Failure assessment of the hard chrome coated rotors in the downhole drilling motors

In this investigation, the premature failure of the hard chrome coated rotors of the downhole drilling motors is studied. The main part in these motors is the power section, composed of a metallic rotor and an elastomeric stator. The rotor is manufactured from 17-4 PH stainless steel and rotates within a stator and displaces the drilling fluid forward. Drilling fluid characteristics change upon circulation and influxes, and cause erosion and corrosion of the rotor. Different types of damages such as scratches, deep and shallow cuttings, spalling, pits, micro and macrocracks are observed on chrome plated rotor surface. Effects of working parameters such as bottom hole temperature and solid content on life time of rotors are investigated. Chemical analysis and microscopic examinations of rotor and its coating are also carried out to identify the causes of failure. It was found that, drilling fluids are corrosive in nature and their solid content are too high for chrome coated rotors.

► Spalling of a hard chrome coated rotor is investigated.
► The work is an excellent case study in the drilling industry.
► Embedded solid particles in inner surface of stator caused fatigue and spalling of chrome coating.