The oxidation and corrosion resistance of nitrided iron alloys

The chemistry and mechanical properties of nitrided iron alloys are well understood but their resistance to oxidation and corrosion has received less attention. Thermogravimetric and metallographic results are presented of the oxidation in air of nitrided iron, FeMo and FeCr laboratory alloys, and mild steel. Oxidation resistance is improved after nitriding by formation of a fine grain oxide which has a higher fracture strain than the coarser oxide on the corresponding annealed alloys. No void formation occurs at the oxide/metal interface. The results are discussed in terms of nucleation of oxide on dispersed incoherent nitride particles in the metal surface. The improved cohesion of the oxide on nitrided alloys is however not reflected in the results of thermogravimetric studies as the reduced interfacial voidage on the dispersoid-containing alloy allows unrestricted cation transfer from metal to oxide and the conditions are insufficiently aggressive to cause spalling on un-nitrided alloys. Experiments on nitrided mild steel demonstrate the resistance to fracture of oxides formed on nitrided alloys. The criteria for resistance to aqueous corrosion are different from those for resistance to high temperature oxidation but nitriding is shown to produce significant improvements in resistance to general and pitting attack. Potentiokinetic measurements are reported for nitrided laboratory alloys and a commercial austenitic stainless steel containing nitrogen in solid solution. This behaviour is not fully understood but is not simply related to oxide nucleation and cohesion as is the case in high-temperature oxidation.