Nickel alloys intergranular precipitates

Another type of nickel alloy with which problems of intergranular corrosion may be encountered is that based on Ni-Cr-Mo containing about 15% Cr and 15% Mo. In this type of alloy the nature of the grain boundary precipitation responsible for the phenomenon is more complex than in Ni-Cr-Fe alloys, and the precipitates that may form during unfavourable heat treatment are not confined to carbides but include at least one inter-metallic phase in addition. The phenomenon has been extensively studied in recent years . The grain boundary precipitates responsible are molybdenum-rich M C carbide and non-stoichiometric intermetallic ix... 

As with alloys of other metals, nickel alloys may suffer stress-corrosion cracking in certain corrosive environments, although the number of alloy environment combinations in which nickel alloys have been reported to undergo cracking is relatively small. In addition, intergranular attack due to grain boundary precipitates may be intensified by tensile stress in the metal in certain environments and develop into cracking. Table 4.28 lists the major circumstances in which stress corrosion or stress-assisted corrosion of nickel and its alloys have been recorded in service and also shows the preventive and remedial measures that have been adopted, usually with success, in each case. 

Nickel alloys may be attacked by intergranular corrosion in certain very aggressive environments after incorrect heat treatment. In NiCr alloys, ehromium carbide is precipitated in the same temperature range as for the austenitic stainless steels. The NiCr alloys are primarily attacked by strong oxidizers such as hot nitric acid. The prevention measures are mainly the same as for the austenitic stainless steels. 

The most common microstructural effect on the corrosion resistance of nickel alloys is intergranular sensitization, as previously mentioned. This is the result of chromium carbide precipitation in many Ni-Fe-Cr alloys but can result from intermetallic Mu-phase precipitation in low-carbon highly alloyed materials such as alloy C-276 (UNS N10276). Several standard IGA tests (discussed in the Intergranular Corrosion section) are available for determining (1) if stabilized alloys have been properly annealed to prevent subsequent sensitization, and (2) if nonstabilized alloys are free from significant sensitization as produced. 

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. 

Intergranular corrosion can also occur in other metals and alloys in connection with grain boundary precipitation, for example, in aluminum magnesium (AlMg) 5 alloy, nickel chromium 30 iron (NiCr30Fe), or Ti, and in methanol with traces of water and HCl. Solution annealed austenitic chromium-nickel steels can suffer intergranular corrosion from strong oxidants such as chromium... 

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