Microstructure and corrosion behavior of Cr and Cr-P alloy coatings electrodeposited from a Cr(III) deep eutectic solvent

Cr and Cr-P coatings were electrodeposited on Fe substrates from non-aqueous deep eutectic solvent-based electrolytes containing Cr(III). The optimized deposition parameters for the coating process were explored. A two-step process of Cr(III) reduction occurred, i.e. Cr(III) -> Cr(II) -> Cr(0), and the controlling step was promoted by adding NH4H2PO2. It was found that an electro-brush plated Ni underlayer was essential to obtain a smooth and compact Cr or Cr-P coating on the Fe substrate. The structure and composition of the as-deposited coatings were thoroughly analyzed. The corrosion behavior of the Cr and Cr-P coatings is quite different in 3.5 wt% NaCl and 0.1 M H2SO4 solutions. The diplex effects of the layered structures and ion-selective components in the as-prepared Cr-based coatings are suggested to be responsible for the different corrosion mechanism in different corrosion media.