Rust Nucleation Sites in 18Cr-Ti Stainless Steel

Abstract

The aim of the present work is to study the sites of rust formation and mechanism of rusting on 18 Cr-Ti steels under the salt spray test. Rusts are preferentially initiated at cubic inclusions or precipitates with titanium (esp. TiN-TiC-Ti₄C₂S₂ system, TiO₂ (or Al₂O₃) -TiN-TiC-Ti₄C₂S₂ system, TiN-TiC system). But no rusting occurs either at separate particles of titanium carbosulfide or at titanium cabonitride surrounded by a rim of carbosulfide. There exist primary microscopical crevices between the cubic inclusions and the matrix. At these sites crevice corrosion is able to initiate and causes the annular rusting around the cubic inclusions. Additions of titanium (about 0.2% or more in free Ti) in extra low carbon and nitrogen 18 Cr steels show a very pronounced effect on the resistance to rusting. The role of titanium in rust resistance is (1) to prevent the formation of manganese sulfide related to rusting and to lead the formation of carbosulfide resistant to the dissolution and (2) to shift the pitting potential to the noble direction.