A critical look at interpretation of electrochemical impedance spectra of sol-gel coated aluminium

A proper assignment of time constants is important for the correct interpretation of the electrochemical impedance spectra of coated substrates. In this work, several model aluminium samples with different anodic oxide thicknesses were prepared and characterised. The aluminium samples were coated by a hybrid sol-gel formulation and studied by electrochemical impedance spectroscopy (EIS) during immersion in NaCl solution. A hierarchical model containing three R-C elements adequately described the impedance spectra of the coated samples. The R-C elements described the properties of sol-gel coatings, aluminium oxide layers and corrosion process. The contribution of the aluminium oxide layer to impedance was unambiguously proven to be at middle frequencies on the spectra. Furthermore, the “real” capacitances of oxide layers were obtained from two capacitance–CPE equations and their applicability was scrutinized. The obtained results presented an opportunity to determine the adhesion behaviour of the sol-gel coatings to the oxide layers, which was studied by analysing the ratio of the oxide layer capacitance of the sol-gel coated samples to the capacitance of the uncoated ones. It was demonstrated that the exposed surface area of the oxide to the electrolyte was about 60–68% and was not changing during immersion. Based on these findings, a degradation model of the sol-gel coating on anodised aluminium was proposed.publishe

Influence of sol-gel process parameters on the protection properties of sol-gel coatings applied on AA2024

The present work explores effects of shelf-life, temperature and the presence of cerium nitrate additive on the protective performance of sol-gel coatings applied on AA2024. These are important parameters when industrial applicability of coating formulations is considered. The studied sol-gel system is based on a combination of metalorganic precursor titanium(IV) propoxide and hybrid epoxy silane 3-glycidoxypropyltrimethoxysilane. The synthesized sol-gel formulations without or with addition of 0.007 M Ce(NO3)(3) were aged during different times at temperatures 25 degrees C or 6 degrees C. FTIR results show that the intensities of the bands associated to Si- O-Si increase with respect to Si-OH groups during the aging process as a result of extended condensation. However, such transformations occur less rapidly at low temperature. The electrochemical impedance spectroscopy measurements clearly show that sol-gel coatings corresponding to the sol-gel aged at low temperature exhibit similar corrosion protective capabilities compared to the coating obtained without sol-gel aging. It is also observed that the protection performance of the sol-gel coatings with cerium nitrate obtained after two weeks of sol-gel aging is lower compared to the coatings without cerium additive. This effect is attributed to the enhanced reactivity of the sol-gel due to cerium(III) presence. (C) 2014 Elsevier B.V. All rights reserved

Active Corrosion Protection by Nanoparticles and Conversion Films of Layered Double Hydroxides

The active corrosion protection of metallic substrates can be achieved by the addition of corrosion inhibitors to protective coatings. However, direct mixing of inhibitors with coating formulations can lead to important drawbacks, decreasing barrier properties of the coating and diminishing activity of the inhibitor. Nanocontainers for corrosion inhibitors were developed in this work using layered double hydroxides (LDH) as nanocarriers. The LDH were used as nanosized particles, which are then included in a sol-gel hybrid coating, and as a LDH layer directly formed on the metal surface (conversion film) and covered by a sol-gel hybrid layer. Electrochemical impedance spectroscopy (EIS) has been used for evaluation of corrosion protective properties of the coatings. The results show that the LDH with incorporated vanadate inhibitor provide efficient active corrosion protection in both cases

One-step synthesis and growth mechanism of nitrate intercalated ZnAl LDH conversion coatings on zinc

An approach for the synthesis of ZnAl-NO3 LDH conversion coatings on zinc in an aqueous acidic Al(NO3)3/NaNO3 solution is demonstrated for the first time. The growth mechanism has been investigated using time resolved structural, microstructural and analytical methods. A LDH growth model involving both electrochemical and chemical processes is suggested.publishe

Active corrosion protection coating for a ZE41 magnesium alloy created by combining PEO and sol-gel techniques

An active protective coating for ZE41 magnesium alloy was produced by sealing an anodic layer, loaded with 1,2,4-triazole, with a sol-gel film. An anodic oxide layer was formed using PEO in a silicate-fluoride alkaline solution. This thin (1.8 mu m) porous PEO layer was impregnated with corrosion inhibitor 1,2,4-triazole and sealed with a silica-based sol-gel film modified with titanium oxide. For the first time it was demonstrated that this relatively thin PEO-based composite coating revealed high barrier properties and provided superior protection against corrosion attack during 1 month of continuous exposure to 3% NaCl. A scanning vibrating electrode technique showed a sharp decrease (100 times) of corrosion activity in micro defects formed in the 1,2,4-triazole doped composite coating, when compared to blank samples