Valutazione del comportamento a corrosione di differenti trattamenti termochimici su substrati in acciaio

Nitriding and carbo-nitriding are thermochemical treatments of surface hardening aimed at improving the resistance of steels to wear and fatigue. So-treated components usually exhibit high tribological properties, showing at the same time low corrosion resistance in the form of localized attacks. It is now common practice, to overcome this drawback, to submit the nitrident/carbonitrided components to a post-oxidation treatment. Post-oxidized components can be used in several industrial field, ranging from automotive to building, from energy technology to industrial machinery. It follows that there is an increasing interest in this kind of treatments with the aim to develop optimized process conditions to obtain at the same time mechanical properties, tribological features and corrosion resistance as good as possible. The present work issue from a purely productive context as well as from the security of the literature. There is actually the need for thermochemical treatments that can guarantee the requested wear resistance, at the same time responding to specific corrosion behaviours. Some authors report that nitriding/carbo-nitriding provides a simple barrier protection against corrosion as the approach of aggressive species from the environment to the steel surface is hindered. The corrosion resistance improves significanty only after a post-oxidation treatment. Also, post-oxidation on a carbo-nitrided substrate appears to favour the conversion of the very first layers of carbonitrides into iron oxides, mainly magnetite, and the partial occlusion of the pores. Electrochemical studies conducted in 3.5% NaCl solution showed that corrosion behaviour is strictly connected to the kind of thermochemical treatment to which steel was subjected. With reference to post-oxidized specimens, slight ennoblement of the treated steel with respect to the untreated one has been reported from open circuit potential measurements in 0.1M HCl. Also, the percentage of oxygen present in the working atmosphere can play a role in the corrosion behaviour of the resulting conversion layer. 10-15% of oxygen appears to be needed to guarantee the best corrosion performance. The aim of the present work has been to develop a reliable procedure to discriminate the corrosion behaviour of several steels subjected to different thermochemical treatments. Conventional steels, i.e., 42CrMo4, ST52 and C45, were used as substrates and subjected to the following thermochemical treatments: ion nitriding and carbo-nitriding followed by post-oxidation in nitrogen protoxide, and carbo-nitriding in salt baths. Morphology and crystalline nature of the conversion layers were examined by optical microscopy, SEM, GDOES and XRD. The electrochemical behaviour was studied by potentiodynamic polarization in aerated 0.1M NaCl solution, while long-term exposure tests were performed in 3,5% NaCl solution. The choice of using testing solutions containing chloride was imposed by the need of reproducing the effect of conventional operating environments