The Corrosion Behavior Of Pipeline Steel Under Disbonded Coatings

Steel pipeline buried in soil environment is generally protected from corrosion by combination of polymeric coating and cathodic protection (CP). A coating, which is a physical barrier, has the basic function of letting the steel away from the corrosive environment close to the pipeline. CP inhibits or prevents the corrosion of metals by reducing. However, coatings may result in current shielding and are disbonded because of external force during the process of buried installation and use. Corrosion often takes place in disbonded coatings. A local corrosive micro-environment is formed once corrosive media enters into the crevice between coating and the steel, which leads to corrosion of steel pipelines under disbonded coatings. The ambient CO2 derives from decaying organic matter in the soil, and then dissolves in dilute red soil solution forming carbonic acid. In this environment, pipeline steels with stress are susceptible to near-neutral stress croosion cracking (SCC), which may cause large economic losses and people injuring.In this paper, a rectangular crevice corrosion cell was constructed to simulate coating disbondment and W/WO3 pH microelectrodes were placed on it. The pH values were in-situ measured under disbonded coating. The variation of current densities, potential and chemical/electrochemical environment and corrosion behavior of pipeline steel X80 under disbonded coating were studied by electrochemical impedance spectroscopy (EIS), microelectrode technology, microscopic analysis. The results may offer a theoretical basis for corrosion and protection of X80 pipeline steel under disbonded coating.The corrosion behaviors of pipeline steel with and without stress under disbonded coating without cathodic protection were studied. Severe corrosion attack was found to occur preferentially at open holiday region, which was aggravated by tensile stress. General corrosion from strongly preferential anodic dissolution was dominant the surface of free steel specimen (without stress). Whereas, initiation of microcracks was observed on tensile specimen and some microcracks had started to merge, forming small clusters along scratch lines. Whereas inside the shielding, anodic dissolution trended to lessen under the shielding over the distance away from opening, and no microcrack was not observed.The distribution of potential, current densities and micro-environment under disbonded coatings at different cathodic potentialswere investigated. The distance effectively protected by CP increased with the decreasing of applied potentials. However, when the applied potential was decreased to-1200 mVvs.SCE or more negative levels, hydrogen evolution dominated at opening region of disbonded coatings, which may shield cathodic current and induce hydrogen embrittlement (HE) of the steel.Corrosion behavior of X80 pipeline steel in an acidic red soil from Southern China was studied at a variety of temperatures (20-75℃) by EIS. The process and kinetics of the corrosion reaction were ananlyzed by dynamics and transition state theory. The results showed that, EIS of X80 steel in soil contains the soil capacitivearc at the high-frequency region and the capacitivearc from the interface process at low-frequency region. Temperature has significant effect on corrosion of X80 steel. With increasing temperature, the soil resistivity and the charge transfer resistance decreases, and the corrosion rate increases. Kinetics analysis showed that the corrosion reaction of X80 steel in the acidic red soil is an endothermic reaction with the degree of disorder decreasing.Galvanostatic pulse measurement (GPM) was introduced as a new non-destructive technique to monitor corrosion of X80 pipeline steel in a red soil environment. Corrosion rate and double layer capacitance of the metal/soil interface for X80 pipeline steel in red soil were obtained from GPM, and the results were compared with that from polarization method. Results showed that the galvanostatic pulse method is useful for evaluating soil corrosion and it can provide a nondestructive technology for characterizing double layer. The polarization resistance indicated that corrosion rate of the steel decreased with time in the water-saturated red soil.