Stress Corrosion Cracking and its Protection Method of Invar Alloy for LNG Piping

摘要

Stress corrosion cracking (SCC) and its protection method of Invar alloy for LNG piping were studied. SCC behavior of Invar alloy was investigated in simulated marine atmosphere and in chloride solutions. TG-SCC was initiated by pitting corrosion in the atmosphere with adhered airborne salt and by crevice corrosion in chloride solutions. This result was discussed from the relationship among the pitting potential (V_C'), the re-passivation potential for crevice corrosion (E_(R,CREV)) and the corrosion potential (E_(corr)). In a solution containing more than 200 ppm chloride ion, (E_(R,CREV)) was almost the same as E_(corr) = (-0.5/ - 0.4 V vs. SCE), which means that the crevice corrosion is easy to occur. E_(corr) became more noble than V_C, showing the occurrence of pitting corrosion, because of the ennoblement of E_(corr) by the thin condensed electrolyte layer which contained adhered airborne salt. Therefore, the breakdown of the passive film was considered to be closely related to SCC susceptibility of Invar alloy. The protection method of SCC was evaluated. Sand blasted Invar alloy showed a good SCC resistance. Inorganic zinc-rich painting after sand blasting also prevented Invar alloy from SCC even if there were defects of painting.