Investigation of fracture behavior of annealed electroless Ni-P coating on pipeline steel using acoustic emission methodology

Electroless Ni-P coatings have been a popular option for a variety of applications because of their autocatalytic nature and excellent resistance to wear and corrosion. These properties of Ni-P coating suggest an additional potential application as a pipeline coating. The objective of this study, is to determine the suitability of using as-deposited and annealed electroless Ni-P coating in the oil industry by investigating the toughness and dent resistance of the coating. In this study, high phosphorus content electroless Ni-P coating was deposited on API X100 pipeline steel, which was then annealed at 400, 600, and 800 °C. Structural characterization using XRD indicates that as-deposited Ni-P coating is semi-amorphous, while annealed coatings are crystalline mixture of nickel and nickel phosphide phases. Three points bending tests and Hertzian-type indentations were conducted on specimens. Acoustic emission was used to monitor crack initiation. The annealed coatings are found to have a higher hardness and Young's modulus compared to the as-deposited coatings. However, the toughness of the as-deposited coating is higher. It is also found that the indentation behavior of electroless Ni-P coating is significantly influenced by microstructure and hardness variations.