Intergranular corrosion prevention

Addition of niobium to austenitic stainless steels inhibits intergranular corrosion by forming niobium carbide with the carbon that is present in the steel. Without the niobium addition, chromium precipitates as a chromium carbide film at the grain boundaries and thus depletes the adjacent areas of chromium and reduces the corrosion resistance. An amount of niobium equal to 10 times the carbon content is necessary to prevent precipitation of the chromium carbide. 

This alloy has a nominal composition of 65% nickel, 28% molybdenum and 6% iron. It is generally used in reducing conditions. It is intended to work in very severely corrosive situations after post-weld heat treatment to prevent intergranular corrosion. These alloys have outstanding resistance to all concentrations of hydrochloric acid up to boiling-point temperatures and in boiling sulfuric acid solutions up to 60% concentration. 

Intergranular corrosion can be prevented or minimized by the following considerations ... 

In practice, three methods are available for preventing sensitisation and intergranular corrosion of austenitic stainless steels ... 

At temperatures above 300°C, low-carbon nickel (0-02% C) is preferred to avoid the possibility of intergranular attack developing after long exposure if material of higher carbon is employed it should be annealed after fabrication and before exposure to caustic alkalis to prevent stress-assisted intergranular corrosion. 

Prevention. In North America, susceptibility to intergranular corrosion and sensitization can be avoided generally by the use of low-carbon grades such as type 316L (0.03% C maximum) in place of sensitization-susceptible type 316 (0.08% C maximum). In Europe, it is more common to use 0.05% C (maximum) steels, which are still reasonably resistant to sensitization, particularly if they contain molybdenum and nitrogen these elements appear to raise the tolerable level of carbon and/or heat input. (Wahid)61, (Krysiak)14 However, this method is not effective for eliminating sensitization that would result from long-term service exposure at 425-815°C. 

The Fe-Cr-C Equilibrium Relationships in Stainless Steels. The metallurgical processes occurring in austenitic stainless steels causing susceptibility to intergranular corrosion (sensitization) and methods to either prevent or remove susceptibility, are illustrated by the physical metallurgy of the selected alloys in Table 7.5. These are all austenitic stainless steels, and after quenching from elevated temperatures are es-... 

Because of the greater carbon and nitrogen contents of the intermediate-purity ferritic stainless steels, prevention of susceptibility to intergranular corrosion is more difficult than with the ultrahigh-purity alloys. Small amounts of niobium and/or titanium are added to combine... 

Prevention of intergranular corrosion of ferritic stainless steels is done by the same methods as for austenitic steels, with the differences in annealing temperature and... 

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. 

For austenitic steels that are resistant to transformation on cold working (e.g., type 310), nitrogen is the element largely responsible for stress-cracking susceptibility, whereas additions of carbon decrease susceptibility (Fig. 19.10) [60]. The effect is related to alloy imperfection structure rather than to any shift of either critical or corrosion potential [59]. Stabilizing additions effective in preventing intergranular corrosion, such as titanium or columbium, have no... 

To prevent halide-induced intergranular corrosion which could occur in aqueous environment with significant quantities of dissolved oxygen, flushing water is inhibited via additions of hydrazine. Results of tests have proven these inhibitors to be completely effective. Operational chemistry specifications restrict concentrations of halide and oxygen, both prerequisites of intergranular attacks (refer to Section 9.3.4). 

The aim was to lower the phosphoric acid content this acid prevents intergranular corrosion but leads to undesirable phosphate in the waste. 

In RBMK reactors, oxygen (up to 0.2mg/kg) is injected into the feedwater circuit downstream from the condensate polishing system in order to prevent corrosive attack on the perlitic steels, and then removed from the water in the deaerator (Dragunov et al., 1992). Attempts have also been made to inject hydrogen as a remedy for intergranular corrosion attack on 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. 

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