Natural Conditions for Caustic Cracking of a Mild Steel

Stressed and unstressed specimens of a mild steel (0.25% C, 1.0% Mn) were studied in aqueous caustic solutions ranging in concentration from 0.5 to 60% NaOH. Natural corrosion potentials were measured while (1) the caustic concentration was varied, (2) oxidizing and reducing agents were added, (3) the solution was aerated and deaerated, and (4) the electrode was polarized by impressed cathodic currents. Potentiostatic anodic polarization curves were determined for several caustic concentrations and temperatures. Stress corrosion cracking (SCO behavior was examined at both natural and imposed potentials. U-bend specimens were exposed to environments which induce SCC without potentiostatic stimulation, and small cylindrical tensile specimens were pulled at slow strain rates under potentiostatic potential control. The data support the concept that caustic cracking occurs in solutions in which mixed active and passive control of corrosion is operative. Caustic cracking is inhibited under conditions favoring formation and rapid repair of a protective passive film or under conditions favoring active dissolution of the steel.