Effect of High-Temperature Nitriding on the Corrosion Resistance of Ferrite Chromium Steels

Abstract

High-temperature nitriding with subsequent annealing allows changing the structure and increasing the mechanical properties of the entire volume of thin-walled samples from corrosion-resistant high-chromium ferrite steels without the formation of a continuous nitride layer on the surface. At the same time, the influence of high-temperature nitriding on the corrosion resistance of the steels has not been studied. In this paper, a comparative study of the effect of high-temperature nitriding on the electrochemical corrosion resistance and resistance to prolonged high-temperature oxidation of high-chromium ferrite steels with different content of Cr and V (17Cr–1Ti (AISI 439 type), 25Cr–1Ti (AISI 446 type), and Fe–20Cr–5V) has been carried out. High-temperature nitriding of ferrite steel at a temperature of 1075–1100°C with subsequent heat treatment (annealing at 550°C and annealing at 700°C with/without degassing) provides a different content of Cr and V in a solid solution and changes the corrosion resistance of materials. It is ascertained that high-temperature nitriding has a positive effect on the corrosion resistance of the steels in an aqueous solution of 1 mol/L NaCl + 0.5 mol/L H₂SO₄, and in an aqueous solution of 3% NaCl, the effect is negative. In this case, the degassing of the samples from their surface leads to a sharp displacement of the steady-state corrosion potential toward electropositive values, which indicates an increase in the corrosion resistance of the materials. The steels investigated are characterized by a high resistance to oxidation at high temperature (700°C for 30 h). For Cr20V5 steel in the initial (non-nitrided) state, the specific weight gain is several times greater than for the other steels. Nitriding with annealing both at a temperature of 550°C and at 700°C significantly increases the resistance of Cr20V5 steel to oxidation.