Stress-corrosion cracking of Ni-Cr-B-Si coatings under cyclic stress

The authors have demonstrated that Ni-Cr-B-Si coatings on sucker rod pump plungers in Bashkortostan and Udmurtia can be prone to corrosion cracking under cyclic stresses in conditions with hydrochloric acid, sulfides, and chlorides, significantly reducing plunger reliability and durability. It has been established that the nearly perpendicular arrangement of developing cracks in the coating leads to its fragmentation and flaking, forming the typical damage morphology of the Ni-Cr-B-Si layer obtained by flame spraying. The authors suggest a multi-stage mechanism for the initiation and development of cracks: the formation of corrosion cavity centers; the initiation of radial stress corrosion cracks within these cavities, which propagate deeper into the coating; the development of subsurface axial contact fatigue cracks. The coating’s failure process concludes with its delamination down to a thin, lightly etched layer that remains on the surface of the investigated plungers. The microstructure of the coating consists of a matrix solid solution based on nickel, which represents an almost closed network of mixed hard phases, including chromium carbides and borides. A transition zone has been found at the boundary of the coating and base metal with a modified structure, resistant to damage, caused by the diffusion of elements from the coating into the base material. Measures to reduce embrittling factors are recommended, including the adjustment of the content of chromium carbides and borides in the coating and modifying the length of the plungers. An evaluation of the economic efficiency of improving the resistance of the coating to hydrogen sulfide, including cost reduction, is conducted.The study was supported by JSC ELKAM-Neftemash.The authors express special gratitude to the head of the chair for Metal Science, Thermal and Laser Processing of Metals of PNRPU, Doctor of Engineering Sciences, Professor Yu. N. Simonov.