Chromium-rich carbides

Susceptibility to intergranular corrosion also can occur in ferritic stainless steels (Ref 86-90). As with the austenitic stainless steels, the extent of the susceptibility is a function of the chemical composition and the thermal history of the steel. Also, the mechanism of intergranular attack is essentially the same for both classes of stainless steels, specifically, attack of lowered-chromium-content regions adjacent to precipitated chromium-rich carbides and nitrides. However, there are... 

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. 

An example of weld effects is shown in Fig. 6. Four different 18Cr-8Ni stainless steel panels were joined by welding and were then immersed in a solution of nitric-hydrofluoric acids. This solution produces rapid attack on grain boundaries containing chromium-rich carbide precipitates, which are surrounded by chromium-depleted zones. The attack is similar to that shown in Fig. 3. Dislodgement of grains in the weld-decay zones leads to the formation of grooves on both sides of the panel of Type 304 (about 0.06 % C) steel that are parallel, but not adjacent to the weld bead. If immersion had been continued, the panel would have dropped off from the assembly. 

All of the alloys listed in Tables 1 and 2 may become sensitized that is, form various precipitates at grain boundaries when exposed to certain temperatures and thereby become subject to intergranular attack. Chromium-rich carbides are the most common precipitates in the alloys of Tables 1 and 2, except in the Ni-Cr-Mo alloys in which molybdenum carbide is formed. All of the evaluation tests discussed below detect susceptibility to intergranular attack associated with chromium and molybdenum carbide precipitates. 

In the selection of materials, the possibility of intergranular corrosion must also be taken into account. This type of corrosion is selective and is based, for example, on the precipitation of chromium-rich carbides on the grain boundaries (e.g. by welding). The chromium depletion can lower the corrosion resistance so that the grains disintegrate. This modification of the material is known as sensitisation [30]. Thus, for example, steel no. 1.4301 (Table 4) can be sensitised and, according to [32], the use of this material is not recommended. 

Although columbium (niobium) stabilized alloy G from formation of chromium-rich carbides in the heat-affected zones of the welds, secondary carbide precipitation still occurred when the primary columbium carbides dissolved at high temperatures, and the increased carbon in the matrix increases the tendency of the alloy to precipitate intermetallic phases. Alloy G-3 has lower carbon (0.015% maximum vs. 0.05% maximum for alloy G) and lower columbium (0.3% maximum vs. 2% for alloy G). The alloy also possesses slightly higher molybdenum (7% vs. 5% for alloy G). 

Sensitization occurs in a zone subject to a critical thermal cycle in which chromium-rich carbides precipitate and in which chromium diffusion is much slower than that of carbon. The carbides are precipitated on grain boundaries that are consequently flanked by a thin chromium-depleted layer. This sensitized micro-structure is much less corrosion resistant, because the chromium-depleted layer and the precipitate can be subject to preferential attack. Sensitization can be avoided by the use of low-carbon grades such as Type 316L (0.03% C max.) in place of sensitization-susceptible Type 316 (0.08% C max.). 

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