Intergranular attack in ferritic stainless

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... 

A 763 Practices for Detecting Susceptibility to Intergranular Attack in Ferritic Stainless Steels... 

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... 

Table 8 provides a summary of the redox potentials of various tests for austenitic and ferritic stainless steels. Because both carbides and molybdenum-rich sigma-phase may result in intergranular attack in highly oxidizing solutions, the nitric acid test should be specified for materials to be used in nitric acid or other highly oxidizing environments. 

FIG. 21—Intergranular attack on weldmant In ferritic stainless steel caused by high nitrogen content (5x). Autogenous weld In 28.5 %Cr<4.2 %Mo alloy with 22 ppm carbon and 388 ppm nitrogen after exposure In ferric sulfate-50 % sulfuric acid test. 

Sensitization in ferritic stainless steels is introduced by high temperature heat treatment (above 925°C) and relieved by heating for a short time between 650 and 815°C, which is opposite to the observations on austenitic steels. Annealing sensitized ferritic steel at 788°C for several minutes will eliminate intergranular attack. The susceptibility is reduced by the addition of titanium or niobium. The presence of carbon or nitrogen is necessary to cause sensitization. 

Why intergranular attack is not prevented in ferritic stainless steel even if carbon is reduced to 0.3% ... 

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... 

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