Abstract

To avoid corrosion problems, different methods have been used such as protective varnishes, corrosion inhibitors and coatings. Compared to zinc coatings, the use of zinc alloy coatings has shown better corrosion resistance. However, their long-term protection remains a challenge where pre-treatments of the conversion surface are suggested. Chromate conversion processes show the best results with regard to corrosion of galvanized steels, but the use of Cr (VI) generate environmental problems as hexavalent chromium is toxic, carcinogenic and mobile in the environment. One of the very promising alternative passivation treatments uses rare earth oxides, especially cerium oxide. The passivation treatment is carried out by immersion in cerium salts solutions or by cathodic or anodic deposition. In the case of cathodic deposition”CELD”, the formation of thin films is generally associated with the formation of cerium oxides or hydroxides at the cathode sites of the metal surface. Our study was developed in this context.
In this work we studied the effect of experimental parameters related to the electrodeposition process (current density) and those related to the deposition bath (temperature, additive concentration and the solution pH) on the electrodeposition of the cerium oxide coatings on electrogalvanized steel from 0.01 M cerium chloride solution. The feasibility and the kinetics of deposit formation were studied using voltammetric and chronopotentiometric curves. The surface morphology, topography and microstructures of the coatings were characterized by optical microscopy (OM), scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) and white light optical profilometry (WLI). The electrochemical characterization of the deposits was evaluated in an aggressive medium (0,5 M NaCl) by using d.c polarization (linear polarization resistance, Tafel curve) and non-stationary techniques (EIS electrochemical impedance spectroscopy).
The obtained results show that the cerium oxide coatings greatly improve the corrosion resistance of the electro-galvanized steel substrate. This resistance has significantly increased by adding an organic additive to the deposition solution which in our case is acetic acid (HAc). Different HAc concentrations were tested (10-3, 10-2, 10-1, 1 and 2 M) where the structural and electrochemical analyzes revealed that the coatings prepared from an equimolar mixture of cerium chloride 10-2 M and 10-2 M acetic acid at pH 6 and 25°C were less cracked, with the smallest particle size and offered the best corrosion resistance. The influence of the deposition current density, was tested in a range of densities between 0.25 mA/cm2 and 3 mA/cm2. The best results were obtained with the current density about 1.5 mA/cm2. The third parameter in our study is the temperature of the deposition bath. In this study we used a temperature range of 15°C to 55°C with a step of 10°C. The results obtained were in perfect agreement with numerous previous studies which showed that the ambient temperature (in our case 25°C) gives the best results. Then, the prepared coatings at the three optimized study parameters: equimolar solution (10-2 M CeCl3 + 10-2 M HAc), current density 1.5 mA/cm2 and 25°C with a deposition time of 20 min (optimized elsewhere previous studies) and pH 6, were exposed to an aggressive medium 0.5 M NaCl for a period of 30 days in order to evaluate their effectiveness against corrosion. The results of the d.c. polarization techniques and EIS showed that tested coatings were able to resist the entire immersion period where the both surface morphology and electrochemical results confirm the existence of the cerium oxide film deposited during all the test period and higher polarization resistance compared to the bare substrate even after 15 days of immersion was found. In addition, raising the pH to 6 with the same solution and the same current density and temperature conditions resulted in a better coating free from corrosion products.
Key words:
Electrogalvanized steel, cerium oxide coating, acetic acid, polarization, SEM, WLI, EIS.

Information

Item Type:	Thesis
Divisions:	» Laboratory of Physics for Matter and Radiation » Faculty of Science and Technology
ePrint ID:	2725
Date Deposited:	2021-09-19
Further Information:	Google Scholar
URI:	https://www.univ-soukahras.dz/en/publication/article/2725

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