Volume 8, Issue 7 (July 2011)
Effect of Plasma Nitriding on the Wear and Corrosion Properties of Hastelloy CW2M Alloy
Hastelloy C alloys are widely used in chemical and nuclear industries in corrosive environments. The predominant alloying elements of these nickel-based alloys are typically chromium and molybdenum, although other elements are added. Plasma nitriding is an efficient surface treatment to increase wear resistance. The objective of this work was to evaluate the influence of nitriding treatment on the wear and corrosion properties of the Hastelloy CW2M super-alloy. Hastelloy CW2M was plasma nitrided using the dc method with the following conditions: a gas mixture of 80 vol. % H2 and 20 vol. % N2, a pressure of 500 Pa, temperature of 450 and 500 °C during 5 h. As-cast and plasma treated samples were analyzed using scanning electron microscopy, wear and corrosion tests. An EDS line scan performed on the samples cross- section indicates that Cr and Mo content are the largest on the layer, with decreasing amounts of Ni and with stable amounts of Fe. Plasma nitriding of Hastelloy CW2M produced a homogeneous layer with high hardness. X-ray diffraction revealed a γ (fcc) phase for the un-nitrided CW2M. The nitriding performed at 450 °C yielded chromium and molybdenum nitride peaks and an expanded phase. At 500 °C, an increase on the chromium and molybdenum nitride peaks was observed. The wear of samples nitrided at 450 and 500 °C were almost 10 times lower than the substrate’s wear, which indicates a great improvement on the tribological properties. The corrosion parameters showed that although the corrosion potential goes toward positive potentials, by increasing nitriding temperature, the corrosion current moves in the opposite direction. Nitriding at 450 °C promoted good corrosion protection with more positive open circuit and corrosion potentials.