Inorganic–organic sol gel hybrid coatings for corrosion protection of metals

Inorganic–organic hybrid coatings by sol–gel process are very suitable for fighting corrosion. Inorganic sols in hybrid coatings not only increase adhesion by forming chemical bonds between metals and hybrid coatings, but also improve comprehensive performances of polymer in the coatings. Different organic polymers or organic functionalities are introduced into gel network to achieve tailored properties, such as hydrophobic properties, increasing cross-linking density, etc. As for corrosion protection of metals organic components of hybrid coatings are selected to repel water and form dense thick films and reduce coating porosity. The factors, such as the ratio of inorganic and organic components, cure temperature, pigments in hybrid coatings, need to be optimized for attaining hybrid films with the maximum corrosion resistance. Electro-deposition technique offers relatively thick homogeneous defect-free hybrid coatings in comparison to dip or spin coating techniques. Green cerium ions and non-ionizable organic inhibitors are more developed in hybrid coatings nowadays than other corrosion inhibitors. Long-term corrosion resistance techniques of inhibitors are discussed. The inhibitors entrapped in the nanocontainers are doped in hybrid films to prolong release of the inhibitors to damaged zones, which is discussed in detail. Among all the nanocontainers of corrosion inhibitors the prospective techniques which show superior corrosion protection are cyclodextrin/organic inhibitor inclusion complexes and layer by layer assembly of organic corrosion inhibitors in nanocontainers. Super-hydrophobic property of hybrid coatings derives from low surface tension and surface roughness of hybrid coatings, which endues the films with excellent corrosion protection for metals, but the durable property of super-hydrophobic coatings needs to be improved for industrial application. An ideal multiple model of hybrid coatings for superior anti-corrosion of metals proposed is a combination of super-hydrophobic hybrid coatings and underlying hybrid coatings doped with sustained release of corrosion inhibitors on metal substrates.     

Enhancing corrosion resistance of zinc-filled protective coatings using conductive polymers

Organic coatings containing zinc are amply used for the protection of metals, particularly steel structures. Ways to reduce the zinc content in the coating materials are sought for environmental and financial reasons. Our previous work (Kohl, Prog Org Coat 77:512–517, 2014; Kohl and Kalendová, Mater Sci Forum 818: 171–174, 2015a) suggested that one of the options consists in the use of conductive polymers in the formulation of the zinc coatings. The benefits of conductive polymers include nontoxicity, high stability, electric conductivity and redox potential. Previously we focussed on the effect of conductive polymers added to the organic coatings so as to complete the zinc volume concentration to 67%. The anticorrosion efficiency of the organic coatings was found to improve with increasing polyaniline phosphate or polypyrrole concentrations. Zinc content reduction in the system, however, did not attain more than 5%. The present work focusses on systems where the organic coatings are prepared with zinc having a pigment volume concentration PVC = 50%. Zinc content reduction in the system attains up to 20%. This work examines the mechanical and anticorrosion properties of the organic coatings with reduced zinc contents. The present work was devoted to the feasibility of using of conductive polymers in the formulation of coatings with reduced zinc contents. The conductive polymers included polyaniline, polypyrrole and poly(phenylenediamine); these were synthesised and characterised using physico-chemical methods. Polyphenylenediamine as a potential corrosion inhibitor has not been paid adequate attention so far. Subsequently, organic coatings with reduced zinc contents and containing the pigments at 0.5, 1 and 3% volume concentrations were formulated. The coatings were subjected to mechanical tests and accelerated corrosion tests to assess their mechanical and corrosion resistance. The corrosion resistance of the organic coatings was also studied by linear polarisation. The results of the mechanical tests, accelerated corrosion tests and linear polarisation measurements indicate that the organic coating properties get affected by the conductive polymer type as well as by the pigment volume concentration. The important finding is that the use of conductive polymers in coatings with reduced zinc contents was beneficial in all cases.     

