Nickel-molybdenum-chromium alloys intergranular corrosion

The composition of this alloy (54% nickel, 15% molybdenum, 15% chromium, 5% tungsten and 5% iron) is less susceptible to intergranular corrosion at welds. The presence of chromium in this alloy gives it better resistance to oxidizing conditions than the nickel/molybdenum alloy, particularly for durability in wet chlorine and concentrated hypochlorite solutions, and has many applications in chlorination processes. In cases in which hydrochloric and sulfuric acid solutions contain oxidizing agents such as ferric and cupric ions, it is better to use the nickel/molybdenum/ chromium alloy than the nickel/molybdenum alloy. 

Precipitation processes of this kind are always caused by heat treatments, snch as sensitizing annealing, that are inappropriate for the alloy in question. For the austenitic chronuum-nickel-molybdenum steels used for the fabrication of chemical plant equipment, the critical tanperature range is 400-800°C. Chromium depletion through formation of chromium-rich carbides, mostly of the type (M23Cg), is the main cause of intergranular corrosion in these steels. The precipitation of chromium nitrides of importance only that the chromium-rich nitride (CrjN) can initiate intergranular corrosion, especially in ferritic steels. Since the intermetalUc phases in stainless steels contain appreciably less chromium than carbides and nitrides and their deposition is far slower, the chromium depletion related to these phases is minimal. 

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