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Cracking of mild steel in NaOH and caustic aluminate solutions Sriram, Rajagopal


Stress corrosion cracking (SCC) of ASME SA 516 Grade 70 steel was studied in hot (92°C) NaOH solutions of different caustic concentrations, caustic aluminate solutions and an Industrial Bayer solution (IBS). The potential regimes of stress corrosion cracking susceptibility were assessed using the slow strain rate technique (SSRT). At anodic potentials, SCC tendency was most pronounced in the active-passive transion region. Under these conditions, the largest number of secondary cracks and highest apparent crack velocities were observed in the IBS. The inhibition of corrosion of steel in caustic solutions by the addition of alumina trihydrate was studied by using anodic polarisation tests, cyclic voltammetry, electron diffraction and energy dispersive X-ray spectroscopy (EDS) techniques. The results were consistent with the formation of an amorphous iron-aluminate film on the electrode surface. Fracture mechanics techniques were used to study the kinetics of stress corrosion crack propagation in caustic solutions of different concentrations ( 2, 4 and 8 mols/Kg NaOH), 4 mols/Kg NaOH + 1 mol/KG of alumina and IBS. Both stress intensity dependent (region I) and stress intensity independent (region II) behaviour were observed. Crack fractography was studied by scanning electron microscopy. The results suggested that cracking at anodic potentials appears to be consistent with the general principles of the film-rupture dissolution model and that the dissolution processes in the crack-tip region were under mixed activation-diffusion control.

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