STUDY OF THE SUSCEPTIBILITY OF API 5L X80 GIRTH WELDS TO SULFIDE STRESS CORROSION CRACKING AND HYDROGEN EMBRITTLEMENT

The growing demand for natural gas and oil, as energy sources, has driven industry's need for ever-increasing strength levels in oil and gas transmission pipeline materials in order to achieve safe and economic transportation. The current world trend points to the use of pipes with larger diameters and thinner wall thicknesses, operating under high pressure. In addition, pipeline steels for sour service must exhibit good Hydrogen Induced Cracking (HIC) and Sulphide Stress Corrosion Cracking (SSCC) resistance. This study evaluates the susceptibility of API 5L-X80 girth welds to SSCC and Hydrogen Embrittlement (HE). Slow strain rate tensile (SSRT) tests and Hydrogen Permeation tests were performed at room temperature, in different acidic environment containing sodium thiosulfate solutions. Most of the SSRT tests undertaken in solution, showed a loss of ductility and a decrease in the reduction of area, as compared with tests conducted in air. The susceptibility to HE and potentially SSCC was evidenced by a reduction in ductility in the SSRT tests and an increase in the hydrogen permeation current values, for almost all welded joints. This was observed with greater intensity for the more acidic test solutions (pH= 3.4), while for the less acidic test solutions (pH= 4.4) little loss of ductility was observed and the hydrogen permeation current remained at values close to zero, indicating little or no permeation of hydrogen through the metal for the testing times applied. The behaviour exhibited by the samples tested in the more acidic solutions was attributed to the dissolution of material from the sample together with hydrogen embrittlement. These results confirmed that the use of sodium thiosulfate solutions to generate H2S, permits the study of phenomena related to SCC in environments containing sulphides.