In heavy oil wells, carbon steel casing and tubing often suffers from severe corrosion due to the presence of corrosive gases, chlorine compounds and solid particles in the corrosive medium. An electroless Ni-P coating (EN coating) has been applied to the carbon steel surface, which presents better corrosion resistance than the substrate. However, defects such as microscopic pores, as the corrosion paths, will be inevitably present in EN coating, which can cause the corrosion of carbon steel substrate despite the coating. To solve this problem, passivation treatment can be used to create a passive film on EN coating surface and improve the corrosion resistance of the coating. However, erosion-corrosion also occurs when solid particles are suspended in the corrosive medium, and the erosion-corrosion mechanism of the passivated Ni-P coating is not known.

In this work, a passive film was formed on the EN coating in the K₂Cr₂O₇-containing bath with and without an applied passivation potential. The corrosion properties of passive films were evaluated by means of electrochemical measurements and compared to the non-passivated EN coating. SEM, XPS, and EDS were employed to analyze the chemical composition and surface morphology of the films. The results showed an obvious passive region of passive film on EN coating in the potential range of -0.1 – 0.45 V (vs. SCE). The corrosion current of the passivated EN coating through chemical passivation treatment (CPT) was reduced by 61 % than that of non-passivated EN coating film, while it was reduced by 79 % through a combination of CPT and electrochemical passivation treatment (EPT) with an applied passivation potential of 0.2V (vs. SCE). These results demonstrate how EPT can significantly improve the corrosion resistance of the passive film.

Erosion-corrosion of passivated EN coatings through CPT and EPT was studied under an applied passivation potential by means of a single particle impingement technique. The transient current response to disruption of the passive film was obtained at different impact angles and velocities, which was the result of different damaged surface area produced by solid particle impingement. Correspondingly, the hydrodynamic effects on erosion-corrosion of passivated EN coating were predictable with a theoretical model based on kinetic analysis.