Corrosion analysis of G105-coated drill-pipe washout and electroless plating Ni-P as drill-pipe protection in freshwater mud

G105 (API SPEC 5D group 3)-coated drill pipes suffer from washout after drilling for 70,000 m to 80,000 m. The micro-structure, micromorphology, and corrosion products near the washout were analyzed by metallurgical microscopy, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). Electroless plating Ni-P as drill pipe protection in freshwater mud was studied. Results showed the metal-lographic microstructure of the material was typical tempered sorbite. No fatigue crack was observed. Drill-pipe washout was caused mainly by the inclusion of manganese sulfide (MnS) in steel because of the excess S and by damaged coating, both of which induced pit nucleation and promoted the pitting corrosion process. Electroless plating was cystiform in structure on the surface, and phosphorus content was 13.23%, which was high phosphorus and corrosion-resistant plating. Maximum corrosion rate of electroless plating was a quarter of the minimum of the corrosion rate of G105. Organic coating was damaged by enhanced abrasion after 10 h, and electroless plating did not change basically after 50 h. Organic coating bubbled after 10 h under low velocity because of high-temperature corrosion, but electroless plating had no change.