Abstract
The structure to electrolyte potential survey is a useful and important test to evaluate cathodic protection systems. The structure to electrolyte potential values on carbon steel pipelines should be negative when the voltmeter positive terminal is connected to the structure and the negative terminal is connected to the copper sulfate reference electrode, however in this case study many of those values were electropositive.
No operational mistakes in the measurements were found. Causes such as reversed rectifier connections, interference from other systems were not found. At locations with positive potentials at coupon test stations indicated severe corrosion with coupons connected to the pipeline, and showed moderate corrosion on coupons expose to soil with no connection to the pipeline. These results were confirmed by the direction of the current, from the coupons to the electrolyte, evidence of an anodic condition.
The minimum of 100 mV of cathodic polarization criterion does not guarantee adequate protection in the case of positives potentials. It was determined that there was the possible existence of a corrosion macro cell along the pipeline with unknown cause.
The study concluded that it was necessary to complete additional actions to guarantee pipeline integrity, and to assure low corrosion rates.
