Development of amorphous Ni-P coating over API X70 steel for hydrogen barrier application

Abstract Hydrogen embrittlement of high strength steels due to the presence of a few ppm of diffusible hydrogen is a serious problem. The present work investigates the use of an amorphous Ni-P coating laid by electroless method over API (American Petroleum Institute) X70 line pipe grade steel for H-barrier applications. It has been shown using Transmission Electron Microscopy (TEM) and X-ray Photo-electron Spectroscopy (XPS) that the coating structure is predominantly amorphous with some nano-crystalline phases. The Ni-P coating displays a slower and delayed H-permeation compared with crystalline Ni-electroplated steel at similar coating thickness levels. Furthermore, potentiodynamic polarization and salt fog exposure tests in NaCl solution showed improved corrosion resistance of Ni-P coating compared with crystalline Ni coating. Excellent resistance to H-permeation as well as corrosion in chloride environment has been attributed to the amorphous structure of the Ni-P coating.