Experimental and DFT investigations on the influence of cetrimonium bromide (CTAB) on surface morphology and anti-tarnishing performance of pulse galvanostatically deposited gold coatings from a cyanide-free electroplating solution

Abstract

In the present study, pulse galvanostatic electroplating method is used to produce mirror-bright gold (Au) deposits from a non-cyanide electroplating solution. Electrodeposition is carried out from two different solutions in the presence and absence of cetrimonium bromide (CTAB) to study the influence of the quaternary ammonium surfactant on the microstructure and the anti-tarnishing behaviour of the Au deposits. Electrochemical studies (cyclic voltammetry (CV) and chronoamperometry) are carried out to get an insight into the type of nucleation and growth occurring during the electrodeposition of gold. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies show that in the presence of CTAB, fine-structured coating with a compact and uniform morphology is obtained. The X-ray diffraction (XRD) studies show that in the presence of cetrimonium bromide, the growth of Au is favoured along the (111) lower index crystal surface, and with no cetrimonium bromide in the electroplating bath, the growth of Au is favoured along the (200) higher index crystal surface. The influence of cetrimonium bromide on the resistance to the tarnishing of the Au coating is initially predicted by quantum chemical calculations based on density functional theory (DFT). Electrochemical impedance spectroscopy (EIS) and static immersion test also confirm the less severity of tarnishing for the coating electrodeposited from the solution containing cetrimonium bromide (CTAB). Visual observation and the glossiness analysis confirm the difference in the physical appearance of the coating surface in the as-deposited condition and after tarnishing. The findings of this study demonstrate that the electrodeposition of Au with a crystal surface control, by using CTAB as a bath additive, can be used as a plausible technique for enhancing the anti-tarnishing performance of the coatings.