Correlation Between Microstructure and Corrosion Properties of Electrodeposited Nickel and Zinc Based Coatings
Sai Jyotheender, Kompella
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Conventional metallic coatings such as Ni and Zn are extensively used in corrosion protection applications. In most studies on the corrosion behavior of metallic coatings, the emphasis is on correlating morphology and corrosion behavior. The focus on relating the microstructure (phase fraction and spatial location of phases), micro-texture, grain boundary constitution, and strain in the coatings with its corrosion performance remains relatively less explored. Corrosion initiates at the surface and propagates through the coating to reach the substrate-coating interface; therefore, the coating's microstructural attributes play a significant role in deciding the degradation kinetics. In this work, such correlation between microstructure and corrosion properties has been investigated for the following systems: Ni and Zn coatings electrodeposited using surfactants of different polarity, Ni and Zn coatings electrodeposited with carbon-nanotubes (CNTs) as a foreign additive, Ni-Co alloy coatings with different concentrations of Co, and Zn-Ni alloy coatings with different concentrations of Ni. In all the cases, a direct correlation between the coating micro-texture, grain boundary constitution, strain, and corrosion behavior was noticed. This is the main conclusion of this research work. The key observations were: (a) in the case of Ni coatings deposited using surfactants of different polarities (cationic (CTAB) surfactant, anionic (SLS) surfactant, and non-ionic (Triton X-100)) surfactant, highest corrosion resistance was obtained in case of anionic (SLS) surfactants containing coating because of lesser density of geometrically necessary dislocations (GNDs) and lower coating strain, (b) in the case of Zn coatings electrodeposited using different surfactants (cationic (CTAB) surfactant, anionic (SLS) surfactant, and non-ionic (Triton X-100) surfactant), highest corrosion resistance was observed for coating with cationic (CTAB) surfactant due to a low fraction of high angle grain boundaries (HAGBs), a high number of low energy special boundaries, and relatively lesser defective morphology, (c) in the case of Ni-CNT composite coatings produced form electrolyte bath containing different concentrations of dispersed CNTs, highest corrosion resistance was observed for Ni coating produced from an electrolyte bath containing 50 mg/L of CNTs, this was due to high coincidence site lattice (CSL) fraction (greater than 40%) when compared to the other Ni-CNT composite coatings (d) in the case of Zn-CNT composite coatings produced form electrolyte bath containing different concentrations of dispersed CNTs, highest corrosion resistance was observed for Zn-CNT coating produced form the electrolyte bath containing CNT in concentration of 8 mg/L. This was due to the presence of low energy basal plane texture, low fraction of HAGBs and a high number of low energy special boundaries, (e) in the case of electrodeposited Ni-Co coatings with different concentrations of Ni, lowest corrosion rate was observed in the case of Ni-20 wt.% Co due to uniform distribution of Ʃ3 CSLs along the coating cross-section, (f) in the case of Zn-Ni alloy coatings containing different minor concentrations of Ni (1-4 wt%), high corrosion resistance was observed in case of Zn-1.34 wt.% of Ni due to the presence of γ-phase at the grain boundary regions and less micro-strain in coatings.