The Influence of Alternating Current on the Polarization Behavior of Stainless Steels

In later years direct electrical heating (DEH) based on the use of alternating current (AC) has replaced traditional chemical based methods for hydrate formation control in subsea flow lines. The problem with the AC-method is the increased risk of corrosion. A number of publications have been presented on the effect of AC on carbon steel since the early 1900s and there is a general understanding that higher alternating currents lead to higher risk of AC corrosion. However the mechanism is still not completely understood and besides - little is known about the AC-induced corrosion on stainless steels. The purpose of this study is to provide accurate and thorough understanding of the mechanism and severity of applied AC on steels in general and stainless steels in particular. The cathodic polarization behavior of 316L stainless steel has been studied and compared with the polarization behavior of X65 carbon steel, at the direct current (DC) potentials -800 mV and -1050 mV (SCE) for a range of applied AC current densities - from 0 to 1000 A/m2 . An investigation of the passivity of stainless steels with and without AC was carried out on 25Cr under open circuit conditions and under anodic bias. A total of 55 experiments were performed in 3.5 wt% sodium chloride (NaCl) for this study. The experiments were conducted at 25 oC with a stagnant working electrode and lasted 96 hours at the most. The current responses were recorded. Weight losses were measured. Surface morphologies were studied both visually, with Scanning Electron Microscopy (SEM) and Energy-dispersive spectroscopy (EDS). Cathodic current densities for 316L were found to increase with one order of magnitude at -800 mV and two orders of magnitude at -1050 mV when AC was increased from 0 to 1000 A/m2. The highest cathodic current density, 3.35*10-3 A/cm2, was found for 316L at -1050 mV and 1000 A/m2 AC. The highest corrosion rate (CR) for 316L - 0.113 mm/y - was measured at -800 mV and 1000 A/m2AC. Cathodic current densities for X65 were found to be of same magnitude at -800 mV, but increased with one order of magnitude at -1050 mV when AC was increased from 0 to700 A/m2. The highest cathodic current density for X65 was 2.18 *10-3 A/cm2 at -1050 mV and 700 A/m2 AC. The highest CR for X65, 0.205 mm/y and 0.236 mm/y, were measured when 500 A/m2 AC was applied at -800 mV and -1050 mV respectively. When AC densities higher than 100 A/m2 were applied on 25Cr, hydrogen evolution was found to control the corrosion kinetics and the passive region disappeared.The results indicate a significant acceleration of cathodic kinetics with applied AC. Weight loss measurements indicate also an increase in the corrosion rate with applied AC. However, the cathodic reaction rates were much higher than the anodic reaction rates at the applied DC potentials. The results implicate that the presence of AC decreases the effectiveness of cathodic protection systems. A possible model for the AC mechanism has been proposed