Thick Chromium Electroplating From Trivalent Chromium In Sulfate System

As a functional coating, chrome is often used to provide high hardness and achieveenhanced wear resistance in performance. Traditional hexavalent chromiumelectroplating is too toxic, so trivalent chromium technique is used as an alternative. Inchloride system, trivalent chromium electroplating produces chlorine which is harmfulto environment and health. Comparatively speaking, the sulfate trivalent chromiumelectrodeposition has less negative environment and health impact.It is of high importance to develop a trivalent bath for continueous electroplatingdue to thickness increasing problem of the trivalent chromium. This paper studies theperformance of ammonium formate and carboxylic acid （SS I） on coating thickness andbrightness range in different chromium sulfate concentrations and the optimumconcentration of chromium sulfate is selected. The paper also finds the ammoniumformate, carboxylic acid （SS I）, pH value, bath temperature and current density have thesignificant impacts on coating roughness, which results in the determination of theoptimum process.Sodium formate and carboxylic acid （SS I） system is developed to increasedeposition speed. The deposition rate is reached to0.76μm/min without ammoniumsulfate, but the speed decreases rapidly with enhanced brightness when ammoniumsulfate is added into the bath. Bath composition and process conditions have a greatinfluence on the coating thickness and brightness range in sodium formate system Andprocess condition influencing factors are compared in ammonium formate and sodiumformate systems. In order to improve the continuous plating capability of the bath, acomposite buffer is developed for electroplating at room temperature. A thickness of20μm bright chromium coating is achieved with the optimum sodium formate,carboxylic acid （SS II） concentrations and proportioned composite buffer obtained inthe orthogonal test and single factor experiment.A large number of hemispherical protrusions can be seen in the picture of SEM andthe protrusions grow bigger as the electrodeposition time extends. The protrusionsdisappear after heat treatment with the advent of micro-cracks, and the coatinghardness is enhanced to835HV at200°C. XRD pattern indicates that the electroplatedcoating is an amorphous structure which is crystallized after heat treatment at200°C.Equilibrium shifting method is employed to determine the proportion of trivalentchromium with ammonium formate, carboxylic acid （SS I） and carboxylic acid （SS II）,and the stability constants are calculated. The distance between Cr³⁺and complexingagents are simulated and calculated through Material Studio after they are coordinated.which provides a basis for choosing complexing agents. According to the calculationof MS, ammonium ion shows different coordination distances in sulfate and chloride systems due to the roles the hydrogen bond plays in the solutions, which explains thedifferent functions of ammonium ion in the two systems. This study also indicates thedensity impacts of ammonium formate, sodium formate and carboxylic acid （SS I） onthe cathodic polarization curves. The cyclic voltammetry test verifies that chromiumreduction process is divided into two steps, Cr³⁺â†'Cr²⁺and Cr²⁺â†'Cr. carboxylic acid（SS I） makes the potential of the second reduction step negative shift, so that itaccelerates hydrogen evolution while the evolution of hydrogen decreases the currentefficiency. The electrochemical impedance spectroscopy verifies that trivalentchromium reduction is divided into two steps and in the reduction process appears theadsorption of the active substance which is present in the form of inductance in theequivalent circuit.