Corrosion protection of steel 30KhGSN2A in hydrogen sulphide-containing media using diffusion nickel coatings

This paper investigates the effect of diffusion alloying from a medium of low melting liquid metal solutions (DALMMS) of 30KhGSN2A steel with Ni–Cu and Ni–Cr elemental compositions on the structure, elemental composition of surface layers and corrosion resistance in hydrogen sulphide containing media. When DALMMS of 30KhGSN2A steel with Ni–Cu is used, coatings consisting of 3 layers are formed on the surface of the steel: a surface layer, a transition layer saturated with copper, a transition layer of a base layer. The coatings have a low microhardness (133 HV), the maximum concentration of nickel is 44 %, of copper – 80 %. The total thickness of the coatings is up to 30 μm. When DALMMS of 30KhGSN2A steel with Ni–Cr is used, coatings consisting of 2 layers are formed on the steel surface: a surface carbide layer, a transition layer of a base coating. The coatings are characterized by high surface microhardness (2000 HV). The total thickness of the coatings is up to 20 μm. The maximum concentration of chromium was 75 %, of nickel – 20 %. When tested for hydrogen cracking resistance, samples with both types of coatings showed no corrosion. The average value of total (continuous) corrosion for the Ni–Cu coating was 0.031 mm/year and for the Ni–Cr coating 0.048 mm/year. When tested for stress-sulfide cracking test, the maximum time to fracture for a Ni–Cr coated sample was 313 hours, while the Ni–Cu coated samples withstood the test completely (720 hours) without destruction or coating failure. It was revealed that diffusion Ni–Cu coatings were found to be effective in protecting the base metal from general hydrogen sulphide corrosion at the temperatures up to 150 °C, with a protection rate of over 98 % (tested according to the standard NACE MR0175/ISO 15156). The research was carried out under financial support of Kuban scientific fund within the framework of the scientific-investing project No. NIP-20.1/22.17.