Effect of pH on nucleation mechanism of Zn–Mn coatings electrodeposited at different deposition potential

Abstract

Zn-Mn alloy coatings were galvanostatically deposited from an acidic chloride bath at pH 2?0 and 4?9. Effects of pH on nucleation mode, surface morphology and corrosion resistance of Zn-Mn coatings were studied. Analysis of current-time transients indicated that the nucleation mode of Zn-Mn coatings at pH 2?0 is instantaneous. With increasing the pH to 4?9 at potentials of 21?08 and 21?59 V, the experimental data were observed to be near the theoretical instantaneous nucleation, but at higher potentials of 21?12 and 21?2 V, the progressive mode of nucleation was dominant. From AFM images, the average nuclei density N values for Zn-Mn deposition at pH 2?0 and 4?9 were 9?2610 9 and 10610 9 cm 22 respectively. At pH 2?0, this value was near that obtained by the model, but it was found to be far away at pH 4?9. Potentiodynamic polarisation indicated that the corrosion resistance of Zn-Mn coatings at pH 4?9 is slightly improved.

The Formation of Passive Layers on Zinc Based Platings

“…ZnFe plating was carried out using a bath based on 23.14 zinc sulfate (Zn-SO 4 Oxalate and tartrate layers were synthesized by means of cyclic voltammetry technique. These layers obtained in the 0.20 M sodium oxalate (NaOX) or 0.20 M sodium tartrate (NaTart) solutions were coated on ZnFe or ZnFeCo alloy deposited carbon steel electrode by applying a scan rate of 50 mV s -1…”

“…In particular, because electrodeposited ZnFe alloy coatings possess several superior properties such as excellent paintability, good welding properties and good corrosion resistance. [3][4][5][6][7][8][9][10] At the same time, zinc based coating surfaces are still coated with thin chromate or phosphate layers. There have been several studies on the application of chromatings and phosphate coatings to improve the corrosion resistance of certain coatings used in automotive industry, industrial pipes and household items.…”