STRESS-CORROSION CRACKING OF CARBON-STEEL IN CAUSTIC ALUMINATE SOLUTIONS OF THE BAYER PROCESS

The influence of impurities (SO32-, SO42-, AlO2-, organic species) of Bayer liquor on stress corrosion cracking (SCC) of ASTM A516 steel is evaluated by the slow strain rate tests. For this purpose, the imposed critical potential in each case is determined precisely by the transients. Results show that among all these impurities, aluminate ions have the most important effect on the susceptibility of A516 steel and embrittlement is increased more than 25% of that obtained in plain NaOH solution. The crack propagation rates, as evaluated from the transients, in the active, active-passive and passive states are close to that obtained by the slow strain rate tests. The anodic dissolution controlled model describes well the caustic embrittlement of A516 steel. The obtained SCC in the passive region demonstrates that anodic protection, used for preventing SCC, does not completely eliminate the danger of cracking in Bayer solutions.