XANES investigation of the role of cerium compounds as corrosion inhibitors for aluminum

This paper reports work aimed at finding an alternative treatment to chromate conversion coatings (CCC) that could meet all the requirements of industrial applications. A first step was made by carrying out the deposition of films of CeO 2 ·2H 2 O on aluminium alloys in a few minutes at room temperature with or without a catalyst. Different types of pretreatment (acidic or alkaline) led to drastically different results. A model for the catalytic deposition mechanism is suggested which is consistent with scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) observations. The anticorrosive properties of these coatings were investigated by potentiokinetic curves measurements and by electrochemical impedance spectroscopy (EIS). Encouraging results were obtained though the performances still do not equal those of CCC. Some of the coating features (adhesion, sealing of the anodic sites, etc.) which should still be improved are highlighted.

Study of the deposition of cerium oxide by conversion on to aluminium alloys

Effect of cerium pretreatment on the corrosion inhibition characteristics of AA6061 T6 alloy-10% Al 2 O 3 metal-matrix composite was investigated in aerated 3.5% NaCl solution after different immersion times ranging between 1 and 60 days at room temperature. Series of specimens were prepared under the following conditions: as-polished; directly treated in CeCl 3 ; alkaline etching followed by CeCl 3 treatment; immersion in boiling water followed by CeCl 3 treatment; and alkaline etching followed by immersion in boiling water and then CeCl 3 treatment. Electrochemical impedance spectroscopy (EIS) was used to investigate the corrosion behaviour of treated and untreated specimens. The EIS was measured by perturbing the free corrosion potential of the specimens with 10 mV a.c. with a frequency decreasing from 10 mHz to 65 kHz. Polarization tests were used to investigate the perfect passivity domain after corrosion. Optical microscopy and SEM were used to investigate the occurrence of localized corrosion due to chloride ion attack. Specimens untreated, directly treated in CeCl 3 and alkaline etched followed by CeCl 3 treatment suffered from pitting as well as crevice corrosion after 30 days of immersion in NaCl. Conversely, the specimens after alkaline etching followed by oxide thickening in boiling water before cerium treatment showed promising corrosion protection even after 60 days of immersion in NaCl. The degree of protection depends to a high extent on the specimen preparation prior to cerium pretreatment.

Corrosion protection of aluminium metal–matrix composites by cerium conversion coatings

The effect of surface preparation prior to cerium treatment on the corrosion protection of AA6061 T6–10% Al2O3 in NaCl solution was investigated using electrochemical impedance spectroscopy and DC polarization techniques. A new surface preparation method consisted of pre-etching followed by oxide-thickening is proposed. Optical microscope and scanning electron microscopy showed a marked decrease in the number and depth of pits for the pre-etched as compared to the as-polished specimens. X-ray photoelectron spectroscopy revealed that pre-etching eliminated the active sites where the chloride ions can adsorb. Simultaneously, the adsorption of cerium ions is uniformly on the surface. The oxide-thickening step has a vital role in the protection mechanism. It was found that presence of a thick oxide layer completely changed the mechanism of protection.

Effect of surface preparation prior to cerium pre-treatment on the corrosion protection performance of aluminum composites

The sorption of Cs(I), Sr(II) and Co(II) from aqueous solutions on alumina under various experimental conditions has been studied by batch techniques. Freundlich, Langmuir and Dubnin-Raduskevich equations have been used to interpret the sorption data. The values of various thermodynamic parameters have been determined. The sorption of Cs(I) and Sr(II) on alumina is exothermic in nature while that of Co(II) is an endothermic process. The ΔHo values for Cs(I), Sr(II) and Co(II) were−23.29 KJ/mol at 298K,−35.3 KJ/mol at 293 K and 13.49 KJ/mol at 293 K, respectively. Negative values of ΔGo show the spontaneity of the sorption processes; ΔGo values of Cs(I) and Sr(II) becomes less negative at higher temperatures while the ΔGo values of Co(II) become more negative with increasing temperature. At higher temperatures, less amounts of Cs(I) and Sr(II) and greater amounts of Co(II) are sorbed on alumina. The values of the mean free energies of sorption,E, for Sr(II) and Co(II) at various temperatures were within the range of 7–10 KJ/mol which show that these metals are sorbed on alumina predominantly by an ion-exchange process.

Adsorption of Cs(I), Sr(II) and Co(II) on Al2O3

The sorption of microamounts of trivalent lanthanides (Ln3+) on freshly precipitated Al(OH)3 has been measured in dependence on pH and the time of sorption. Also, the influence of organic complexing ligands and inorganic electrolytes on the sorption process has been investigated. The mechanism of sorption is discussed. Freshly precipitated Al(OH)3 can be used for the preconcentration of microamounts of trivalent lanthanides. However, the preconcentration is not quantitative in the presence of high concentrations of complexing ligands (citrate and similar) which form strong complexes with Ln3+ ions.

S. Musić

Papers

Sorption of microamounts of Ce3+, Pm3+, Eu3+, Gd3+ and Yb3+ on aluminium hydroxide