This paper is a review of the most recent and relevant achievements (from 2001 to 2013) on the development of organic-inorganic hybrid (OIH) coatings produced by sol-gel-derived methods to improve resistance to oxidation/corrosion of different metallic substrates and their alloys. This review is focused on the research of OIH coatings based on siloxanes using the sol-gel process conducted at an academic level and aims to summarize the materials developed and identify perspectives for further research. The fundamentals of sol-gel are described, including OIH classification, the interaction with the substrate, their advantages, and limitations. The main precursors used in the synthesis of OIH sol-gel coatings for corrosion protection are also discussed, according to the metallic substrate used. Finally, a multilayer system to improve the resistance to corrosion is proposed, based on OIH coatings produced by the sol-gel process, and the future research challenges are debated.

Corrosion degradation of materials and metallic structures is one of the major issues that give rise to depreciation of assets, causing great financial outlays in their recovery and or prevention. Therefore, the development of active corrosion protection systems for metallic substrates is an issue of prime importance. The promising properties and wide application range of hybrid sol-gel-derived polymers have attracted significant attention over recent decades. The combination of organic polymers and inorganic materials in a single phase provides exceptional possibilities to tailor electrical, optical, anticorrosive, and mechanical properties for diverse applications. This unlimited design concept has led to the development of hybrid coatings for several applications, such as transparent plastics, glasses, and metals to prevent these substrates from permeation, mechanical abrasion, and corrosion, or even for decorative functions. Nevertheless, the development of new hybrid products requires a basic understanding of the fundamental chemistry, as well as of the parameters that influence the processing techniques, which will briefly be discussed. Additionally, this review will also summarize and discuss the most promising sol-gel coatings for corrosion protection of steel, aluminium, and their alloys conducted at an academic level.

This study is focused on the electrochemical behavior and surface analysis of an eco-friendly organic-inorganic hybrid (OIH) coating for hot dip galvanized steel (HDGS) in contact with cementitious media. This treatment is a proposed alternative to replace toxic Cr(VI)-based pre-treatments used to control reactions between the zinc and wet concrete. HDGS samples were coated with two different sets of OIH gels obtained by a sol-gel process using a dip-coating method. Five distinct OIH matrices were obtained by reaction of functionalized metal-alkoxide (3-isocyanatopropyltriethoxysilane) with five different molecular weight diaminealkylethers. One set of HDGS samples was coated with each of the five pure OIH matrices and another was coated with similar matrices doped with Cr(III). The morphology of OIH coatings over HDGS surface was characterized by SEM/EDS. Similar films were prepared separately and the respective resistivity was measured by electrochemical impedance spectroscopy. Polarization resistance and macrocell current density were used to evaluate the corrosion protection properties of the HDGS coated samples in contact with cementitious media for a period of 74 days. Results showed that the produced coatings provide barrier properties that withstand the high pH of the electrolyte, protecting the HDGS when it first contacts cementitious media.The corrosion of steel in concrete is one of the major causes of structures degradation, requiring expensive rehabilitation. The use of hot dip galvanized steel (HDGS) has been recognized as an effective measure to increase the service life of reinforced concrete structures exposed to carbonation or to chloride ions. 1-3 The galvanized coating is a physical barrier that hinders the contact of aggressive agents with the steel substrate and the zinc layer acts as a sacrificial anode, protecting the steel against corrosion. 4,5 Immediately after the HDGS is embedded in fresh concrete, a highly alkaline environment, the zinc coating corrodes for a limited period (from several hours to a few days) until passivating surface layers are formed and concrete hardens. This initial corrosion process may lead to zinc consumption between 5 to 10 μm. 2 At the same time hydrogen is produced which may lead to the loss of adhesion between steel and concrete.Several corrosion studies reported the behavior of HDGS in contact with concrete media and in alkaline solutions. 2-21 However, uncertainties concerning the initial corrosion behavior of the galvanized coating when embedded in concrete still remains. The main literature about corrosion and passivation mechanisms of zinc in concrete environments, suggest that the formation of the protective layer due to zinc oxidation takes place with water reduction and subsequent hydrogen evolution. 9-11 Other authors claim that the formation of protective layer is related to the presence of oxygen at the concrete/rebar interface. 12,14,18 Andrade and co-workers, 11,22,23 found that at pH ≥ 12.5 zinc dissolution and hydrogen evolution takes place p...

