Galvanic Corrosion Following Local Breakdown of a Scale Formed on X-65 in CO2 Saturated Solutions

Under some conditions in sweet environments, the precipitation of corrosion products (primarily FeCO3) on the surface of X-65 pipeline steel can decrease the corrosion rate of the metal, this precipitated film acting like a protective scale. However, when these scales are damaged due to effects such as solid- particle erosion or mechanical stress, a galvanic pair can form between the bare steel (anode) and the surrounding undamaged scale (cathode). The aim of this work is to evaluate the magnitude of galvanic coupling that arises when the protective scale is broken and a corrosion cell is established between the defect and the surrounding scale. This study has been conducted in two stages: firstly, the formation of a protective scale on the surface of X-65 samples in CO2 saturated conditions (80 oC and a CO2 pressure of 0.53 bars); secondly, a scaled sample of X-65 steel was coupled with a fresh (non-scaled) X-65 sample in a brine solution saturated with CO2 at 60oC. Zero resistance ammeter (ZRA) measurements were then conducted using this galvanic couple. According to the results, there was galvanic coupling between the bare metal and a scaled surface where the magnitude of the galvanic pair was found to be dependent on the anode:cathode ratio.