Inhibition of Stress Corrosion Cracking of AISI 4340 Steel in 10% Potassium Nitrate Solution at 100 C

A study of the action of chemical inhibitors of stress corrosion cracking (SCC) of AISI 4340 steel has shown that such cracking in boiling KNO₃ solution is more efficiently prevented by a suitable combination of complementary or synergistic inhibitors than by either inhibitor alone. The efficiency is estimated in terms of the total mole concentration required for full protection. Examples of the effect are inhibitor pairs consisting of NaNO₂ and a basic compound such as NaOH, Na₂CO₃, Na₃PO₄, or Na₂SiO₃. The synergism results because the members of a proper pair act against stress corrosion through different mechanisms, and may thus block the corrosion reactions at both electrodes of the microcorrosion cell. The effectiveness of a given inhibitor usually approaches some asymptotic limit with increasing concentration so that small increments of two complementary inhibitors have a greater effect on the corrosion rate than the same total number of moles of either alone. Sodium silicate appears to suppress (SCC) by two mechanisms: (1) it raises the pH of the system to a range where hydrogen evolution and the resulting hydrogen embrittlement do not occur, and (2) it deposits an insoluble silicate film which interferes with ion transport to the corrosion sites.