THE PHYSICS AND CHEMISTRY OF CHROMIUM (III) DEPOSITION

Abstract

The study of physics and chemistry of Chromium (III) deposition commenced on two selected complexants, sodium thiocyanate and malonic acid. The cathode current efficiency of sodium thiocyanate bath is higher than the malonato chromium (III) bath. However, the deposits show darker color and sulfur was detected as a major contaminant. The morphology of deposits were reviewed with their respective plating parameters. To determine the optimum solution composition and operational conditions for malonato-chromium (Ill) system, the effects of Cr(III)/ligand ratio, concentration of chromium complex, current density, pH and temperature on the current efficiency have been studied. With the optimum solution composition and operational conditions, silver white and smooth decorative chromium deposit was obtained. It is almost impossible to differentiate the color of the deposits from chromic acid bath. The role of additives in malonato-chromium bath was also reviewed through cyclic voltammetry study. Malonic acid serves as a complexing agent; sodium sulfate acts as a conductivity salt and boric acid is a buffering agent of the electrolytes. The mechanisms of chromium deposition were studied by cyclic voltammetry and it was suggested that Cr+3 + e → Cr+2 followed by two electrons transfer as Cr+2 + 2e → Cr⁰. The diffusion coefficient was calculated to be in the order of 10-5 cm 2/sec. The study of influence of pulse plating parameters indicates the activation overpotential becoming larger with increasing pulse current density thus producing fine grain deposits. It also revealed that the ON/OFF time ratio affects the grain size significantly, the higher ratio producing larger grain size of deposits. However, if trivalent chromium electrolyte contains no additives, it is impossible, to produce good metallic coating even by adopting pulse electrodeposition technique. Further study has been made on surface structure and impurities in chromium deposits as well as some physical properties. SEM micrographs revealed that the surface structure of thick chromium deposit from hexavalent and trivalent chromium are quite different. The former shows an unoriented dispersion type whereas the latter a striated type or globular cell structure. Surface composition analysis by EDX (Energy Dispersive X-ray Spectrometer) and WDX (Wavelength Dispersive X-ray Spectrometer) indicates that some organic liqancls are included in the coating from trivalent chromium baths. The malonato complex system is quite exceptional in that it has similar composition to the chromium deposits from the hexavalent chromium system. ESCA (Electron Spectroscopy for Chemical Analysis) study shows that chromium deposits obtained from hexavalent and trivalent chromium systems always contain oxides and chromium elements present in three chemical states. The oxygen is distributed throughout the deposits which is shown in WDX and ESCA spectrum. X-ray diffraction study indicates the crystal structure of both hexavalent and trivalent chromium have the same bbc structure with the lattice constant of 2.8793Å and 2.8973Å respectively. Corrosion and hardness evaluation of deposits show that the trivalent chromium deposit exhibits equally good corrosion resistance (vs Cr(VI) deposits) during the 144 hours of salt spray test but surface becomes tarnished if it was exposed to longer hours. Hardness of deposits from hexavalent and trivalent chromiums are different. The deposit of trivalent chromium shows lightly lower hardness as plated condition. However, hardness increases significantly after heat treating at about 400⁰C, whilst the hardness behaviour of hexavalent chromium deposit is reversed. Therefore this hardness behaviour is a unique characteristic of trivalent chromium deposits.