Corrosione ad umido di acciai con strati superficiali nitrurati prevalentemente costituiti da fase epsilon

Previous studies in the department on nitriding and carbonitriding processes dealt with the equilibrium relationship between the pases of Fe-N and Fe-C-N systems, the morphology and the formation kinetic of the layers. Among the characteristics of surface layers mainly constituted by epsilon phases, the corrosion resistance in slightly aggressive media is not a secondary problem. In order to avoid difficulties in the interpretation of the electrochemical data, due to the limited thickness and the different porosity of the layers, the following sequence of experiments has been made: first of all,. the behavior of layers obtained by nitriding in ammonia at 700 degree C has been considered, because of their noticeable thickness ( greater than 100 mu m) and their reduced porosity. Then the corrosion resistance of samples 'soft' nitrided in salt baths or gas mixtures has been examined. From the results obtained the following conclusions may be drawn: Thick, continuous and compact layers mainly constituted by the epsilon phase protect UNI C 40 steel against corrosion in neutral solutions; the corrosion rate of the layer is lower than the corrosion rate of the matrix; and, depending on the oxidizing properties of the carbonitriding medium, the layer becomes nobler.