Corrosion mechanism of electroless Ni–Sn–P coating in multi thermal fluid

Multi thermal fluid generated by burning of fuel and oxygen-containing gas has been applied to enhance the production of heavy oil. While the thermal fluid containing carbon dioxide, nitrogen gas, oxygen and steam has caused serious corrosion damage of metallic tubing. Electroless Ni-P coatings show excellent corrosion resistance in many environments and Sn seems to have the potential to improve coating toughness and corrosion resistance. In the present work, Ni–Sn–P coating was prepared on N80 tubing substrate by electroless method, and its corrosion performance in multi thermal fluid was investigated. The results revealed that electroless Ni–Sn–P coating containing ∼2 wt% Sn was dense, defect-free and uniform, and possessed amorphous structure. After 240 h exposure, the bare N80 tubing suffered from severe corrosion and formed a thick, complex, and porous corrosion scale mainly composed of iron oxides. In contrast, N80 tubing substrate with Ni–Sn–P coating showed much more excellent corrosion resistance by forming an extremely thin NiO/Ni(OH)₂/SnO₂ corrosion layer and maintaining the amorphous structure. (paper)