Abstract
Carbon steel pipes used to transport hydrocarbons and dangerous fluids are provided with corrosion prevention systems, namely an insulating coating and a cathodic protection (CP) system which reduces (or halts) corrosion rate below 10 µm/y, which is the maximum accepted corrosion according to standards.
The presence of AC interference may cause serious corrosion damage on metallic structures, even under CP condition. AC interference can take place by a conduction or an induction mechanism causing corrosion corresponding to coating defects, even if the -0.850 V CSE criterion is matched. Field failures and laboratory investigations have confirmed that at industrial AC frequencies (50 or 60 Hz) corrosion can occur even if CP is correctly applied and that AC corrosion is less than that caused by the equivalent direct current. Nevertheless, uncertainties still exist on protection potential interval to reach acceptable corrosion protection if AC interference is present.
In order to assess CP criteria in the presence of AC, weight loss tests on cathodically protected carbon steel specimens were carried out in soil-simulated conditions in the presence of AC densities ranging from 1 to about 800 A/m2. Samples were cathodically polarised in the presence of AC interference for four months: protection potential, protection current density and AC density were periodically monitored. Based on experimental results, a corrosion risk map is proposed: overprotection (potential more negative than -1.1 V CSE) seems to be the most dangerous condition in the presence of AC.
