Fabrication of a pH microsensor for local pH measurement during chromium electrodeposition from a trivalent chromium-based electrolyte

Abstract

In this work, the pH conditions during chromium electrodeposition are investigated using a home-made pH microsensor and scanning electrochemical microscopy (SECM). Few studies have focused on measuring local pH during metal electrodeposition and none has been conducted during chromium electrodeposition. The novelty of this work is the study of the local pH during chromium electrodeposition in very acidic (pH < 1) and concentrated media (>1 mol·L−1). Along with several other determinants, pH is a key factor in achieving an even metallic deposit. pH buffers and complexing agents are often used for this purpose but in the vicinity of the electrode, at the electrode/electrolyte interface, strong variations are highly likely to occur. For this reason, a pH microelectrode has been developed to scan the pH gradient during the electrodeposition of chromium. This is based on an iridium oxide-modified microelectrode, since iridium oxide has been reported to be suitable for local pH sensing. The thermal treatment of iridium oxide (IrOx) has also been studied. The deposition of a Nafion layer on top of the oxide has also been shown to achieve good selectivity and good stability while maintaining a reasonable level of sensitivity. An optimized IrOx microelectrode was obtained and used as a SECM microelectrode to investigate the evolution of the pH at the electrode/electrolyte interface during electrodeposition. The results are promising and will enable us to further develop our understanding of chromium electrodeposition.