Carbon steel casing and tubing, utilized for oil and gas production, may potentially experience severe corrosion due to the presence of corrosive gases (e.g., CO₂ and H₂S) and chlorine compounds in the production wells. To mitigate the corrosion of carbon steel, the Ni-P coating has been applied to the carbon steel surface using electroless deposition method. The extensive studies have suggested that Ni-P coating can effectively protect the substrate through isolating the substrate from the corrosive environment, and exhibits good corrosion resistance in the environment containing brine, acid, CO₂ or even H₂S. However, some internal microdefects derived from the deposition process as well as some external defects originated from the mechanical effect during the production process will inevitably be present in the coating. These defects are likely to pose great risks to the reliability of the coating and negatively affect its durability in the corrosive environments, especially in the coexistence of CO₂ and Cl^-.

In this work, the degradation behavior of a high-phosphorus Ni-P coating with microdefects or an artificial defect in CO₂/Cl^- environments was systematically investigated using electrochemical methods and surface characterizations. The results show that although the corrosion occurs at the microdefects and extends towards the inside of Ni-P coating, the coating has a good resistance to corrosion disbonding in the CO₂/Cl^- environment, even with an artificial defect in the coating. Under the cathodic polarization condition (to accelerate the corrosion process), the defects in the coating provide effective pathways for the electrolyte to transport through the coating and along the coating/substrate interface laterally from the defects, thereby, causing the localized corrosion and disbonding of the coating. Finally, a corrosion model is proposed to well interpret the degradation process of the coating with microdefects in CO₂/Cl^- environment. The electrolyte penetrates into the micropores and causes the corrosion of coating at the micropores, promoting the initiation of the pits. As the corrosion proceeds, the accumulation of corrosive species in the pits increases the localized corrosion rate. After the pits penetrate through the entire coating, the corrosion process is governed by the substrate dissolution and the mass diffusion between the substrate interface and the electrolyte. The corrosion of the substrate propagates along the coating/substrate interface laterally and towards the depth direction due to the accumulation of the electrolyte at the exposed substrate surface, which causes local corrosion disbonding of the coating.