In subsea oil and gas systems, steel linepipes (typically API X65) have been welded to 8630M low alloy steel hubs, using a buttering technique and nickel alloy filler materials (e.g. Alloy 625). After buttering, postweld heat treatment (PWHT) is carried out to temper the heat-affected zone of the low alloy steel. However, while the vast majority of subsea joints provide successful service, a small number of in-service failures has had significant ramifications. During PWHT of the buttered joints, carbon migrates from the ferritic steel to the high alloy ‘austenitic’ weld metal. The resulting microstructures and consequent localised hardening cause sensitivity to hydrogen induced cracking.
Subsea systems employ cathodic protection (CP) using aluminium based anodes in order to prevent corrosion. However, while cathodic protection has proved successful as a means of preventing corrosion of the steel components, complications can arise due to hydrogen charging.
This paper describes the detailed characterisation of such dissimilar interfaces from several joint types, including examples recovered from the seabed after exposure under CP and newly fabricated joints, using electron microscopy, microanalysis and nanohardness testing.