Different electrochemical techniques (recording of the corrosion potential as a function of time, polarization curve and electrochemical impedance spectroscopy measurements) are applied to monitor the evolution of the corrosive attack suffered by various surface-nitrocarburized and -oxy-nitrocarburized carbon steel discs in a neutral aerated sodium chloride (5%) solution. The results are discussed in relation to the protection given by the iron oxide or iron carbonitride layers to the steel substrate and in relation to the penetration of the solution through these layers. The iron oxide (magnetite) and iron carbonitride layers are more corrosion resistant than iron. The corrosion potential of magnetite is more positive (about 200-300 mV) than that of the iron carbonitride, which, in turn, is more positive (about 300 mV) than that of the base steel. Therefore it may be possible to monitor the evolution of the corrosive attack suffered by these discs in 5% NaCl solution by recording the trends of their free corrosion potentials as a function of time: the more or less rapid potential shift from the value proper of the magnetite or of the iron carbonitride to that proper of the untreated base steel may give an indication of the penetration of the aggressive solution as far as the metal substrate and, consequently, of the efficiency of the oxy-nitrocarburizing treatment towards the corrosion behaviour.
ELECTROCHEMICAL CHARACTERIZATION OF NITRO-CARBURIZED AND OXIDIZED LAYERS
FRIGNANI, Alessandro;COLLEDAN, Alessandra;
2001
Abstract
Different electrochemical techniques (recording of the corrosion potential as a function of time, polarization curve and electrochemical impedance spectroscopy measurements) are applied to monitor the evolution of the corrosive attack suffered by various surface-nitrocarburized and -oxy-nitrocarburized carbon steel discs in a neutral aerated sodium chloride (5%) solution. The results are discussed in relation to the protection given by the iron oxide or iron carbonitride layers to the steel substrate and in relation to the penetration of the solution through these layers. The iron oxide (magnetite) and iron carbonitride layers are more corrosion resistant than iron. The corrosion potential of magnetite is more positive (about 200-300 mV) than that of the iron carbonitride, which, in turn, is more positive (about 300 mV) than that of the base steel. Therefore it may be possible to monitor the evolution of the corrosive attack suffered by these discs in 5% NaCl solution by recording the trends of their free corrosion potentials as a function of time: the more or less rapid potential shift from the value proper of the magnetite or of the iron carbonitride to that proper of the untreated base steel may give an indication of the penetration of the aggressive solution as far as the metal substrate and, consequently, of the efficiency of the oxy-nitrocarburizing treatment towards the corrosion behaviour.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
