Electrochemical and microscopic investigation on passive behavior of ductile iron in simulated cement-mortar pore solution

Abstract Electrodeposition and stripping of Zn, Mn and Ni metals and their alloys were investigated in an aqueous sulfate solution. The effect of coating processes and its impact on the substrate were evaluated by investigating the prepared alloys with X-ray diffraction analysis, scanning electron microscopy and atomic absorption spectroscopy. The electrodeposition mechanism of the Zn-Ni-Mn alloy was found to be an anomalous type. The high corrosion resistance of the Zn-Ni-Mn alloy, as well as its preferable surface appearance, has been ascertained compared with the electrodeposited Zn-Ni alloy under homogeneous conditions. It was found that Mn can be codeposited with Ni or with Ni and Zn understudied conditions where it does not deposit in its pure form. During electrodeposition of manganese alone or manganese alloys, a dark brown deposit was formed on the counter platinum electrode surface as a result of oxidation to manganese dioxide.

Fabrication of Zn-Ni-Mn alloy by electrodeposition and its characterization

Codeposition of Zinc with Manganese from Different Gluconate Baths

The aim of this investigation is to evaluate the influence of glycerol (a nontoxic additive) on Zn-Mn coatings formed by electrochemical deposition. The influence of glycerol (GLY) on the electrodeposition of Zn-Mn alloys on Pt from acid baths was studied by cyclic voltammetry. The coatings were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The Zn-Mn alloys prepared in the absence or presence of this additive showed similar voltammograms except when using GLY and boric acid (BA) concentrations at 0.63 and 0.24 M, respectively. The current efficiency of the Zn-Mn electrodeposition process ranged from 5 to 41%, due to the lowest and highest concentrations of GLY and BA, respectively. SEM images of deposits obtained at -1.53 V from baths containing 0.08-0.48 M GLY were dendritic, while those formed in the presence of 0.63 M GLY showed that GLY acted as a brightener at this concentration. The XRD analysis showed that the electrodeposits contained Zn, Mn, oxides as well as alloys of various compositions. GLY acted as a grain refiner and inhibited the codeposition of Zn with Mn and the formation of oxides.

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The influence of glycerol as an additive in Zinc-Manganese alloy coatings formed by electrodeposition

Zinc-manganese alloy coatings were electrodeposited using sulfate baths in the presence of a superimposed magnetic field up to 1 T parallel to the electrode surface. The effect of ratio [Zn2+]/[Mn2+] on the elaborated coatings has been investigated using chronoamperometric measurements with potentiostatic technique. Crystallographic structure, morphology, roughness, and chemical composition of the coatings have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy analysis (EDXS), and profilometry. XRD diffraction method revealed that the coatings consisted of zinc (η phase) and/or epsilon phase (MnZn3). It has been found that the electrolytic bath with higher manganese ion concentration and lesser zinc ion concentration promotes the formation of MnZn3 phase. Moreover, the magneto-hydrodynamic-convection (MHD) generated by the magnetic field superimposition favors the diffusion of zinc and manganese species on the working electrode surface, the electrochemical kinetics, and the content of manganese in the deposit.

Zn-Mn alloy coating elaboration by magnetoelectrodeposition

In order to improve the protective performance of Zn coating on reinforcing steel in concrete, the electrochemical deposition of Zn–Mn coatings was conducted on steel surface. The morphology, chemical and phase compositions of Zn–Mn coatings obtained from sulfate–citrate bath were investigated in the first part of paper. In the second part, the obtained deposits were tested in solution simulating carbonated concrete, consisting of NaHCO3 and Na2CO3. Data obtained from Tafel analysis showed higher corrosion resistance for Zn–Mn alloy deposits obtained at –1700 and –1800mV versus SCE, when compared to pure Zn deposit. Impedance spectroscopy investigations revealed that the total impedance of Zn–Mn coatings increased steadily with time, and was significantly higher as compared to pure Zn after 24h in corrosion solution. On the contrary, for pure Zn, the impedance increased in the first 12h, and then decreased during prolonged exposure time, which can be explained by rapid growth of nonprotective white rust and the degradation of zinc coating, as was confirmed by optical microscope after 24h of immersion in carbonated concrete pore solution.

A. Legat

Papers

THE ELECTROCHEMICAL BEHAVIOR OF Zn–Mn ALLOY COATING IN CARBONATED CONCRETE SOLUTION