Investigation of filiform corrosion of coated aluminium

Filiform corrosion on different aluminium alloys, coated with a clear varnish has been studied. Image analysis and metallographies were used to characterize the corrosion. It has been shown, that filiform corrosion consists of a lateral propagation of filaments and an attack on the aluminium under the filaments. The extent of filiform corrosion is dependent on the sort of alloy and the pretreatment of metall surfaces.     

On the application of electrochemical impedance spectroscopy to study the self-healing properties of protective coatings

Active corrosion protection based on self-healing of defects in coatings is a vital issue for development of new advanced corrosion protection systems. However, there is a significant lack of experimental protocols, which can be routinely used to reveal the self-healing ability and to study the active corrosion protection properties of organic and hybrid coatings.The present work demonstrates the possibility to use EIS (electrochemical impedance spectroscopy) for investigation of the self-healing properties of protective coatings applied on a metal surface. The model EIS experiments supported by SVET (scanning vibrating electrode technique) measurements show that an increase of low frequency impedance during immersion in the corrosive medium is related to the suppression of active corrosion processes and healing of the corroded areas. Thus, EIS can effectively be employed as a routine method to study the self-repair properties of different protective systems. The 2024 aluminium alloy coated with hybrid sol–gel film was used as a model system to study the healing of artificial defects by an organic inhibitor (8-hydroxyquinoline).     

Analysis of deterioration process of organic protective coating using EIS assisted by SOM network

Under cyclic wet–dry conditions, the deterioration process of the organic coating on carbon steel surface has been studied using electrochemical impedance spectroscopy (EIS) assisted by self-organizing feature map (SOM) network. According to the EIS characteristics, changing rate of impedance and the classification results by SOM network, the entire deterioration process can be divided into three main stages shown as follows. Stage I is the medium penetration into coatings, which is a slow process. Stage II is the corrosion initiation under coatings, which is a relatively fast transition period. Stage III is the corrosion extension which causes coating delamination and makes the coatings lose its corrosion protection eventually. Besides, the results indicate that SOM network is a very simple and effective technique for analyzing the deterioration process of organic coating.     

SR-XRD and SR-FTIR study of the alteration of silver foils in medieval paintings

Altarpieces and polychrome carved wood from the fifteenth century AD usually exhibit golden and silvery areas by the application of a very thin foil of metal. The metal foils were normally protected from the atmosphere by a varnish or resin which maybe either preserved or absent. Moreover, they were glued to the background surface by adhesive substances (egg yolk, drying oil or animal glue). The high proportion of the glueing substances often renders the development of reaction compounds. With time, silver alters blacken or simply disappear completely. In this paper, we study the alterations to metal foils from a selection of fifteenth century artworks showing different glueing agents, organic coatings and several degrees of conservation of the organic coatings and metal leafs. The submillimetric layered structure and the high variability and low amount of most of the compounds present in the different layers, as well as their differing nature (organic and inorganic) make the use of micron-sensitive high-resolution techniques essential for their study. In particular, the high resolution, high brilliance and small footprint renders synchrotron radiation most adequate for their study. SR-XRD was performed to identify the reaction compounds formed in the different layers; μFTIR was used at to identify the silver protecting organic coatings, the metal foil glueing layers and the corresponding reaction compounds. The results obtained suggest that atmospheric corrosion is the dominant mechanism, and therefore that the degree of corrosion of the metal foils is mainly related to the conservation state of the protecting coatings.     

