Corrosion protection mechanism of Ce4+/organic inhibitor for AA2024 in 3.5% NaCl

Cerium is a rare earth element that has been widely proposed for the corrosion protection of aluminium alloys (AA). Both cerium salts, Ce³⁺ and Ce⁴⁺, have been used in combination with other compounds to offer synergistic inhibition, however, the inhibitive corrosion mechanism when using Ce⁴⁺ with organic compounds is still not clear. In this study, the synergistic inhibition effect of Ce⁴⁺ and melamine (M) on the corrosion of aluminium alloy 2024 (AA2024) in 3.5% NaCl solution was investigated. Potentiodynamic Polarization (PDP) and Electrochemical Impedance Spectroscopy (EIS) techniques were used to study the synergistic effect of different Ce⁴⁺/M ratios on the corrosion behaviour of AA2024. The PDP study showed that a combination of 50% Ce⁴⁺ and 50% M leads to the lowest corrosion rates, both acting as cathodic inhibitors. Both PDP and EIS results indicated that M or Ce⁴⁺ in isolation did not offer effective corrosion protection, while the combination of M and Ce⁴⁺ significantly enhances the corrosion protection with a synergism parameter equal to 3.5. SEM and EDX observations confirm the findings from the electrochemical techniques. XPS was used to investigate the mechanism of protection, revealing that the reduction of Ce⁴⁺ to Ce³⁺ occurs during protection of AA2024. A new mechanism of corrosion synergistic inhibition by Ce⁴⁺ and organic compounds is postulated where the role of the organic compounds is to enhance the reduction of Ce⁴⁺.

Proposed corrosion protection mechanism of Ce⁴⁺/ organic inhibitor for AA2024.

Unusual kinetics of poly(ethylene glycol) oxidation with cerium(iv) ions in sulfuric acid medium and implications for copolymer synthesis

Complex aqueous phase equilibria influence the kinetics of polyalcohol–cerium(iv) reaction in sulfuric acid medium. Zeroth-order kinetics, not previously reported in such systems, results for certain sulfate ion concentrations.

Oxidation of benzoic acid by electrochemically generated Ce(IV)

This paper presents a study of the oxidation of benzoic acid in aqueous solutions by Ce(IV), which functions as an electrogenerated mediator. In this reaction, benzoic acid is oxidized to CO2, and the reaction displayed pseudo-first-order kinetics for the conditions 0.15-0.45 mM Ce(SO4)2, 0.5-3.0 mM benzoic acid, and 0.2 M H2SO4 at 25 degrees C. The k1 values were in the range 0.40 x 10(-4)-0.59 x 10(-4) s(-1) and increased with temperature within the range studied, 25-45 degrees C. The activation parameters for the reaction, deltaH++, deltaG++, and deltaS++, were determined to be 10.5 kcal/mol, 23.3 kcal/mol, and -43 cal/(mol K), respectively. The electrolysis of Ce(III) in a stirred solution at 25 degrees C and with 1.5 V constant potential generated a 96% Ce(IV)-4% Ce(III) distribution in 140 min. The k for the generation of Ce(IV) was 0.0003 s(-1). Electrolysis under similar conditions but with a large excess of benzoic acid in solution produced a steady-state conversion of 87% Ce (IV) and the corresponding k was 0.00027 s(-1). This reduction in k is consistent with the relative magnitudes of the oxidation reactions for Ce(III) and benzoic acid. Correspondingly, the electrolysis of benzoic acid (0.594 mM) in a 0.2 M H2SO4 solution containing 0.25 mM Ce2(SO4)3 was accomplished by applying 1.5 V to the stirred solution at 25 degrees C. The benzoic acid concentration was reduced to 0.026 mM, or by 96%, in 456 h. In this experiment the potential was applied for 3 h and then turned off for 48 h, and the sequence was repeated 8 times.