Potentiostatically controlled slow-strain-rate tension tests (∼ 10-6s-1) were conducted on the parent steel, simulated heat-affected zone, and weld metal specimens obtained from a sample of Canadian grades 483 and 414 high-strength low-alloy (HSLA) line pipe. All stress corrosion cracking (SCC) tests were conducted in 0.5 mmol/cm3 Na2CO3 + 1.0 mmol/cm3 NaHCO3 solutions at 75°C in a stressed-electrode cell. Percentage reduction in area and apparent secondary cracking velocity were used as measures of SCC susceptibility. The potentials at which cracking occurred were correlated with fast and slow scan anodic potentiodynamic polarization curves in the same environment.
Secondary cracks occurred along the gage length normal to the stress in a narrow potential range with crack growth rate reaching a maximum in the range from — 700 mV to — 650 mV versus saturated-calomel electrode. This maximum in the crack depth versus potential curves occurred near the primary passive potential. Scanning electron microscopy provided unambiguous evidence for the presence or absence of SCC in the parent steel, HAZ and weld metal samples in a CO3-2/HCO3-1 environment.
The results support a mixed dissolution-hydrogen embrittlement model for SCC of HSLA line-pipe steel.