Sensitization times for intergranular corrosion in Alloy 800

Abstract

Transmission electron microscopy has been used to determine the T-T-T curve for precipitation of Cr-rich intergranular carbides during direct ageing in one batch of low-C, high- Ti/(C + N)-ratio Alloy 800 annealed at 1050°C. T-T-S data were also determined using test solutions which conformed to ASTM A 262-70 (B) and (E) for the 120 h ferric sulphate-sulphuric acid and 24 h copper-copper sulphate-sulphuric acid tests, respectively, with samples annealed at 1050 and 950°C and directly aged. The present work demonstrates that the alloy can show sensitization to inter granular corrosion after appropriate heat-treatment. A suppressed sensitization response was observed in the alloy after annealing at 950°C with respect to material annealed at 1050°C. Metallography showed that this suppressed sensitization response corresponded with (i) an increased spatial density of TiC precipitates, and (ii) a decrease in grain size. This correspondence is readily explained in terms of the Cr-depleted zone theory of intergranular corrosion coupled with a self-healing model for the alloy. Transmission electron microscopy revealed the presence of M₂₃C₆ intergranular precipitates throughout the sensitization/self-healing régime.