We have mesured the carrier recombination dynamics in InGaN/GaN multiple quantum wells over an unprecedented range in intensity and time by means of time-resolved photoluminescence spectroscopy. We find that, at times shorter than 30 ns, they follow an exponential form, and a power law at times longer than 1 µs. To explain these biphasic dynamics, we propose a simple three-level model where a charge-separated state interplays with the radiative state through charge transfer following a tunneling mechanism. We show how the distribution of distances in chargeseparated states controls the dynamics at long time. Our results imply that charge recombination happens on nearly-isolated clusters of localization centers.

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A new type of photochromic naphthopyran with a fused structure was embedded in an organic-inorganic amine-alcohol-silicate hybrid gel producing colourless, transparent and flexible photochromic films. Under UV-vis light these materials developed, in less than 30 s, intense yellow-orange colours that faded completely in the dark in 3 min. This behaviour contrasts with the usual performance of common naphthopyrans that always generate two coloured species which fade with different kinetics, leading to the formation of a residual colour that persists for several minutes/hours. The structural design of this new naphthopyran prevents the formation of the long-lived coloured species and therefore fully reversible photo-switchable materials can be easily obtained. Scheme 1 Photochromic equilibrium for 2H-naphtho[1,2-b]pyran 1 and 3Hnaphtho[2,1-b]pyran 2.

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Organic-inorganic hybrid (OIH) matrices were synthesized by sol-gel method and deposited on hot-dip galvanized steel (HDGS) using a dip-coating process. These OIHs, generally called amino-alcohol-silicates, were synthesized using a functionalized siloxane, 3-glycidoxypropyltrimethoxysilane, and five oligopolymers (Jeffamine) with different molecular weights: 230, 400, 600, 900 and 2000. Besides the five different pure OIH matrix coatings, a similar set of HDGS samples were coated with the OIH matrices doped with Cr(III), which was tested as a corrosion inhibitor. The OIH coatings were assessed using electrochemical studies, namely electrochemical impedance spectroscopy, macrocell current density, open circuit potential monitoring and polarization resistance. The studies were carried out in mortar. Analysis of the results obtained by optical and scanning electronic microscopy methods were consistent with the data obtained by electrochemical techniques. The HDGS samples coated with OIH matrices showed better performance when compared with HDGS uncoated samples. During the last three decades, the corrosion of steel reinforcement has been widely studied and reported. [1][2][3][4][5][6][7] In spite of the majority of the reinforced concrete (RC) structures showing high durability, serviceability and long-term performance as well as intense research being performed in the last few years, a large number of failures have been reported due to the corrosion of the reinforcement embedded in the concrete.The most effective way to minimize the risk of corrosion reinforcement concrete is to ensure that the cover of the metallic reinforcement parts is of an adequate thickness. The concrete should be of high quality, with a proper mixing ratio, good compaction and curing. 1,6,7 Several proven methods are known for preventing and controlling corrosion. Improving the concrete quality and increasing the concrete cover are the most economical protection measures yet are not always enough. A method that is widely accepted and recognized as efficient at providing corrosion protection of the RC structures is the application of hot-dip galvanized steel (HDGS), since it is considered economically favorable when compared to others. The use of HDGS has been recognized by several authors 7,[8][9][10][11][12] as an effective measure to increase the service life of RC structures exposed to carbonation or to chloride ions. However, zinc in contact with fresh concrete (high alkaline environment) is oxidized and hydrogen evolution occurs, for a limited period, until passivating formation layers occur and concrete hardens. To avoid zinc corrosion and hydrogen evolution, chromate and similar hexavalent chromium compounds are among the most common substances used as inhibitors or incorporated into the preparation of the fresh concrete. However, these compounds are toxic and carcinogenic, causing serious environmental hazards and their incorporation in protective coatings is heavily regulated by most environmental legislation. Therefore, intense re...