Determination of cathodic protection potential criteria for thermally insulated pipeline in synthetic groundwater

Although the application of cathodic protection (CP) on thermally insulated pipeline is ineffective, a CP system could be a valuable method for preventing corrosion at areas that are difficult to access, where the steel surface would be exposed to severe corrosive environments due to the excess of water ingress and/or damage of the insulation materials by hydrolysis reaction. Thus, as an initial step of the design of a CP system for thermally insulated pipeline, the protection potential range was established by electrochemical tests and slow strain rate tests (SSRT). The potential range was set on the basis of our previous result (-1.35 V vs. saturated calomel electrode [SCE]) considering insufficient protection and over-protection. Since the corrosion rates were reduced to the negligible value below the protection potential of -1.15 V_SCE, a potential of -1.15 V_SCE could be applicable to the upper (positive) limit of cathodic protection criterion for thermally insulated pipelines. The lower (negative) limit of protection potential was determined by considering the ductility of base metal obtained from SSRT because the ductility of weldment continuously decreased with increasing applied potential so that it is hard to set the potential limit. The ductilities for the base metal showed that a significant decrease in ductility occurred when the tests were made under a cathodic potential of -1.65 V_SCE. Thus, the lower limit of protection potential was determined as -1.55 V_SCE. From these results, it can be suggested that the high-temperature CP potential range for thermally insulated pipelines when considering the insufficient protection and overprotection was -1.15 V_SCE - 1.55 V_SCE.