Effect of grain boundary relaxation on the corrosion behaviour of nanocrystalline Ni-P alloy

Abstract

Grain boundary relaxation (GBR) in nanocrystalline materials is an approach to reduce the excess free energy of the material without any change in the grain size of the material and is carried out through controlled heat treatments at relatively low temperatures. In this work, we are investigating the effect of grain boundary relaxation on the corrosion behaviour of nanocrystalline Ni-P, a topic which has not been explored before. Nanocrystalline Ni-P foils were produced using the pulsed electrodeposition route using the modified Watts bath and were characterized by X-Ray Diffraction and Transmission Electron Microscopy. Differential scanning calorimetry (DSC) characterization technique was used to establish the GBR regime, and the same was validated by measuring the grain size post the grain boundary relaxation treatment. Once the relaxation regime was established, systematic heat treatments at a temperature of 473 K were carried out, which enhanced the hardness of the nanocrystalline material without any change in the initial grain size. Corrosion studies using potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) test were performed to understand the effect of GBR. The corrosion studies revealed an improvement in corrosion resistance of the relaxed structure vis-à-vis the as deposited nanocrystalline Ni-P. An increase in Ecorr and a decrease in icorr and ip were found for the relaxed structure. The EIS tests too revealed an improvement in the polarization resistance for the relaxed structure.