Electrochemical mechanism of stress corrosion cracking of API X70 pipeline steel under different AC frequencies

Electrochemical and SCC behaviour of X70 pipeline steel samples were investigated under various AC frequencies to elucidate the mechanistic aspects of the alternating current (AC)-assisted stress corrosion cracking (SCC) of pipeline steels in simulated seawater solutions. Results indicate that AC serves as a critical influencing factor on SCC of X70 pipeline steel by accelerating the mass transfer and in-situ-O2-generation, and hydrogen evolution due to the non-Faraday and Faraday potential of AC, respectively. The electrochemical reactions show a maximum rate around 30 Hz, which is attributed to the periodic effect of AC frequency on the different processes; a similar trend is observed in SCC behaviour with increase in AC frequency. AC catalyses both the anodic dissolution (AD) and hydrogen evolution reactions, and greatly increases the SCC susceptibility resulting from the combined effect of increased AD and hydrogen embrittlement (HE) on the SCC process.