The Corrosion Enhancement due to Plasma Post-Oxidation Subsequent to Plasma Nitriding of a Steel AISI 4140

The corrosion enhancement due to plasma post-oxidation subsequent to plasma nitriding of the samples of low alloy annealed steel AISI 4140 was evaluated. The plasma nitriding was carried out at approximate to 570 degrees C for 1-3 h in an atmosphere of 75% H-2 and 25% N-2. After this process post-oxidation at approximate to 450 degrees C in a gas mixture, air and dry air-like was conducted for an hour. The treated samples were characterized using the XRD, SEM, AFM, potentiodynamic polarization and Vickers microhardness tests. The outcome of combined processes showed the presence of epsilon and gamma' nitrided phases in the inner compound layer and in the outer one mostly of magnetite phase and barely visible of hematite one without using H-2 in the post-oxidation process. Also in these processes, the corrosion potential proved to be lower in the treated specimens than the untreated ones, and the corrosion current decreased up to one order of magnitude when treated in environment air and air-like ambient.