Electroactive conducting polymers for corrosion control

There is an intensive effort underway to develop new corrosion control coatings for structural metals. In part, this effort has been motivated by the desire to replace chromium(VI)-containing coatings currently used for corrosion control of iron and aluminum alloys. Cr(VI) has been shown to be hazardous to the environmental and to human health, and its use in many countries will be sharply curtailed in the coming years. Electroactive conducting polymers (ECPs) represent a class of interesting materials currently being explored for use in corrosion control coating systems, possibly as a replacement for Cr(VI)-based coatings. The electroactivity and the electronic conductivity (or semiconductivity) of ECPs set them apart from traditional organic coatings. As with chromate, interesting and potentially beneficial interactions of ECPs with active metal alloys such as steel and aluminum are anticipated, with concomitant alteration of their corrosion behavior. A review of this active research area will be presented in two parts. Here in Part 1, a general introduction to the topic of corrosion control by ECPs will be presented, including an overview of corrosion and its control by traditional methods, an introduction to ECPs and their properties, and a discussion of the processing issues surrounding the use of ECPs as coatings. Part 1 also includes a review of the literature on the use of ECPs as coatings (or components of coatings) on non-ferrous active metals, principally aluminum and aluminum alloys, although some work on zinc, copper, silver, titanium and silicon will also be described. In Part 2 of this review (to be published in the next issue of this journal), the rather extensive literature on the use of ECPs for the corrosion control of ferrous alloys (steels) will be reviewed. Electronic Publication     

有机涂层在浸泡和干湿循环条件下劣化过程的EIS对比研究

应用电化学阻抗谱(EIS)技术以对比法研究了浸泡在3.5%NaCl溶液中的铁基有机涂层在浸泡和干湿循环条件下的劣化过程.干湿循环实验步骤为12h浸泡和12h干燥(298 K、50%RH).结果表明,根据EIS响应特征,浸泡和干湿循环下的涂层劣化过程均可分为3个主要阶段:涂层渗水阶段,基底金属腐蚀发生阶段和基底金属腐蚀发展与涂层失效阶段.与单一浸泡过程相比,干湿循环明显加速了腐蚀反应的发生,但减缓了其渗水、腐蚀发展以及涂层失效劣化过程.根据实验结果,深入讨论了干湿循环加速涂层劣化和涂层下基底金属的腐蚀机理.     

Crack‐free sol‐gel coatings for protection of AA1050 aluminium alloy

Organically modified sol‐gel coatings have been investigated as potential replacements for chromate conversion treatments of an AA1050 aluminium alloy. The coatings were prepared by combination of a completely hydrolysable precursor of tetra‐n‐propoxyzirconium (TPOZ), with a partially hydrolysable precursor of glycidoxypropyltrimethoxysilane (GPTMS). GPTMS contains an organic functional group, which is retained in the sol‐gel coatings after the hydrolysis–condensation process. Different GPTMS/TPOZ ratios and withdrawal speeds were studied. Coatings produced using a low GPTMS/TPOZ ratio and a high withdrawal speed generated significant cracks due to the shrinkage of the coatings, with no corrosion protection of the alloy. It was found that increase of organic moieties reduced the shrinkage of the coatings and the tendency for crack formation. By control of process parameters and ratios of organic and inorganic moieties, crack‐free sol‐gel coatings above 1 µm thick, with improved corrosion protection, can be produced on the alloy surface. Copyright © 2010 John Wiley & Sons, Ltd.     

Protective Properties of a Sol-Gel Coating on Zinc Coated Steel

Galvanised and galvannealed steels are widely used due to their good corrosion resistance in aqueous solutions. However, when additional protection is required, organic coatings, corrosion inhibitors or conversion coatings are used to improve their corrosion protection. In this work, sol-gel coating was used to improve the corrosion behaviour of these two materials. This paper analyses the final protective properties of a sol-gel coating prepared by basic catalysis and its dependence on the sintering temperature and time of treatment. The influence of the sintering conditions on the galvanised and galvannealed substrates is a decisive factor for the coating quality and for the barrier affect against the aggressive media. While heat treatment time is the controlling factor for the galvannealed steels, the temperature is determining in the case of the galvanised. Corrosion mechanisms for sol-gel galvanised steels did not changed with respect to the uncoated steel. However for galvannealed steel, after coating the mechanism is not purely cathodic.     

