Effect of TiO2 addition on crack propagation of alloy 600 and its penetration into crevices in caustic solution

A number of tubes of nuclear steam generators have experienced stress corrosion cracking in environments thought to be caustic. Wedged CT specimens of Alloy 600 were used to measure crack propagation rates with and without TiO2 in a 10% NaOH solution at 315 °C. The crack propagation rate was reduced by 1/2 with 2 g/l TiO2 addition. Polarization curves showed an extensive decrease of the anodic peak current and passive current with TiO2 addition in a 10% NaOH solution at 315 °C. The TiO2 penetration into crevices was investigated using Alloy 600 specimens having different gap sizes in a 1% NaOH solution at 315 °C with TiO2 addition. The Ti concentration of the specimens was analyzed in crevices by Auger electron spectroscopy after a 3-day exposure. The Ti concentration was 42% or more in crevices wider than 0.2 mm and 20% in the narrowest crevice with 0.05 mm.