Film and pH Effects in the Stress Corrosion Cracking of Type 304 Stainless Steel

Rapid stress corrosion cracking of 304 stainless steel in MgCl2-FeCl3 solutions at 125 C has been shown to occur only when the pH of the corrodent liquid within the crack lay between 1.2 and 2.5. A film of more acidic corrodent solution is raised to pH = 1.2 by reaction with the metal within a few seconds after isolation in a pit, crack, or crevice. MgCl2 solutions of pH higher than 2 became more acidic when in contact with stainless steel as a result of corrosion processes. The pH of small amounts of such solution isolated in pits or crevices eventually fell to near 1.5, where stress corrosion cracking could occur. This pH range is considered to be critical for stress corrosion cracking of 304 stainless steel because it is the range in which a corrosion resistant protective film is formed in the presence of the corrodent solution. This film is essential to crack propagation. If there is added to a corrodent , solution in this pH range an organic complexing agent such as glycerine or glycol which prevents-formation of the protective oxide film, the general corrosion process continues unchecked but no stress corrosion cracking occurs. The data support a model in which stress corrosion cracking is driven by a highly localized galvanic cell within the crack. The cell operates in such a way that there is no large change in pH of the solution in the crack. These results emphasize the importance of the corrosion resistant film in the chemical aspect of the stress corrosion mechanism.