Ferritic stainless steels intergranular corrosion

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)... 

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... 

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... 

Dundas HJ, Bond AP. Niobium and titanium requirements for stabilization of ferritic stainless steels. In Steigerwald RF, editor. Intergranular Corrosion of Stainless Steel. STP 656. Philadephia ASTM, 1978. 

Ferritic stainless steel has the reputation of being less sensitive to intergranular corrosion than austenitic stainless steel. This type of corrosion can nevertheless take place under certain conditions of thermal treatment [20]. The diffusion coefficients of both carbon and chromium in ferrite are larger than in austenite. Grain boundary precipitation of carbides and nitrides of chromium can therefore occur at temperatures of 540-600 °C already. The behavior differs from that of austenitic stainless steel, which becomes sensitized at higher temperatures only. Because of the larger diffusion... 

It was soon found that among the variables affecting corrosion rates were certain heat treatments that made not only the ferritic stainless steels subject to intergranular attack, but also the austenitic, 18Cr-8Ni alloys. From this simulated service test in boiling 65 % nitric acid, there evolved Practice C of ASTM A 262. Its large-scale use by one... 

Conventional 400 series ferritic stainless steels such as AlSl Types 430,434, and 446 are susceptible to intergranular corrosion and to embrittlement in the as-welded condition. Corrosion in the weld area generally encompasses both the weld metal and weld HAZ. Early attempts to avoid some of these... 

---