Fondmental Research Of SCC Behaviour And Detection Technology Of Pipeline Steel

Stress corrosion cracking (SCC) of X70 pipeline steel in Na2CO3/NaHCO3 solution and in simulating ground water (NS4 and NOVA) has been investigated in this paper. The possibility of applying the Metal Magnetic Test (MMT) in detection of SCC in buried pipeline was also discussed. Firstly, the possibility and the sensitive potential region of SCC in X70 steel were studied with polarization curve. Secondly, the component, structure and characteristic of film on the surface of X70 steel were observed by electrochemical method and surface analysis. Thirdly, SCC of X70 steel was studied by SSRT. Mechanism of SCC was also identified by fracture morphology of steel. Forth, the regions of stress concentration were obtained by MMT, which provide the feasibility of using MMT on detecting SCC of buried pipeline.The electrochemical behavior of X70 steel was related to SCC. The results showed that the passive film was generated on the surface of X70 steel in Na2CO3/NaHCO3 solutions. Sensitive potential region was -640mV～-570mV, which predicted by boundary condition of Parkins and SCC parameter Pi. There was no passive film on the surface of X70 steel in simulating ground water (NS4 and NOVA). NS4 solution was more sensitive than NOVA solution. The solution with CO2 is more sensitive, too. The characteristic of passive film of X70 steel was related to SCC. The results showed that the concentration of HCO3- was related to anodic polarization behavior of X70 steel in Na2CO3/NaHCO3 solutions. When HCO3- is lower than 0.5mol/L, there were prepassive film and passive film on the surface. The prepassive film has double-layer structure, where Fe(OH)2 was main composition of inner layer while FeCO3 covered the outer layer of the prepassive film. There were many hollows and micro-cracks existed on the prepassive film, which could result in SCC. Passive film was mainly consisted of Fe2O3 and Fe3O4, the capability of protection was enhancing with the increasing of HCO3-.The study of sensitivity to SCC of X70 steel showed that the characteristic of IGSCC was showed by fracture morphology of X70 steel in Na2CO3/NaHCO3 solutions. There is nothing with the concentration of HCO3-. The characteristic of TGSCC was showed in simulating ground water (NS4 and NOVA). Stress played the important role in generating and developing of SCC. The results of soaking experiment and using MMT to detect the region of stress concentration showed that the potentials of samples of U forms were lower than the un-bend samples'. The pitting corrosion generally generated on the position of bending place. The evaluate parameter was used to determine the regions of stress concentration. The regions were sensitive to SCC, which were proved by soaking experiment. There was the feasibility of using MMT on detecting SCC of buried pipeline.