Cathodic protection and DC non-stationary anodic interference: criteria and electrochemical processes

摘要

Non-stationary DC anodic interference on buried carbon steel pipelines under cathodic protection, mainly caused by public traction systems, is an unavoidable phenomenon in several practical cases. International standards provide criteria to assure corrosion protection, even under interference, based on potential and current readings. ISO 15589-1 standard states that interference is not acceptable if the structure potential is more positive than the protection one, namely -0.85 V Cu/CuSO_(4,sat) for steel in aerated condition. On the other hand, standard EN 50162 reports a criterion based on current: the maximum acceptable interfering period is a function of the current measured on a coupon. The aim of the present research is opening a pathway toward quantification of the DC non-stationary interference interpretation for improving EN 50162 standard approach. Laboratory test has been performed on carbon steel in simulated soil solution under protection with suitable protection current. Period of anodic interference changes from 2 to 20 hours and range of anodic current density changes from half to five times of protection current. The results confirm that the alkaline pH established at the metal surface during cathodic protection reduces the extent of corrosion promoted by anodic interference. The average of maximum potential in each day for a long term measurement could be a representative of the average of time that surface is out of protection. Moreover, the maximum potential of the one-day measurement could be representative of time out of protection, if the potential is not more positive than corrosion potential and time out of protection is not more than interference time.