Nitriding of Duplex Stainless Steel for Reduction Corrosion and Wear

Duplex stainless steel (DSS) which has a dual nature of ferrite and austenite with nearly equal ratio has found a good application in oil and gas industries because of its excellent corrosion resistance, high yield strength, good weldability, and relative low life cycle costing from reduced operating cost. Dual phases of DSS or combination of two-phase existence to be more characterize with better mechanical properties than a single-phase metal of ferrite or austenite. Nitrogen is a good strengthening alloy for DSS because it forms a solid solution of (Fe, Cr)₂N which is responsible for the hardness and wear resistance in DSS. Spontaneous formation of oxide or passivated layer is responsible for shield and direct exposure of surface of stainless steel to corrosive medium though is susceptible to depletion and deterioration in high chlorine water and low pH level in production environment. Total damage of passivated region on the surface of DSS prompts initiation of localized defect such as pit which grows over a time to form crack. Oil and gas environment is characterized with high H₂S content which is a constraint in application of DSS in this environment because of its insignificant resistance to sulfide stress corrosion cracking (SSCC) and hydrogen-induced cracking (HIC).