Chromium-nickel 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. 

Table 19.3 Summary of chemical tests used for the determination of susceptibility to intergranular corrosion of iron-nickel Chromium alloys ...

Cowan, R. L. and Tedmon, C. S. (Jr.), Intergranular Corrosion of Iron-Nickel-Chromium Alloys , in Advances in Corrosion Science and Technology, Vol. 3, (eds. M. G. Fontana and R. W. Staehle), Plenum Press (1973)... 

Recommended practice for applying statistics to analysis of corrosion data Practice for operating light- and water-exposure apparatus (carbon-arc Type) for exposure of nonmetallic materials Method for detecting susceptibility to intergranular attack in wrought nickel-rich, chromium-bearing alloys... 

Intergranular and transgranular cracking often occur simultaneously in the same alloy. Such transitions in crack modes are observed in alloys with high amounts of nickel, iron chromium and brasses. In corrosion under tension, ruptures are fragile and are sometimes characterized by the presence of cleavages, notably in the case of hydrogen embrittlement.16... 

ASTM Standard G 28, Standard Test Methods of Detecting Susceptibility to Intergranular Corrosion in Wrought, Nickel Rich, Chromium-Bearing Alloys, Annual Book of ASTM, 1999, 03.02. 

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... 

Intergranular corrosion can result from an enrichment or depletion of one of the alloying elements at the grain boundary. Stainless steels, chromium, and nickel-rich alloys are all susceptible to intergranular corrosion. 

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. 

As described before, intergranular corrosion of stainless steels occurs when the material is sensitized. This condition produces a chromium-depleted envelope around each grain, which is less corrosion-resistant. This results in intergranular corrosion of the stainless steel in certain environments. The two ASTM standards that describe how to test stainless steels for susceptibility to this form of corrosion are ASTM A 262 and A 763. A third standard, G 28, is applicable to nickel-rich, chromium-bearing alloys. 

The types of chromium-containing nickel alloys that owe their corrosion resistance to passivity, viz. Ni-Cr-Fe, Ni-Cr-Fe-Mo and Ni-Cr-Fe-Mo-Cu alloys, may become susceptible to intergranular corrosion in circumstances broadly similar to those that produce susceptibility in stainless steels ... 

---