Evaluating localised under deposit corrosion and inhibition of carbon steel in CO2 saturated environments

When sand in present in low velocity energy transport pipework it tends to settle on internal areas interacting with the natural corrosion processes. In carbon dioxide (CO2)-saturated environments the presence of settled sand can severely impact corrosion rates. This type of corrosion is termed under deposit corrosion. In long-standing in-service pipelines, the natural corrosion processes can result in large areas of localised corrosion termed “pits”. Sand particles naturally gravitate to these areas manifesting pits under a deposit termed “under deposit localised corrosion”. Several problems exist surrounding the presence of under deposit localised corrosion, including an inability to effectively stifle corrosion in these environments using chemical corrosion inhibitors. A lack of understanding exists on the chemical properties within these environments and the mechanisms of inhibition in these environments which forms the basis of the work in this thesis.
A three-step approach to determine the effectiveness of corrosion inhibitors on X65 carbon steel in under deposit localised corrosion is developed within this thesis. Stage 1 evaluates the effect of two corrosion inhibitors and their blend in reducing the corrosion rate of X65 carbon steel in CO2-saturated 20 g L-1 NaCl electrolyte at 50 °C, pH 4. The sulphur-based inhibitor produced an inhibitor efficiency (IE) of 94 %, whereas the imidazoline-based inhibitor was slightly less effective at 83 %. The blend showed synergistic capabilities with an IE of 99 %.
Stage 2 evaluates the inhibitor transport properties of the inhibitors and their blend. A test cell was designed to direct the flow of inhibitor through a SiO2 deposit layer of 8 mm thickness (100 g dry deposit). The corrosion rate of X65 carbon steel was monitored. The sulphur-based inhibitor was somewhat effective with an IE of 47 %. The imidazoline and surprisingly the blend were ineffective in reaching the specimen in quantities sufficient to alter electrochemical behaviour.
A bespoke under deposit localised corrosion test cell was used to evaluate the corrosion rate of X65 carbon steel at varying specimen recession (pits) in the presence and absence of a deposit. The corrosion rate reduced considerably with the presence of 100 g SiO2 deposit. Post-experimental analysis confirmed the precipitation of iron carbonate (FeCO3) under the deposit layer. Precipitation was related to an increase in near-surface pH measured using a thin shaft pH probe, at increased recession depth under deposit.
Stage 3 evaluates inhibition in under deposit localised corrosion scenarios. The corrosion rate of specimens at recession depths of 0 mm and 9 mm under a thick SiO2 deposit layer was measured. The sulphur-based inhibitor was able to effectively reduce corrosion rates, however, IE reduced with recession depth. The imidazoline and the blend were ineffective in all under deposit scenarios. The galvanic coupling effect between an exposed and under deposit specimen was also evaluated and showed all inhibitors caused a drastic increase in corrosion rate with the introduction of inhibitor. In all cases, the under deposit specimen acted as the net cathode and the increased corrosion rate was sustained for the duration of the 20 h test.
Overall, results showed the three-step approach could effectively be used to screen corrosion inhibitors for their performance in under deposit localised corrosion scenarios.