Corrosion analysis of G105 coating drill-pipe washout

Purpose – The purpose of this study was to investigate the reason of G105 coated drill-pipes suffering from washout after drilling for 70000–80000 m. Design/methodology/approach – The microstructure, micromorphology and corrosion products near the washout were analyzed by metallurgical microscopy, SEM and EDS. Findings – Results showed the metallographic microstructure of the material was typical tempered sorbite. No fatigue crack was observed. Drill-pipe washout was caused mainly by the inclusion of MnS in steel because of the excess S and by damaged coating, both of which induced pit nucleation and promoted the pitting corrosion process. The corrosion hole extended from the interior to the exterior, which resulted in the fracturing of the external drill-pipe surface under pressure. Originality/value – This paper can give practical help to the selection of drill pipe materials in the future.