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 °C and 700 °C provided material with average grain size values of 660 nm and 4.8 μm, 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 °C and 700 °C annealed samples. The icorr of the nanocrystalline material was 9 × 10−6 A/cm2 which reduced to 5.3 × 10−6 A/cm2 on the addition of the inhibitor. On the other hand, the icorr of the 500 °C and 700 °C annealed samples were 10 × 10−6 A/cm2 and 10.3 × 10−6 A/cm2, respectively, which reduced marginally to 9.1 × 10−6 and 10.2 × 10−6 A/cm2 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 °C and 700 °C annealed samples. © 2020, Qatar University and Springer Nature Switzerland AG.