The Effect of Cerium Ions on the Structure, Porosity and Electrochemical Properties of Si/Zr-Based Hybrid Sol-Gel Coatings Deposited on Aluminum

Abstract

This study was focused on the synthesis and characterization of Si/Zr-based hybrid sol-gel coatings with and without the addition of cerium(III) ions. The coatings were deposited on aluminum aiming to act as an effective and ecologically harmless alternative to toxic chromate coatings. The chemical composition, structure, thermal properties and porosity of the non-doped and Ce-doped coatings containing various Zr contents were examined by Raman spectroscopy and photothermal beam deflection spectroscopy. The corrosion properties of the coated aluminum substrates were studied using AC and DC electrochemical methods in 0.1 M NaCl electrolyte solution. Barrier and protecting properties of the coatings were monitored upon long-term immersion in chloride solution using electrochemical impedance spectroscopy. The effect of cerium ions was twofold: on the formation of a more condensed Si−O−Zr network structure and on the formation of Ce-based deposits, which diminish the rate of cathodic reaction at the coating/metal interface. These effects acted synergistically and resulted in the creation of the coatings with effective barrier and active corrosion protection.

The Effect of the Methyl and Ethyl Group of the Acrylate Precursor in Hybrid Silane Coatings Used for Corrosion Protection of Aluminium Alloy 7075-T6

“…The CPE 1 and R po are related to the high frequency (f ) region corresponding to the coating/electrolyte interface; in other words, these parameters correspond to the coating resistance to water uptake at the outer interface resulting from the formation of ionically conducting paths across the coating (also referred to as pore resistance). The CPE 2 and R 2 are related to the low frequency region corresponding to the response of the inner layer adjacent to the metal/coating interface [8,11,14]. Fitted parameters including Q as the pseudocapacitance of CPE, R as the resistance and n as the CPE power are presented in Table 5.…”

“…By increasing the ratio between thermal initiator benzoyl peroxide (BPO) and MMA, the polymerization efficiency can be improved [8]. Some sol-gel coatings were doped in order to increase their cross-linking, increase adhesion to the metal substrate, or to improve scratch resistance and thermal stability [7,[9][10][11][12][13][14].…”

Contemporary Modes of Corrosion Protection and Functionalization of Materials

“…82 It is noteworthy, however, that an optimal content of cerium should be incorporated since too large concentration may destruct the structural integrity of the coating. 86,87 At the same time, it has to be assured that doping of the coating with cerium does not reduce the barrier properties of the coating, as cerium ions are bigger in size (1.978 Å) compared to silicon atoms (1.068 Å). Larger Ce addition can cause lattice deformation and formation of nanopores and consequently change dramatically the characteristics of the coating.…”

“…Larger Ce addition can cause lattice deformation and formation of nanopores and consequently change dramatically the characteristics of the coating. 86 Self-healing of the coating can be followed electrochemically by measuring the open circuit potential (E oc ) 86 or impedance at low frequency range. 87 Upon immersion of TMZ coating, E oc was cca.…”

Corrosion Inhibition

“…Employing a combination of electrochemical measurements and X-ray photoelectron spectroscopy (XPS), they are able to gain insight into the functionality of the inhibitive species. Driven by the imperative to remove chromate from coatings, the experimental study of Rodič et al [3] focuses on the impact of cerium ions on the structure and corrosion resistance of Si/Zr-based sol-gel coatings applied to aluminium. A combination of electrochemical probes and interfacial characterisation reveal the benefits of cerium addition through improving both barrier and active inhibition properties.…”