On the evolution of protective oxide chemistry and corrosion resistance of electrodeposited FeCrMnNiCo-carbon nanotube composite coatings

Abstract

ABSTRACT Reinforcement of carbon nanotube (CNT) into FeCrMnNiCo High Entropy Alloys (HEA) was carried out using electrodeposition method. Different volume fractions of functionalised CNTs (0.5, 1.0, 5.0, 10.0, 20.0, 30.0 mg/L) were dispersed in the aqueous electrolyte bath to synthesise HEA-CNT composite coatings. Effect of CNT addition on the phase constitution, morphology, composition, roughness, contact angle and ultimately on corrosion resistance of the pristine coating was investigated. The study revealed that the corrosion behaviour of the composite coatings was highly sensitive to the volume fraction of CNTs in the coatings. The change in the corrosion rate was, however, not monotonic and it was observed that the corrosion rate decreased progressively with CNT addition reaching a minimum followed by a sharp increase in corrosion rate thereafter. The protection efficiency of pristine coating was enhanced remarkably by 74.27% with the addition of an optimum amount (10 mg/L) of CNTs. The enhanced corrosion resistance was due to fine-grained morphology, reduced surface roughness from 1194. 9 ± 3.6 nm for pristine coating to 721.3 ± 1.8 nm for the highest corrosionresistant coating, enhanced hydrophobicity with a tremendous increase in water contact angle from 85.2° to 152.8°. Additionally, the dominance of more stable oxides of chromium (Cr2O3) and nickel (NiO) on the passive oxide layer made the coating less susceptible to corrosive media. Beyond the optimum CNT volume fraction, the drastic deterioration in corrosion-resistance observed beyond the optimum was attributed to increased surface defects in the forms of cracks due to the incorporation of agglomerated CNTs into the coating matrix.