Improvement of the corrosion behaviour of mild steel by deposition of Zn-Ni-Cu-Fe-Cd from an acetate sulphate bath

Abstract

Zn-Ni-Cu-Fe-Cd quinary alloy was successfully electrodeposited on mild steel from acetate-sulphate baths with only cathodic potential applied during linear sweep voltammetry. The influence of scan rate (1 mV/s, 5 mV/s and 10 mV/s) as well as the effect of organic additives: 4-(4-nitrophenyl)-3-thiosemicarbazide or 4-(3-methoxyphenyl)--3-thiosemicarbazide, on the surface appearance and deposit composition were investigated. Three electrochemical techniques, potentiodynamic polarisation (PDP), linear polarisation resistance (LPR) and electrochemical impedance spectroscopy (EIS) were used to investigate the corrosion behaviour of the alloy composition after immersion in a solution containing 1 M HCl. Energy dispersive X-ray (EDX) and scanning electron microscopy (SEM) were carried out after deposition to provide a visual aspect of the coating. The lowest corrosion current density, the highest corrosion resistance, and the highest impedance were observed in each alloy system at a scan rate of 1 mV/s. These values were: 67 mu A/cm-2, 51 mu A/cm-2, and 7.6 mu A/cm-2 for PDP, 965 Omega cm(2), 1117 Omega cm(2), and 2853 Omega cm(2) for LPR, and 1602 Omega cm(2), 12938 Omega cm(2), and 1595 Omega cm(2) for EIS, corresponding to coatings without additives, coatings with the additive 4-(4-nitrophenyl)-3-thiosemicarbazide, and coatings with the additive 4-(3-methoxyphenyl)-3-thiosemicarbazide, respectively. From the SEM and EDX results it was concluded that 4-(3-methoxyphenyl)-3-thiosemicarbazide has a more pronounced effect on the coating than 4-(4-nitrophenyl)-3-thiosemicarbazide, probably due to its better adsorption on the surface of soft steel.