EFFECT OF APPLIED POTENTIAL ON SCC OF X80 PIPELINE STEEL AND ITS WELD JOINT IN KU'ERLE SOIL SIMULATED SOLUTION

Potentiodynamic polarization technique and slow strain rate testing(SSRT) were employed to study the stress corrosion cracking(SCC) behavior of a welded X80 pipeline steel in Ku'erle soil simulated solution.Fracture surfaces were observed by SEM under different applied potential conditions.The results show that the polarization curves of the base metal and weld joint represent the typical characteristics of active dissolution.It is found that cracks are generally initiated at corrosion pits and inclusions under anode polarization and open circuit potential.The crack generation mechanism of X80 pipeline base steel and weld metal are attributed to the dissolution at anode.When the applied potential is -900 mV(vs SCE),the base metal exhibites lower SCC sensitivity due to cathodic protection while under the same condition welded joins higher SCC sensitivity.Both the base metal and weld joins exhibite higher SCC sensitivity under -1200 mV(vs SCE) polarization potential and their cracking generation mechanism is hydrogen induced cracking(HIC) due to the synergistic action of stress and hydrogen.Commonly the weld joint is more sensitive to SCC than the base metal under the same applied potential,and fractures are usually presented in heat affected zone(HAZ) and this is attributed to metallurgical phase transformation and residual stress generated during welding process.