Mikhail L. Zheludkevich

Sol–gel coatings for corrosion protection of metals

Sol–gel derived films exhibit a high potential as substitutes for the environmentally unfriendly chromate metal-surface pre-treatment methods. Inorganic sol–gel derived films offer good adhesion between metals and organic paint. However, they cannot provide adequate corrosion protection due to their high crack-forming potential. Introduction of an organic component to an inorganic sol–gel system leads to the formation of thicker, more flexible and functionalized films with enhanced compatibility to different organic top coatings. Incorporation of nanoparticles in the hybrid sol–gel systems increases the corrosion protection properties due to lower porosity and lower cracking potential along with enhancement of the mechanical properties. Furthermore, the incorporation of inorganic nanoparticles can be a way to insert corrosion inhibitors, preparing inhibitor nanoreservoirs for “self-repairing” pre-treatments with controlled release properties.

Enhancement of active corrosion protection via combination of inhibitor-loaded nanocontainers.

The present work reports the synthesis of layered double hydroxides (LDHs) nanocontainers loaded with different corrosion inhibitors (vanadate, phosphate, and 2-mercaptobenzothiazolate) and the characterization of the resulting pigments by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The anticorrosion activity of these nanocontainers with respect to aluminum alloy AA2024 was investigated by electrochemical impedance spectroscopy (EIS). The bare metallic substrates were immersed in dispersions of nanocontainers in sodium chloride solution and tested to understand the inhibition mechanisms and efficiency. The nanocontainers were also incorporated into commercial coatings used for aeronautical applications to study the active corrosion protection properties in systems of industrial relevance. The results show that an enhancement of the active protection effect can be reached when nanocontainers loaded with different inhibitors are combined in the same protective coating system.

Novel Inorganic Host Layered Double Hydroxides Intercalated with Guest Organic Inhibitors for Anticorrosion Applications

Zn-Al and Mg-Al layered double hydroxides (LDHs) loaded with quinaldate and 2-mercaptobenzothiazolate anions were synthesized via anion-exchange reaction. The resulting compounds were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy/energy-dispersive X-ray spectroscopy. Spectrophotometric measurements demonstrated that the release of organic anions from these LDHs into the bulk solution is triggered by the presence of chloride anions, evidencing the anion-exchange nature of this process. The anticorrosion capabilities of LDHs loaded with organic inhibitors toward the AA2024 aluminum alloy were analyzed by electrochemical impedance spectroscopy. A significant reduction of the corrosion rate is observed when the LDH nanopigments are present in the corrosive media. The mechanism by which the inhibiting anions can be released from the LDHs underlines the versatility of these environmentally friendly structures and their potential application as nanocontainers in self-healing coatings.

Hydroxyapatite microparticles as feedback-active reservoirs of corrosion inhibitors.

This work contributes to the development of new feedback-active anticorrosion systems. Inhibitor-doped hydroxyapatite microparticles (HAP) are used as reservoirs, storing corrosion inhibitor to be released on demand. Release of the entrapped inhibitor is triggered by redox reactions associated with the corrosion process. HAP were used as reservoirs for several inhibiting species: cerium(III), lanthanum(III), salicylaldoxime, and 8-hydroxyquinoline. These species are effective corrosion inhibitors for a 2024 aluminum alloy (AA2024), used here as a model metallic substrate. Dissolution of the microparticles and release of the inhibitor are triggered by local acidification resulting from the anodic half-reaction during corrosion of AA2024. Calculated values and experimentally measured local acidification over the aluminum anode (down to pH = 3.65) are presented. The anticorrosion properties of inhibitor-doped HAP were assessed using electrochemical impedance spectroscopy. The microparticles impregnated with the corrosion inhibitors were introduced into a hybrid silica-zirconia sol-gel film, acting as a thin protective coating for AA2024, an alloy used for aeronautical applications. The protective properties of the sol-gel films were improved by the addition of HAP, proving their applicability as submicrometer-sized reservoirs of corrosion inhibitors for active anticorrosion coatings.

Self-healing protective coatings with “green” chitosan based pre-layer reservoir of corrosion inhibitor

A new type of corrosion protective self-healing coating is reported in the present study. The coating is constituted by a chitosan-based pre-layer deposited onto the metal surface and a barrier hybrid film. The chitosan film doped with cerium ions serves as a reservoir for the corrosion inhibitor. The cerium ions form a complex with the functional groups of chitosan macromolecules providing a prolonged release of the active agent on demand. The developed bi-layer protective coating was applied to aluminium alloy 2024, which is widely used in the aeronautical industry. The results obtained from electrochemical impedance spectroscopy clearly demonstrate a superior corrosion protection when the coating with the cerium-doped biopolymer pre-layer is used. The localized electrochemical study in micro-confined defects showed a well-defined self-healing ability of the developed coating system.

