Corrosion Behavior of Nitride and Cr-Al-N Coatings Formed on AISI D2 Steel

In the present work, the corrosion behavior of Cr-Al-N coating formed on the pre-nitrided AISI D2 cold work tool steel by thermo-reactive diffusion technique in the powder mixture consisting of ferro-chromium, aluminum, ammonium chloride and alumina at 1000C for 2 h was investigated. The phases formed in the coating layers were $Cr_2N$, $(Cr,Fe)_2N_{1-x}$, $AlN$, and $Fe_2N$ which were confirmed by X-ray diffraction analysis. The uncoated, nitrided, and Cr-Al-N coated specimens were placed in corrosive media (3.5 wt% salt solution). A standard saturated calomel electrode was used as a reference and graphite as a counter or auxiliary electrode. The effectiveness of the coatings in preventing corrosion was tested in the NaCl solution by electrochemical impedance studies using the Nyquist plots and potentiodynamic studies as well. Conclusively, the application of nitride and Cr-Al-N layers on AISI D2 steel increased its surface hardness and corrosion resistance. The corrosion resistance of Cr-Al-N coatings is higher than that of uncoated and nitrided steels.