Correlating corrosion inhibition to grain size in electrodeposited Ni-18Co

This work describes corrosion studies on electrodeposited Ni-18Co bearing grain size in the nanocrystalline, ultra-fine-grained and microcrystalline range. The as-received Ni-18Co sheet had a nanocrystalline structure with an average grain size of 30 nm. Controlled annealing experiments at 500 degrees C and 700 degrees C provided material with average grain size values of 660 nm and 4.8 mu m, respectively. Corrosion studies were conducted on the as-received and annealed material in de-aerated 0.1 M H2SO4. An inhibitor tryptamine was also added to the electrolyte for selected experiments to understand grain size-corrosion resistance correlations in the presence of the inhibitor. The inhibitor was found to mildly enhance the corrosion resistance of the nanocrystalline material but had negligible effect on the corrosion behaviour of the 500 degrees C and 700 degrees C annealed samples. The i(corr) of the nanocrystalline material was 9 x 10(-6) A/cm(2) which reduced to 5.3 x 10(-6) A/cm(2) on the addition of the inhibitor. On the other hand, the i(corr) of the 500 degrees C and 700 degrees C annealed samples were 10 x 10(-6) A/cm(2) and 10.3 x 10(-6) A/cm(2), respectively, which reduced marginally to 9.1 x 10(-6) and 10.2 x 10(-6) A/cm(2) on the addition of the inhibitor. Microhardness measurements of the three materials revealed a reduction in the hardness following annealing with the hardness of the nanocrystalline material at 4100 MPa which reduced to 2250 and 1050 MPa for 500 degrees C and 700 degrees C annealed samples.