The localized corrosion resistance of three ferritic stainless steels containing 21% Cr and different amounts of Ca was determined and comparison was made with SS304 as a benchmark. The Ca content had insignificant effect on the resistance to localized corrosion, which was found to be similar to that of SS304. No MnS inclusions were observed in the ferritic steels, and pits were found to initiate in the matrix next to duplex inclusions that exhibited a higher Volta potential than the matrix.
Ferritic stainless steels typically contain 10-18%Cr as well as small amounts of other alloying elements such as Mn, Si and P.1 Production cost is lower than that of austenitic stainless steels owing to the absence of Ni, and they exhibit excellent resistance to stress corrosion cracking and high temperature oxidation. Ferritic stainless steels are widely used in automotive exhaust systems.2 Non-metallic inclusions such as MnS in stainless steels are almost always associated with pitting corrosion and efforts to improve the corrosion resistance of these steels often focus on altering the shape, distribution or size of the MnS inclusions.1, 3-7 Ke and Alkire found that pits did not initiate in SS304 at oxide inclusions or at MnS and mixed MnS/oxide inclusions if they were smaller than 0.7 m.8 Smialowski et al. also found that pits in SS304 did not form at oxide inclusions.9 However, others have reported that pits can nucleate at simple oxide inclusions.5 This paper describes a study on the pitting of a clean ferritic stainless steel, i.e. one that contains virtually no sulfide inclusions and few inclusions in total. Ha et al, described the pitting corrosion of two versions of SS409L, 11% ferritic stainless steel, with very low S and thus almost no MnS particles.10
