THE EFFECT OF NITROGEN ON THE CORROSION OF PLASMA-NITRIDED 4140 STEEL

Alloy 4140 (UNS(1) G410400) steel was plasma nitrided in a commercialized chamber at 560-degrees-C for 10 h and the corrosion behavior was evaluated in chloride environment by potentiostatic and potentiodynamic polarizations. The effect of nitrided nitrogen on the anodic dissolution of iron was studied in terms of the electrode potential and the environment pH. To characterize the nitrided surface, x-ray diffraction (XRD), Auger electron spectroscopy (AES), and x-ray photoelectron spectroscopy (XPS) were employed for analysis of the alloying elements and nitride phase resulting from the electrochemical reactions. Results indicate that nitrogen atoms in the nitrided layer can be electrochemically reduced to NH4+, which favors the anodic dissolution of 4140 steel at pH < 4. On the other hand, NH4+ is capable of neutralizing the acidity in a pit and thus retarding the autocatalytic effect of a pit nucleus in the presence of Cl- at pH > 6. A comparative study also indicated that the domain of passivation is in general extended as a result of nitriding.