Interplay between surface characteristics, initial oxidation behavior, and corrosion performance of ferritic stainless steels under simulated automotive exhaust gas condensate conditions

This paper reports on the interplay between surface characteristics, initial oxidation behavior, and corrosion performance of three ferritic stainless steels, grades EN 1.4512, EN 1.4510, and EN 1.4509, all with three different rough cold-rolled surface finishes, under simulated exhaust gas condensate conditions corresponding to those at the cold end of automotive exhaust system. As-received specimens are characterized with respect to surface roughness and grain size. Initial oxidation behavior of specimens, corresponding to the thermal aging step in the cyclic exhaust gas condensate corrosion test, is examined gravimetrically and regarding morphology and key alloying element profiles. These characteristics are correlated with the specimens' performance in the corrosion tests. Initial oxidation at 300°C introduced slightly greater weight gains and thicker oxide films in 12 wt% Cr grade EN 1.4512 than in higher-Cr grades EN 1.4510 and EN 1.4509, although at 600°C the differences were also evident between the higher-Cr grades and were dependent on the surface characteristics. Indeed, morphology and alloying element profiles were influenced by the surface finish, with the most uniform oxide film morphology and Cr, and stabilizing alloying element profiles were obtained for grade EN 1.4510 with surface finish S2 that featured the highest roughness in this alloy. This was also the alloy-surface finish combination that exhibited the best corrosion performance under simulated exhaust gas condensate corrosion test, with almost all other specimens experiencing intergranular corrosion in such areas of the specimens that were intermittently subjected to simulated exhaust gas condensate solution. The results presented in this paper demonstrate the importance of selection of a proper combination of surface characteristics, e.g., roughness, and alloy composition when pursuing sustainability by the means of materials selection.