Abstract
Carbon steels are widely used in the oil and gas exploration and production service. However, these materials are prone to corrosion and cracking in CO2/H2S. The propensity to cracking increases when higher strength grades are employed. The use of corrosion resistant alloys incurs costs; however, these costs are justified in environments where severe corrosion or cracking of cheaper carbon steel is anticipated. Cost reduction can be attained if these materials are coated by an industrial method, such as weld cladding, onto carbon steel. To understand the behavior of UNS N06625 weld clad on carbon steel, several weld clad specimens were prepared and tested in de-aerated 3.5 wt.% NaCl solution for 30 days in 9.5 MPa CO2 containing 0.5 MPa H2S at 120 °C. Specimens were also tested in stressed conditions. Microstructural characterization revealed that the ‘swirls’ of carbon steel in the clad specimens were susceptible to sulphide scale formation in the test environment. These swirls can extend up a few hundred micrometres from the fusion line. The stressed specimens showed no visible cracks at 10X magnification. Thus, when selecting the thickness of weld cladding on carbon steel one should choose the thickness to avoid swirls reaching the surface of the cladding.
