RESUMO
Ni-Mo alloy coatings were deposited on a copper base material from a non-aqueous plating bath based on a deep eutectic solvent (DES) of choline chloride and propylene glycol in a 1:2 molar ratio containing 0.2 mol dm-3 NiCl2 · 6H2O and 0.01 mol dm-3 (NH4)6Mo7O24·4H2O. Uniform and adherent Ni-Mo deposits with a nodular morphology were obtained at all the deposition potentials investigated (from - 0.5 to - 0.9 V vs. Ag). By shifting the potential from - 0.5 to - 0.9 V, the deposition current density increased from - 0.4 to - 1.5 mA cm-2 and the overall surface roughness increased. It was also accompanied by an increase in the Mo content from ~ 7 to ~ 13 wt% in the potential range from - 0.5 to - 0.7 V. A further change in the potential from - 0.8 to - 0.9 V caused a decrease in the Mo content to ~ 10 wt% and a deterioration in the quality of the coating. For the most uniform coating, deposited at - 0.6 V and having a thickness of ca. 660 nm, the crystallite size did not exceed 10 nm. With the content of Ni (89 at.%) and Mo (11 at.%), the selected area electron diffraction (SAED) analysis allowed us to identify the cubic phase Ni3.64Mo0.36. The corrosion resistance of Ni-Mo coatings in 0.05 mol dm-3 NaCl solution generally increased during exposure of 18 h, as evidenced by ever higher polarization resistance. Finally, regardless of the applied deposition potential, low corrosion currents (in the range of 0.1-0.3 µA cm-2) have been measured for the coatings. EIS revealed that charge transfer resistances were the highest (57-67 kΩ cm2) for coatings deposited at - 0.5 V, - 0.6 V and - 0.7 V. Further increase in the deposition potential in the negative direction was unfavorable.
RESUMO
Deep Eutectic Solvents (DESs) are "green" competitors for some conventional plating baths and electrolytes used for surface modification. Their use allows a material to be obtained with a structure different from that observed in conventional plating or finishing technologies. In this work the titanium anodizing process was investigated in a bath based on a choline dihydrogencitrate salt and oxalic acid (1 : 1 molar ratio) green solvent. Titanium anodized at the lowest voltage applied (10 V) was a deep yellow color, which turned to deep blue at 30 V. The surface morphology and topography of titanium, both anodized and untreated, were monitored by optical, scanning electron (SEM and HR-SEM) and atomic force (AFM) microscopy. Anodizing at 10 V produced a fine granular morphology of the oxide layer, while anodizing at 30 V led to the formation of a probably thicker and quite uneven oxide layer, characterized by a distinct and coarse granular morphology. The average size of the micro-nodules was higher than those at 10 V and porous structures have been also identified. According to X-ray photoelectron spectroscopy (XPS) the stoichiometric TiO2, regardless of the applied voltage during anodizing, was practically the only component of the oxide layer produced on titanium in the DES bath. At 10 V, the oxide layer was thicker (>10 nm) than the natural Ti passive layer (approx. 2.2 nm), which, apart from TiO2, also contained oxides of titanium at lower oxidation states, i.e. +2 and +3. Moreover, the XPS technique was supported by electrochemical impedance spectroscopy (EIS), especially in the context of the structure of the oxide layer and its interaction with a corrosive environment. The corrosion resistance of anodized titanium was assessed in 0.05 mol dm-3 solution of NaCl by the linear polarization resistance (LPR) technique and polarization curves. During interpretation of the impedance spectra, the layers produced by the anodizing process were described using the two-layer model. It was assumed that the inner layer formed directly on the surface of metallic titanium was responsible for the barrier properties (resistance of 2.8 MΩ cm2). The porous outer layer formed on it has a much lower corrosion resistance, i.e. 800-1300 Ω cm2.
RESUMO
Cerium molybdenum oxide hydrate microflakes are codeposited with nickel from a deep eutectic solvent-based bath. During seven days of exposure in 0.05 M NaCl solution, the corrosion resistance of composite coating (Ni/CeMoOxide) is slightly reduced, due to the existence of some microcracks caused by large microflakes. Multielemental analysis of the solution, in which coatings are exposed and the qualitative changes in the surface chemistry (XPS) show selective etching molybdenum from microflakes. The amount of various molybdenum species within the surface of coating nearly completely disappear, due to the corrosion process. Significant amounts of Ce3+ compounds are removed, however the corrosion process is less selective towards the cerium, and the overall cerium chemistry remains unchanged. Initially, blank Ni coatings are covered by NiO and Ni(OH)2 in an atomic ratio of 1:2. After exposure, the amount of Ni(OH)2 increases in relation to NiO (ratio 1:3). For the composite coating, the atomic ratios of both forms of nickel vary from 1:0.8 to 1:1.3. Despite achieving lower corrosion resistance of the composite coating, the applied concept of using micro-flakes, whose skeleton is a system of Ce(III) species and active form are molybdate ions, may be interesting for applications in materials with potential self-healing properties.