THE ELECTROCHEMICAL-BEHAVIOR OF PRESSURE-VESSEL STEEL IN HOT BAYER SOLUTIONS AS RELATED TO THE SCC PHENOMENON

Slow strain rate studies indicate a greater susceptibility of A516 steel to stress corrosion cracking (SCC) in caustic aluminate and Bayer solutions than in 2.25 M NaOH solution. Fracture is generally intergranular. A correlation between caustic cracking susceptibility and the electrochemical behaviour of A516 steel is demonstrated. XPS analysis of the oxide film at peak II reveals the probable formation of Fe3O4. When aluminate ions are present in the electrolyte. Al is incorporated into the oxide film; the formation of an amorphous film of Fe3-xAlxO4 (× ≤ 2) increases the susceptibility of A516 steel to caustic embrittlement. The linear dependence of the cyclovoltammetric peaks on the square root of the scan rate suggests that the pore resistance model and/or the low field-ion migration mechanism for the oxide film growth are in effect. In particular, the resistive mechanism adequately describes the electrochemical behaviour of A516 steel in relation to the SCC phenomenon. The pore resistance model is also in good agreement with the assumption of the formation of an amorphous iron-aluminum oxide film. © 1990.