Determination of hydrogen trapping mechanisms by microprinting in Ni and Co coatings obtained by HVOF

Abstract In this work, the hydrogen embrittlement resistance of API 5CT P110 steel coated with high velocity oxygen fuel thermally-sprayed nickel or cobalt was evaluated. The novelty of this work is in the use of the microprint technique to directly observe the hydrogen trapping loci within the sprayed coatings. The coatings were characterized by scanning electron microscopy, X-ray diffraction, roughness, porosity, microhardness and by monitoring mechanical behaviour of samples in tensile test submitted to previous hydrogen charging. Additionally, CHNS (carbon, hydrogen, nitrogen, and sulphur) elemental analysis was employed to compute the amount of hydrogen present within the thermally sprayed coatings and the API 5CT P110 steel substrate. Results showed that the coatings worked as hydrogen barriers since they displayed a high capacity to absorb hydrogen before it reached the substrate. Microprinting revealed that the hydrogen was trapped in a heterogeneous fashion within the cross-section and tended to accumulate in the interface between layers and in the coating imperfections.