Intergranular Corrosion Mechanism of Alloy 400

Abstract

The objective of this study is to find the reason for the intergranular corrosion (or intergranular attack, IGA) of Monel 400 (70Ni-30Cu) tubes that occurs occasionally in practice. Generally, the hypothesized factors of IGA for Monel 400 tubing could be crevices, dissolved oxygen, low pH, reduced sulfur species, and precipitation of impurities at grain boundaries. Electrochemical techniques including cyclic polarization and long-term potentiostatic polarization were used to test two heats of Monel 400 tubing that had behaved differently in practice. To simulate the situation within a crevice or under a deposit, cupric ions were added to the base solution, which was either neutral or acidic in pH. Artificial crevices without the addition of cupric ions in the base solution and a limiting current model were created which helped to elucidate the mechanism of IGA. The effect of thiourea as a representative reduced sulfur compound was investigated. The results show that in neutral solution IGA occurs with little sensitivity to metallurgy and does not require thiourea, but in acid solution it only occurs with thiourea addition, and particular grain boundary microstructures are more susceptible. Key words: intergranular corrosion, Crevice corrosion, Monel 400, thiourea