Stress Corrosion Cracking of ASTM⁽¹⁾ A516 Steel in Hot Caustic Sulfide Solutions—Potential and Weld Effects

A combination of slow strain rate (tensile) testing (SSRη and double cantilever beam (DCB) fracture mechanics testing techniques have been used to study the stress corrosion cracking (SCC) behavior of welded and unwelded pressure vessel quality steel plate (ASTM A516 Grade 70) in hot (92 C) caustic sulfide solutions (3.35 m NaOH + 0.42 m Na₂S) of general relevance to the pulp and paper industry. Tests were conducted over a range of potentials (E) and stress intensities (K_I). The most susceptible SCC situation occurred at potentials near the active-passive transition, where crack propagation rates were slightly lower in the fusion zone (FZ) of the weld than in the base material (BM). However, crack initiation on smooth specimens was far more frequent in the FZ. More noble potentials did not prevent SCC, but decreased the crack propagation rate and decreased crack initiation (i.e., crack density), particularly in the FZ. A fracture mechanics analysis of partial surface cracks and through thickness cracks was used to assess the cracking of kraft digesters in light of the results. It was concluded that anodic protection could play a beneficial role by decreasing the probability of crack coalescence and the resulting formation of a long critical (catastrophic) crack in the weld region. Instead, only small isolated cracks should form, which penetrate the vessel wall more slowly and lead to leakage before catastrophic failure.