Comparative intergranular corrosion tests have been made on five types of 18-8 steel containing 0.06 and 0.03 per cent maximum carbon, including the molybdenum and molybdenum-columbium variations, to gain a better understanding of their corrosion behavior. The data show that the increased corrosion rate noted in boiling 65 per cent nitric acid must be ascribed to a phase formation other than chromium carbide precipitation when molybdenum is present in steels containing about 0.03 per cent carbon or sufficient columbium to combine with all the carbon.
A study of steels containing 0.01 per cent maximum carbon indicates that the phase is sigma.
The phase has been found to be most damaging to corrosion during its formative period when it exists as an envelope of fine particles around the austenite grains. Although the steel in this condition is subject to intergranular attack by strong nitric acid, it is resistant to such media as boiling acidified copper sulfate, mixed nitric-hydrofluoric acids, and air-free sulfuric acid. Prolonged heating at 750 C., which coalesces the particles, restores the resistance of the steel to intergranular attack in nitric acid.
Of the two causes of intergranular corrosion noted in the molybdenum-bearing 18-8 steels, the precipitation of chromium carbide appears to be the more dangerous. It has produced susceptibility to intergranular attack in both reducing acids and oxidizing acids, whereas the effect of the intermetallic compound has been observed only in nitric acid.
The nitric acid test reveals both types of intergranular attack. To distinguish between these two causes in assessing the molybdenum steels for service, the nitric acid test should be supplemented with at least one other intergranular corrosion test to insure that a high corrosion rate in nitric acid does not prejudice the use of the steels in reducing acid media, if the attack is due to the formation of the sigma phase.