Nanostructured LDH-container layer with active protection functionality

Self-healing protective coatings based on different types of nanocontainers of corrosion inhibitors have been recently developed and reported as promising solutions for many industrial applications. Layered double hydroxide (LDH) nanocontainers are among the most interesting systems since they confer the delivery of inhibitors on demand under action of corrosion-relevant triggers such as the presence of corrosive anions or local change of pH. In this paper, we report for the first time a new approach based on formation of a nanostructured LDH-container layer directly on the metal surface as a result of conversion reaction with a metal (aluminium alloy 2024) surface. The methodology allows the formation of a spatially differentiated LDH structure preferentially on surface sites located at active intermetallics. The LDH container layer loaded with vanadate ions provides an efficient active corrosion protection and can be used as a functional layer in self-healing coating systems.

Feedback active coatings based on incorporated nanocontainers

Development of a new generation of multifunctional coatings, which will possess not only passive functionality but also active and rapid feedback activity in response to changes in the local environment, is a key technology for fabrication of future high-tech products and functional surfaces. These new multifunctional coatings should combine passive components inherited from “classical” coatings (barrier layers) and active components, which provide fast responses of the coating properties to changes occurring either in the passive matrix of multifunctional coatings (e.g., cracks, local pH change) or in the local environment surrounding the coating (temperature, humidity). The coatings should also have several functionalities (e.g., antifriction, antifungal, and anticorrosion) exhibiting synergistic effects.

Lanthanide Salts as Corrosion Inhibitors for AA5083. Mechanism and Efficiency of Corrosion Inhibition

The inhibition effect of trivalent lanthanide (Ce 3+ , La 3+ ) cations toward the localized corrosion of 5083 aluminum alloy in chloride-containing solutions was investigated in the present work. Electrochemical impedance spectroscopy was employed to estimate the inhibition efficiency of the lanthanide species under study. Scanning Kelvin probe force microscopy was used to provide deeper understanding of the corrosion and inhibition mechanisms at the micro level. The results of electrochemical tests demonstrate effective hindering of the localized corrosion attack due to precipitation of highly insoluble deposits on cathodic intermetallics. Trivalent lanthanum cations confer a more effective corrosion inhibition in comparison to that of Ce 3+ . Superior inhibition of La 3+ resulted from the formation of very dense hydroxide domes while cerium-based precipitates have a porous granulated structure.

Oxidation of silver by atomic oxygen

Interaction of silver with atomic oxygen has been studied at low incident-flux conditions (1015–1017 atomsċcm−2sċ−1, corresponding to a pressure of about 10−7 Pa) over the temperature range of 373 to 673 K. The only product of surface oxidation observed for these conditions was silver (I) oxide Ag2O. The parabolic rate law was normally observed, and the parabolic constant increased linearly with the incident flux of oxygen atoms. The parabolic rate constant also shows an unusual temperature dependence. It increases with an activation energy of 33 kJ/mol between 423 and 523 K and reaches a maximum at about 573 K and then decreases until oxidation stops around 673 K. Thermodynamical analysis of chemical reactions between silver and atomic oxygen shows the possibility of a “recombinative reduction” of silver oxide to silver metal by atomic oxygen which offers an explanation of the unusual temperature dependence.

Polyelectrolyte-modified layered double hydroxide nanocontainers as vehicles for combined inhibitors

In this work, nanocontainers based on layered double hydroxide (LDH) loaded with two different corrosion inhibitors, namely 2-mercaptobenzothiazole (MBT) and cerium(III) nitrate, were prepared. MBT was intercalated into LDH galleries in anionic form by anion-exchange, while Ce3+ was fixed between polyelectrolyte layers on the surface of LDH-MBT nanoplatelets by the Layer-by-Layer (LbL) method. Both inhibitors were found in the modified LDHs (LDH-Mod) by X-ray diffraction and scanning electron microscopy coupled with energy dispersive spectroscopy. The release studies performed by UV-vis spectrophotometry indicated that the main triggering conditions for release of MBT from LDHs changed when LDH nanoplatelets were covered with polyelectrolytes. Furthermore, electrochemical impedance spectroscopy and DC polarization were used to investigate the effect in combining MBT− and Ce3+ within the same nanocontainer, for the corrosion protection of 2024-T3 aluminium alloy directly in solution as well as in a hybrid sol–gel coating. The obtained results open prospects for application of these systems as additives in multifunctional smart coatings.