Role of caustic concentration and electrochemical potentials in caustic cracking of steels

Increasing concerns of caustic embrittlement as a result of alumina-processing-industry-wide thrust for improving efficiency of Bayer processes by increasing processing temperature and caustic concentration form the basis for this paper. The paper presents results suggesting influences of caustic concentration and imposed electrochemical potential on caustic cracking of mild steel. Slow strain rate tests (SSRT) were conducted using caustic solutions of 300, 450 and 600 grams per litre (g L−1) sodium hydroxide solutions at 100 °C. SSRTs at a strain rate of 2.5 × 10−7 s−1 using 600 and 450 g L−1 solutions produced intergranular caustic cracking whereas those using 300 g L−1 solution resulted exclusively in mechanical failure. However, an imposed potential in the active–passive transition region produced caustic cracking also in 300 g L−1 caustic solution. The paper discusses the mechanistic aspects of caustic cracking and presents evidences in support of dissolution–repassivation mechanism of caustic crack propagation.