Andrei N. Salak

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.

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.

Ferroelectric-to-relaxor transition behaviour of BaTiO3 ceramics doped with La(Mg1/2Ti1/2)O3

Low-frequency dielectric studies were performed on the fine-grain ceramics of BaTiO3 doped with 2.5 mol% of non-ferroelectric perovskite La(Mg1/2Ti1/2)O3 (BT:2.5% LMT). In addition to the permittivity and P–E hysteresis loop measurements as a function of temperature, local electromechanical properties of BT:2.5% LMT were analysed by piezoresponse force microscopy. It was found that the temperature evolution of the dielectric properties detected by macroscopic methods (average over many grains) is similar to that observed on the nanoscale level (in a single grain). BT:2.5% LMT was revealed to exhibit the typical features of both the ferroelectric and the relaxor. The observed dielectric behaviour is discussed in terms of heterovalent substitution at both A-(Ba2+/La3+) and B-(Ti4+/Mg2+) perovskite sites.

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.

Electrical Properties of Na0.5Bi0.5TiO3 – SrTiO3 Ceramics

At the present time, the most widely-used piezoelectric material belongs mainly to the PZT solid solution system (1-x) PbTixO3 – x ZrTiO3. Because of the possible evaporation of lead oxides during ...

Evolution from Ferroelectric to Relaxor Behavior in the (1 − x)BaTiO3 − xLa(Mg1/2Ti1/2)O3 System

Abstract Ceramics of (1 − x)BaTiO3−xLa(Mg1/2Ti1/2)O3 series (x = 0, 0.025, 0.05, 0.075 and 0.1) were dielectrically investigated. Based on the dielectric and X-ray diffraction data, phase diagram of the system for the studied composition range has been determined. Apparent relaxor behavior was revealed to set in when the BaTiO3-type sequence of three phase transitions is reduced to only diffuse transition. No clear boundary was found between ferroelectric and relaxor compositions of the system. It was observed the continuous crossover from ferroelectric to relaxor behavior.

Control of crystallite and particle size in the synthesis of layered double hydroxides: Macromolecular insights and a complementary modeling tool

Zinc-aluminum layered double hydroxides with nitrate intercalated (Zn(n)Al-NO3, n=Zn/Al) is an intermediate material for the intercalation of different functional molecules used in a wide range of industrial applications. The synthesis of Zn(2)Al-NO3 was investigated considering the time and temperature of hydrothermal treatment. By examining the crystallite size in two different directions, hydrodynamic particle size, morphology, crystal structure and chemical species in solution, it was possible to understand the crystallization and dissolution processes involved in the mechanisms of crystallite and particle growth. In addition, hydrogeochemical modeling rendered insights on the speciation of different metal cations in solution. Therefore, this tool can be a promising solution to model and optimize the synthesis of layered double hydroxide-based materials for industrial applications.

Study of cation ordering and magnetic phase transitions in ternary Fe-containing perovskite oxides by Mössbauer spectroscopy

Nuclear gamma-resonance (NGR) spectra were studied in ceramics of ternary perovskite-type oxides Pb(Fe2/3W1/3)O3, Pb(Fe1/2Ta1/2)O3, and Pb(Fe1/2Nb1/2)O3 and in the Pb(Fe1/2Sb1/2)O3 oxide synthesized under pressure. A singlet corresponding to compositional-ordering regions was observed in the NGR spectra for Pb(Fe1/2Sb1/2)O3. Weak doublets associated with paramagnetic regions (in which the degree of cation ordering is higher than the volume average) were observed in the spectra for Pb(Fe2/3W1/3)O3 and Pb(Fe1/2Nb1/2)O3 at temperatures 30–70 K below the antiferromagnetic phase transition point.

Polar and antipolar polymorphs of metastable perovskiteBiFe0.5Sc0.5O3

A metastable perovskite BiFe0.5Sc0.5O3 synthesized under high-pressure (6 GPa) and high- temperature (1500 K) conditions was obtained in two different polymorphs, antipolar Pnma and polar Ima2, through an irreversible behaviour under a heating/cooling thermal cycling. The Ima2 phase represents an original type of a canted ferroelectric structure where Bi3+ cations exhibit both polar and antipolar displacements along the orthogonal [110]p and [1-10]p pseudocubic directions, respectively, and are combined with antiphase octahedral tilting about the polar axis. Both the Pnma and Ima2 structural modifications exhibit a long-range antiferromagnetic ordering with a weak-ferromagnetic component below TN ~ 220 K. Analysis of the coupling between the dipole, magnetic and elastic order parameters based on a general phenomenological approach revealed that the weak-ferromagnetism in both phases is mainly caused by the presence of the antiphase octahe- dral tilting whose axial nature directly represents the relevant part of Dzyaloshinskii vector. The magnetoelectric contribution to the spontaneous magnetization allowed in the polar Ima2 phase is described by a fifth-degree free-energy invariant and is expected to be small.