Intergranular ferrite

It has been reported that fine oxide, sulfide, oxysulfide, boride and nitride particles uniformly distributed in the steels act as effective nuclei for intergranular ferrite (Funakoshi et al. 1977, Nakanishi et al. 1983, Ito and Nakanishi 1975, Mori et al. 1981, Honma et al. 1986, Ohno et al. 1987, Ricks et al. 1982). Although the mechanism by which the more effective nucleants facilitate the nucleation of intergranular ferrite is not certain, Abson (1989) noted that in order to obtain fine ferrite grains, the crystallographic matching (disregistry), wettability between nucleants and ferrite phase, and size of inclusions must be important. 

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 . 

Bond, A. P. and Lizlovs, E. A., Intergranular Corrosion of Ferritic Stainless Steel , J. Elec-irochem. Soc., 115, 233C (1968)... 

Hodges, R. J., Intergranular Corrosion in High Purity Ferritic Stainless Steel. Isothermal Time-Temp. Sensitisation Measurements , Corrosion, 27, 164 (1971)... 

Practice for detecting susceptibility to intergranular attack in ferritic stainless steels Method of salt spray (fog) testing Method for mercurous nitrate test for copper and copper alloys... 

Austenitic stainless steels are the most significant class of corrosion-resistant alloys for which intergranular corrosion can be a major problem in their satisfactory use. The problem is most often encountered as a result of welding but also may result from stress-relief annealing or incorrect heat treatments. Intergranular corrosion also can occur in ferritic stainless steels and in nickel- and aluminum-base alloys. 

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... 

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... 

---