Investigation by fractography of the high-temperature oxidation of the 20%Cr-25%Ni-Nb stainless steel

Abstract

Simple procedures have been developed for the preparation, by fracture, of transverse corroded metal sections for subsequent examination of the microstructure and microchemistry of the corrosion attack by scanning electron microscopy. Examination of 20%Cr-25%Ni-Nb (20/25/Nb) stainless steel following 120-hr oxidation in carbon dioxide at 1000°C confirmed that the essentially three-dimensional picture revealed by fracture provided additional microstructural information not apparent from optical microscopy of standard metallographically mounted transverse sections. The size and morphology of the oxide grains constituting the various layers of the uniform and the pitting types of external scale were established. Nearest to the underlying steel, a band of the inner Cr₂O₃ layer contained a high but variable silicon concentration as well as substantial voidage. Beneath the scale, silica intrusions dramatically influenced the mechanical properties of the 20/25/Nb stainless steel to the depth of this intergranular attack, as fracture at 78 K occurred by a brittle mode. Steel unaffected by oxidation, however, continued to remain ductile.