Abstract
Hydrogen stress cracking (HSC) of buried pipelines under cathodic protection is one of the major concerns in the maintenance of gas transmission pipelines. It has been acknowledged that slow strain rate technique (SSRT) is a rather qualitative method for evaluation of HSC susceptibility, while it is necessary to evaluate fracture toughness of pipelines with defect under cathodic protection. A series of crack tip opening displacement (CTOD) testing has been employed to evaluate critical CTOD, δC, of four API X65 linepipe materials which gives maximum tolerable defect size for prevention of HSC of pipelines under cathodic protection. The study has clarified the followings: 1) Under cathodic over-protection in buffer solution, δC depends on control modes and also on testing rates; constant displacement rate mode gives smaller δC than constant loading rate mode. δC decreases with decreasing displacement rate, d, and shows a minimum when d≦0.01 mm/min. 2) δC steeply decreases when cathodic current density is larger than 1 mA/cm2 δC decreases with decreasing cathodic potential, e, when e ≦ -1.2 V vs, Cu/CuSO4. 3) Fracture morphology has a good correlation with δC. 4) The CTOD test results give maximum tolerable defect size for prevention of HSC of pipelines. It is concluded that CTOD testing with a slow testing rate is highly effective in evaluation of HSC of pipelines.
