Commercial titanium alloys are subject to stress corrosion cracking in which environments?

• A. Hot salt above 275 C
• B. 10% Hydrochloric acid near room temperature
• C. Nitrogen tetroxide or red fuming nitric acid
• D. All of the above

Answer: (D)

Stress corrosion cracking happens when a tensile stress is present in a material and the environment can break down the protective oxide film on the metal, allowing crack growth to continue under load. Commercial titanium alloys rely on a passivating oxide layer for general corrosion resistance, but certain environments can destabilize that film and promote cracking along the stressed areas.

Hot salt containing chlorides at elevated temperature (above about 275 C) is particularly aggressive because chloride ions attack the oxide film and accelerate localized dissolution at crack tips, enabling SCC to propagate under stress. Even hydrochloric acid at room temperature can compromise passivity in the presence of tensile stresses, especially where there are crevices or impurities that concentrate the solution. Extremely oxidizing media, such as nitrogen tetroxide or red fuming nitric acid, also disrupt the oxide layer and promote anodic attack under tensile loading, leading to SCC.

Since each of these environments can erode or rupture the protective film and allow cracks to grow under stress, titanium alloys are susceptible to stress corrosion cracking in all of the above conditions when the right combination of stress, exposure, and temperature occurs.