pitting corrosion, resulting in the inhibition of SCC initiation.