Influence of Salt-Bath Nitrocarburizing on the Electrochemical Corrosion Properties of Stainless Steels and Nickel-Based Alloys in 3.5-wt.% Sodium Chloride Solution

Stainless steels and nickel-based alloys, despite adequate corrosion resistance for general oilfield services, are restricted in abrasive and fretting wear by their surface hardness. Nitrocarburizing is well known to cause surface hardening, but its effect on corrosion is still not well established on high-chromium iron and nickel-based alloys. Salt-bath nitrocarburizing has been applied on five stainless steels (UNS S41000, UNS S41425, UNS S17400, UNS S30400, and UNS S31600) and four nickel-based alloys (UNS N09925, UNS N09935, UNS N07718, and UNS N07716) to explore the effects of alloy compositions on nitrocarburizing compounds, case depths, and corrosion behaviors in a light 3.5 wt.% sodium chloride brine. For all selected alloys, the nitrocarburized surfaces were mainly characterized by optical and scanning electron microscopy and x-ray diffraction. In addition to pit quantification measurements, corrosion resistance was evaluated by cyclic potentiodynamic polarization testing at ambient temperature on both un-nitrocarburized and nitrocarburized alloys, the latter simultaneously processed in a single molten salt bath. Compared to the nickel-based alloys, the nitrocarburized stainless steels possess thicker and harder compound layers of chromium nitrides, with an iron (III) oxide layer on top. Potentiodynamic polarization tests further indicate that nitrocarburizing increases the corrosion resistance of stainless steels at least temporarily. The nickel-based alloys, characterized by thinner nitride layers, were all observed to lose some corrosion resistance, yet remained comparable in pitting resistance than the un-nitrocarburized stainless steels with a minimum PREN of approximately 20. The nitrocarburized nickel-based alloys were consistently found to be free of both iron (III) oxide at the surface and nitrogen enrichment below the compound layer. Carbon was found to cluster below the nitride-rich compound layer, with its contributions in need of further investigations.