Corrosion behavior of Ni-W coatings with and without macroscopic defects under different corrosion conditions

High pressure and high temperature corrosion performance of Ni-W coatings with and without macroscopic defects were investigated successively in live acid at 100 degree celsius, spent acid and formation water containing CO2&H2S at 160 degree celsius by using weight loss method, surface characterization and high temperature and high pressure electrochemical test. Additionally, the corrosion performance of C110 steels was comparatively studied. The results show that compared with C110 steels, Ni-W coatings have better resistance to each harsh corrosive environment with lower uniform corrosion rate, pit depth and decreased corrosion current density. The corrosion rate of Ni-W coatings in both live acid and spent acid are acceptable for oilfields, and its corrosion rate (0.0978 mm/y) in formation water is categorized as moderate-degree corrosion. The corrosion scale consisting of NiS, Ni3S2 or Ni3S4 and a small amount of Ni(OH)(2) is stable, compact and closely combined with the substrate, thus effectively slowing down the corrosion rate of Ni-W coatings during long-term immersion. However, the preferential dissolution of Ni causes a W-rich layer on the top layer of Ni-W coating, probably forming WO2, which tends to crack and aggravates the coating degradation. For Ni-W coatings with artificial defects, the galvanic effect and the catalytic role of Cl- synergistically accelerate the corrosion of the C110 substrate at the defect, and its pit depth is over 100 mu m after being corroded in formation water. Notably, the corrosion of the C110 substrate extends laterally and deeply along the coating/substrate interface, promoting the local corrosion disbonding of the coating. Therefore, macroscopic defects in Ni-W coatings should be avoided in their practical application.