Studies on the Susceptibility of Modified 9Cr-1Mo Steel to Stress Corrosion Cracking in Sodium Hydroxide Using Slow Strain Rate Testing Technique

Round tensile specimens of modified 9Cr-1Mo steel were subjected to slow strain rate tests in 1-4 M sodium hydroxide (NaOH) solutions at 473 K to evaluate their susceptibility to caustic stress corrosion cracking (CSCC). The results obtained in NaOH solutions were compared with specimen tested in de-mineralized (DM) water. Secondary stress corrosion cracks were observed in all specimens tested in 1-4 M NaOH, the number of secondary cracks increased with increase in concentration up to 3 M. Tensile test data showed that ductility (%TE) decreased with increasing concentration of NaOH up to 3 M and nearly remained the same for 4 M NaOH. Time to failure was highest for specimen exposed in DM water and decreased with increasing concentration up to 3 M and remained constant thereafter. Crack velocity showed a trend of increasing velocity with increasing concentration of caustic media up to 3 M and remained same for 4 M NaOH. Laser Raman spectroscopic (LRS) analysis confirmed dissolution of protective magnetite and accelerated corrosion on the specimens exposed to 1-4 M NaOH solutions, leading to the formation of a number of oxides and oxyhydroxides. Fractographic studies showed typical surface oxide cracking with decohesions in all specimens. The studies showed that P91 steel is susceptible to CSCC in the concentrations from 1 to 4 M NaOH solutions at 473 K at a strain rate of 10−6 s−1. The evidence for dissolution of magnetite and the presence of decohesions indicated an important role for hydrogen in caustic cracking of P91 steel.