Intergranular steels

Cihal V 1984 Intergranular Corrosion of Steels and Alloys (Amsterdam Elsevier)... 

Other methods of metal powder manufacture are also employed for specific metals. Selective corrosion of carbide-rich grain boundaries in stainless steel, a process called intergranular corrosion, also yields a powder. 

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. 

Many instances of intergranular stress corrosion cracking (IGSCC) of stainless steel and nickel-based alloys have occurred in the reactor water systems of BWRs. IGSCC, first observed in the recirculation piping systems (21) and later in reactor vessel internal components, has been observed primarily in the weld heat-affected zone of Type 304 stainless steel. 

These nnstabdized grades of stainless steel have an increasing tendency to intergranular carbide precipitation as the caikon content increases above 0.03 percent. 

Fig. 2-18 J U) curves and critical potential range for intergranular stress corrosion (hatched) for a hardened 10 CrMo 9 10 steel (ASTM P21) in boiling 35% NaOH — potentio-dynamically measured with +0.6 V h - - potential change after every 0.5 h At/ = +0.1 V x-x-x potential change after every 0.5 hAf/ = -0.1 V.

Embrittlement embrittlement and for improperly heat treated steel, both of which give intergranular cracks. (Intercrystalline penetration by molten metals is also considered SCC). Other steels in caustic nitrates and some chloride solutions. Brass in aqueous ammonia and sulfur dioxide. physical environments. bases of small corrosion pits, and cracks form with vicious circle of additional corrosion and further crack propagation until failure occurs. Stresses may be dynamic, static, or residual. stress relieve susceptible materials. Consider the new superaustenitic stainless steels. 

Some of the most obvious examples of problems with gas and materials are frequently found in refining or petrochemical applications. One is the presence of hydrogen sulfide. Austenitic stainless steel, normally a premium material, cannot be used if chlorides are present due to intergranular corrosion and subsequent cracking problems. The material choice is influenced by hardness limitations as well as operating stresses that may limit certain perfonnance parameters. 

Intergranular corrosion depends on the length of time the steel is exposed to the sensitizing temperature (500-750°C), even if made from low-carbon or titanium-or niobium-stabilized steel. 

Figure 4-428. Intergranular corrosion of sensitized (improperly annealed) stainless steel. (From Ref. [185].)...

Intergranular Corrosion of Austenitic Stainless Steels (Section 3.3)... 

Fig. 1.8(a) Intergranular precipitation of chromium carbide particles in a sensitised austenitic stainless steel and the consequent chromium-depleted zones adjacent to the grain boundaries, (b) variation of the chromium content across a grain boundary in a sensitised austenitic stainless steel (l8Cr) and (c) intergranular corrosion of a sensitised austenitic stainless steel... 

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

The use of very low carbon (usually < 0 03 %) austenitic stainless steels, in which intergranular carbide precipitation does not occur within practical time scales. 

A more detailed treatment of sensitisation of austenitic stainless steels, of intergranular corrosion of austenitic stainless steels without sensitisation, and of sensitisation and intergranular corrosion of ferritic stainless steels and high-nickel alloys, is given by Cowan and Tedmon . 

A somewhat similar phenomenon is knife-line attack which may be observed after welding titanium or niobium stabilised austenitic stainless steels. In this case there is a very narrow band of severe intergranular attack along the interface between the parent metal and the fusion zone. During welding, the parent metal immediately adjacent to the fusion zone is heated to just below the melting point and both chromium carbides and niobium or titanium carbides dissolve completely. On cooling rapidly, the conditions are such that when relatively thin sections are welded, neither chromium carbide nor niobium or titanium carbide have time to precipitate. If the weld is now... 

Levin, LA. and Maksimova, G. F., Effect of Cold Work on the Tendency Towards Intergranular Corrosion of Type 18-8 Stainless Steel, Khim Mashinosiroenie, 5, 35 (1%1) C.A., 56, 3219... 

Stickler, R. and Vinckier, A., Electron Microscope Investigation of the Intergranular Corrosion Fracture Surfaces in a Sensitised Austenitic Stainless Steel , Corros. Sci., 3, 1 (1963) von Schwenk, W. and Buhler, H.-E., Beoboshtungen an einem Kornzerfallsen falligen Austenitischen Cr-Ni-Stahl im Aktivzustand , Corros. Sci., 3, 145 (1963)... 

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