Hydrogen Embrittlement

Hydrogen Embrittlement.

Hydrogen can enter most metals. Due to the small size of the hydrogen atom, it can migrate through the metal structure and cause a loss of ductility similar to that experienced in stress corrosion cracking.

Definition.

Hydrogen embrittlement is the severe loss of ductility of a metal when hydrogen has been introduced into the metal structure.

Mechanism.

Hydrogen atoms can enter a metal either from hydrogen gas, usually at elevated temperatures, or from atomic hydrogen that is electrolytically formed on its surface.

This hydrogen can either reduce the energy required for forming cracks under stress or can accumulate at areas of high stress, such as crack tips, and cause pressure, which directly assists crack propagation.

High strength materials in general are the most susceptible to hydrogen embrittlement.

Hydrogen can be formed electrolytically during electroplating, during welding when hydrogen is present in the electrode material, in the electrode coating, in the shielding gas, or simply as moisture on the metal surface, or when excessive cathodic protection is applied (potentials more negative than minus 1.2 volts are normally required for significant hydrogen formation by cathodic protection.)

Examples.

Ferritic and martensitic (magnetic) steels, particularly those with a yield strength in excess of 130 ksi, are particularly prone to hydrogen embrittlement. Austenitic (non-magnetic) stainless steels are less susceptible. When hydrogen pickup is suspected, such as in electroplating or welding, the hydrogen can be removed by baking at 200 to 300°F.

Hydrogen pickup during welding is normally prevented by using low hydrogen electrodes and mild preheating to remove water from the surfaces being welded.

Appearance.

Other than catastrophic failure by cracking, there is often no visible evidence of hydrogen embrittlement. In extreme cases, where hydrogen gas bubbles are formed inside the metal, shinny internal blisters are visible at the fracture surface.

Significant Measurements.

Analysis of the metal for untrained hydrogen can be used to verify hydrogen embrittlement if heating subsequent to failure has not driven off the hydrogen gas.

Due to the difficulty in verifying this form of attack, it is often blamed for other forms of cracking failure, often when simple overload is the actual cause of failure.

Source : "Corrosion Control" NAVFAC MO-307 September 1992