Hydrogen embrittlement of a bimaterial

• Measurements of failure of bimaterials in the presence of hydrogen are presented.
• A failure criterion is deduced from tests on notched and un-notched specimens.
• Failure is shown to depend mainly on the concentration of lattice hydrogen.
• The size effect in the measurements is rationalised using Weibull statistics.

Commonly, within the energy industry, the corrosion resistance of pressure vessel steels is increased by the addition of an overlay coating comprising a nickel-based alloy or a stainless steel. However, the interface between the two alloys is prone to hydrogen-assisted cracking, due to for example carbide precipitation near the interface. In the present study, the sensitivity of the tensile strength of the interface to hydrogen concentration is measured for both notched and un-notched specimens made from the overlay welding of 690 nickel alloy on a low alloy steel A533B. An elastic–plastic finite element analysis is used to determine the stress and strain state near the notch root, and thereby to calculate the local distribution of hydrogen within the lattice and at traps. The observed strength of the notched specimens is best rationalised by assuming that the local cohesive strength of the interface is a function of the lattice hydrogen concentration, with a negligible influence of the trapped hydrogen. The scatter in specimen strength, and the relative strength of the notched and un-notched specimens, are adequately described by Weibull statistics, with a low value of Weibull modulus equal to 3.4.