Mechanism

Due to the electrical potential difference that develops when two dissimilar metals or alloys are connected together in an aqueous solution the base metal will become anodic and the more noble metal will act as a cathode. The noble metal is in effect, cathodically protected by the more reactive metal which is corroded.

Background

Hydrogen can diffuse into metals and alloys from a number of sources during both processing and subsequent service. These sources include the dissociation of moisture during casting and welding, thermal decomposition of gases and pickling and plating operations. Hydrogen can also be generated from cathodic reactions during corrosion in service and from cathodic protection measures by sacrificial anodes and impressed current.
Ferritic and Martensitic Steels

The effects of hydrogen are well known in ferritic and martensitic steels, where it can diffuse to suitable sites in the microstructure and develop local internal pressure resulting in the characteristic form of hydrogen embrittlement.
Low Carbon Steels

In low carbon steels, which have inherent ductility, hydrogen may not give cracking but will cause blisters to develop at inclusions. This can lead to delamination in plate due to the directional nature of the inclusions.
Hydrogen Sulphide Environments

Steels for sour gas service, where the environment contains wet hydrogen sulphide, must have very low sulphur levels or have been treated with additions to control the shape of the inclusions during deoxidation to minimise the danger of hydrogen embrittlement and blistering.
Failure

Failure is time-dependent and occurs at low rates of strain as the load-bearing cross-section is reduced during slow crack growth in the embrittled region. Susceptibility for embrittlement is higher in alloys with higher yield strengths, i.e. those that are cold-worked, age-hardened or in the martensitic form. The sites at which hydrogen is trapped include the original austenite grain boundaries and the interfaces between the matrix and non-metallic inclusions, for example manganese sulphides. These then result in both intergranular cracking (with separation at the prior austenite boundaries) and transgranular cracking (flaking or quasi-cleavage) which is associated with the inclusions.
Other Effects

Hydrogen can assist in the propagation of corrosion fatigue cracks and can also cause sulphide stress corrosion cracking in ferritic and martensitic steels, including the stainless grades.