Electrodeposition of mixed chromium metal-carbide-oxide coatings from a trivalent chromium-formate electrolyte without a buffering agent

The electrodeposition of mixed chromium metal-carbide-oxide coatings on low carbon mild steel from a trivalent chromium-formate electrolyte without a buffering agent is investigated at a rotating cylinder electrode enabling precise control of the mass flux. At equilibrium conditions, i.e. in the bulk of the electrolyte with pH 2.3, Cr(III) mainly exists in the form of [Cr(HCOO)(H2O)(5)](2+). A deposition mechanism is proposed based on a fast, stepwise deprotonation of the water ligands in the Cr(III)-complex ion induced by a surface pH increase due to the hydrogen evolution reaction, which is controlled via the applied current density. Three different regimes can be defined related to the current density and mass flux. At low current densities no deposit is formed on the electrode, because soluble [Cr(HCOO)(OH)(H2O)(4)](+) is formed at the electrode (regime I). Above a certain threshold value of the current density Cr(HCOO) (OH)(2)(H2O)(3) is deposited on the electrode (regime II). A part of the Cr(III) of the deposit is reduced to Cr-metal and formate is broken down leading to the formation of Cr-carbide. The amount and composition of the deposit in regime II strongly depend on the applied current density, mass flux and electrolysis time. At high current densities, a further shift of the acid-base equilibrium to [Cr(HCOO)(OH)(3)(H2O)(2)] results in a deposit on the electrode that is mainly composed of Cr-oxide (regime III). In stark contrast to regime II, the amount and composition of the deposit in regime III are almost invariant of the applied current density, mass flux and electrolysis time. (C) 2015 Elsevier Ltd. All rights reserved.