Cobalt-phosphorous coatings with tunable composition fabricated by additive-controlled electrodeposition from choline chloride-ethylene glycol deep eutectic solvent for anti-corrosion application

Abstract

Cobalt-phosphorous (CoP) coatings are promising alternatives for hard chromium due to their outstanding corrosion protection. An excellent CoP coating can efficiently reduce the effects of a corrosive environment. Herein, CoP coatings with a tunable P content, ranging from 5.1 at.% to 20 at.%, are fabricated via a simple electrochemical deposition approach from choline chloride-ethylene glycol (Ethaline)-based deep eutectic solvent (DES) with saccharin (SA) as an additive. The rational introduction of SA induced the formation of close-growing CoP coatings rich in P on the substrate with reduced grain size and surface roughness. It turned out that the crystal structure of the deposited CoP coatings changed from a crystallized state to an amorphous state upon the addition of SA, which accelerated the nucleation rate of the nanocrystals and modified the nucleation model from progressive to instantaneous. The optimal CoP coating prepared exhibited excellent anti-corrosion and barrier protection properties for mild steel in corrosive NaCl solution, functioning up to 10 times better than its counterparts. Our work reveals the central role of additives in tailoring the structure and composition of the deposited CoP coatings in Ethaline-based DES and offers a new avenue for fabricating high-performance coating materials for anti-corrosion applications.