How does the presence of hydrogen sulfide (H2S) affect the material selection for pipelines?

The presence of hydrogen sulfide (H2S) significantly influences material selection for pipelines because H2S is a highly corrosive gas that can cause several forms of corrosion in pipeline steels, particularly sulfide stress cracking (SSC) and hydrogen induced cracking (HIC). SSC is a form of brittle fracture that occurs in high-strength steels exposed to H2S and tensile stress. HIC is a stepwise cracking mechanism caused by the diffusion of atomic hydrogen into the steel, which then combines to form molecular hydrogen at internal voids, creating pressure that leads to cracking. To mitigate these risks, several material selection strategies are employed. Lower-strength steels are generally preferred over high-strength steels, as they are less susceptible to SSC. Steels with a yield strength below 65 ksi (448 MPa) are often specified for H2S service. Specific steel alloys with enhanced resistance to H2S corrosion are used. These alloys typically contain additions of chromium, nickel, and molybdenum, which improve their corrosion resistance. Examples include duplex stainless steels and high-alloy steels. Internal coatings are used to create a barrier between the steel and the H2S-containing fluid. Coatings such as epoxy, polyurethane, and fluoropolymers are commonly used. The coating must be chemically resistant to H2S and impermeable to water. Cathodic protection (CP) is used to reduce the corrosion rate of the steel. CP provides electrons to the steel surface, making it cathodic and preventing the oxidation reaction that leads to corrosion. Control of the pH of the transported fluid is implemented. Maintaining a slightly alkaline pH can reduce the corrosivity of H2S. Regular monitoring and inspection are performed to detect any signs of corrosion or cracking. This includes visual inspection, non-destructive testing (NDT), and electrochemical monitoring. Material selection is guided by industry standards such as NACE MR0175/ISO 15156, which specifies requirements for materials used in H2S-containing environments.