The exposure of certain carbon steels to sour environments can result in severe hydrogen induced cracking (HIC) damage in the oil and gas industry. Current mitigation techniques in this field have low reliability or are not able to provide long-term protection against such damage. Recent advancements in thermal spray technology have resulted in a promising and cost-effective solution. Improvements in particle velocity and deposition efficiency have enabled coatings to achieve higher density and uniformity. High-velocity air fuel (HVAF) thermal sprayed NiCrMoW coatings are particularly interesting due to their outstanding corrosion resistance and mechanical properties. To ensure that equipment is sufficiently protected against the harsh environment of this industry, a thick coating is desired. However, as coating thickness increases, the performance of thermal sprayed coatings is frequently affected by residual stresses and unfavourable microstructural features.

To identify this effect, three NiCrMoW coatings with thicknesses of 250, 375, and 500 \textmu m were applied with HVAF thermal spray technology on S235JR carbon steel. Samples were analyzed in order to evaluate differences in terms of microstructure, mechanical behaviour, HIC resistance, and corrosion resistance. An AK07 HVAF instrument in a controlled setting at the IOT research centre of the University of Aachen was used to ensure consistency among the coatings during the spraying process. Experiments to evaluate HIC resistance and corrosion resistance involved prolonged immersion in a sour environment, cathodic charging, open circuit potential measurements, and potentiodynamic polarization tests. Microstructural variation was examined with the use of SEM-EDS and optical microscopy. Additionally, subsurface microhardness measurements of the coating and underlying substrate were used to evaluate hardness and give an indication of the presence of residual stresses.

Findings indicate that the coatings exhibit excellent corrosion resistance. A small but noticeable decrease in resistance was however observed with increasing coating thickness. This decline can be attributed to two factors: an increase in the degree of oxidation and accumulation of residual stresses within the thicker coatings. Additionally, it is noteworthy that while the degree of oxidation and residual stresses increased with coating thickness, the porosity fraction decreased. Microstructural features in the coatings varied as a result of differences in thermal input, cooling passes and the influence of shot peening effects. Resistance to HIC of carbon steel in a sour environment was significantly improved by the application of the coatings in comparison with uncoated samples. This can be attributed to the excellent corrosion resistance, uniformity and absence of through-coating porosity in the coatings, the thickness did not have an influence. Furthermore, it was found that the galvanic interaction between the NiCrMoW coating and the S235JR carbon steel significantly accelerates the corrosion of the underlying substrate. Thicker coatings might be able to provide a greater physical defect-free barrier which can resist breaking, damage and erosion to prevent this galvanic effect.