Electrochemical factor and mechanical factor take an important role in the process of TGSCC initiation and early stage propagation. Intermittent loading facilitates crack propagation. And crack propagation rate of I/II mixed mode SCC crack is faster than that of mode I crack in near-neutral pH solution. The disbondment of the coating reduces the cathodic protection effect and the cathodic protection potential should be suitably shifted to negative in order to obtain the same cathodic protection effect as that of the intact coating.
Based on the above results from the laboratory test, suggestions for the practical pipeline operation are as followed:
The magnitude of the inner pressure fluctuation should be reduced as much as possible and the same to the additional load during the installation and service of the actual pipeline. The role of mixed mode load in the process of crack propagation should be considered when the lifetime of the buried pipeline is predicted. The effect of mixed mode load on crack propagation should be considered further especially for the region where earthquake often occurs and the slope where the additional bending stress exists. The accelerating action of the intermittent loading on crack propagation should also be considered. The cathodic protection potential should be shifted suitably to negative, like about 150mVSCE, after the coating disbonded according to the results obtained in the present study.
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