Effect of microstructure on the behaviour of HSLA steels to hydrogen embrittlement under cathodic protection

High-strength low-alloy (HSLA) steels are widely used as components of buried and offshore natural gas, and oil pipelines. Cathodic protection (CP), in conjunction with protective coatings, is generally used to prevent the external corrosion of pipelines. This chapter focuses on hydrogen-embrittlement resistance of HSLA steels that are produced either by controlled rolling and accelerated cooling, or by quenching and tempering, and investigates the effects of strength, microstructure, and chemical composition on the hydrogen-embrittlement behavior of the steels under CP in 3.5% NaCI and NS4 solutions. SSRT tests are conducted with 3 mm diameter tensile specimens at different strain rates. Some tests are conducted in a NS4 solution simulating groundwater and synthetic seawater. The specimens are polarized using a potentiostat, a standard Hg/Hg2Cl2 reference electrode and a graphite counter electrode. Stress-strain curves obtained during slow strain rate tensile tests in aqueous solutions are identical to stress-strain curves obtained in air when applied stresses are less than ultimate tensile strengths of steels. At higher stresses, after necking began, stress-strain curves demonstrate the loss of ductility of steels exposed to the solutions especially at very negative applied potentials and low strain rates.