Application of MMS for the study of corrosion at the interface of organic coatings and iron substrate

A special Mössbauer technique, MMS has been used for the study of corrosion on the surface of⁵⁷Fe layers, under two types of organic coatings. The corrosion processes were followed by the Mössbauer spectra.     

Effect of Sintering Temperature on the Corrosion and Wear Behavior of Protective SiO2-Based Sol-Gel Coatings

Sol-gel hybrid organic-inorganic and inorganic SiO₂-based protective coatings with and without added 3 m glass particles were developed and tested for their corrosion and wear behavior of an stainless steel substrate (AISI316L). The corrosion resistance greatly increases by incorporating glass particles in the sols. The incorporation of particles in the coatings allows the synthesis of thicker crack-free coatings. On the other hand, the corrosion resistance increases for coatings with a higher organic content obtained at lower sintering temperature. These coatings are also highly stable in saline aqueous solutions. However, the wear resistance is badly affected by the hybrid character of the SiO₂ matrix. The optimum coating process in terms of corrosion and wear resistance, appears to be a hybrid system with a dense SiO₂ network achieved at intermediate sintering temperatures.     

Influence of organic corrosion inhibitors on the corrosion performance of organic–inorganic hybrid coatings applied on aluminium alloy

The proposed work aims to develop and study sol–gel derived anticorrosion films for aluminium. To further improve performance of these films, organic corrosion inhibitors were incorporated into the films. The organic–inorganic hybrid films with and without corrosion inhibitors were deposited on an aluminium substrate by dip coating. The films were characterized by electrochemical impedance spectroscopy (EIS), DC polarisation techniques, and neutral salt spray test to evaluate their anticorrosion properties. This study shows that very low and very high MBT concentrations deteriorate the corrosion performance of coatings, and consequently, there is an optimum concentration of MBT. EIS results revealed a higher corrosion inhibitive activity of 2-mercaptobenzothiazole (MBT) compared to that of 2-amino-5-methylthiazole and 1,2,3 benzotriazole.     

有机涂层失效过程的电化学阻抗和电位分布响应特征

结合使用电化学阻抗谱(EIS)和扫描Kelvin探针(SKP)技术研究了在质量分数为3.5%的NaCl溶液中的铁基有机涂层劣化过程特征. 结果表明, 根据EIS和SKP的响应特征, 可将涂层劣化过程分为3个主要阶段: (Ⅰ) 涂层渗水阶段. 此时, 涂层渗水阶段的EIS阻抗持续减小, 但保持单容抗弧特征, SKP特征是电位持续降低, 但分布保持均匀; (Ⅱ) 基底金属腐蚀发生阶段. 此时, EIS阻抗快速下降, 并产生第二时间常数; SKP特征为表面电位差增大; (Ⅲ) 基底金属腐蚀发展与涂层失效阶段. 此时, EIS出现扩散尾, SKP电位差保持较大数值. 实验结果表明, 在研究有机涂层劣化过程中, EIS和SKP的结合使用能够互相补充完善, 获得涂层劣化过程中更为准确\, 可靠的变化信息.     

Zn-Al-[V_(10)O_(28)]~(6-)双层氢氧化物掺杂对LY12铝合金表面溶胶-凝胶涂层性能的影响

采用共沉淀法制备Zn-Al-[V10O28]6-双层氢氧化物(以下简称LDH-V),研究不同添加浓度(0.0、0.25×10-3、0.75×10-3、1.5×10-3、3.0×10-3mol·L-1)的LDH-V对LY12铝合金溶胶-凝胶涂层形貌、耐蚀性的影响.采用扫描电子显微镜(SEM)和傅里叶变换红外(FTIR)光谱研究LDH-V对涂层形貌和结构的影响.运用中性盐雾实验对涂层进行耐蚀性评估.利用电化学方法对涂层在0.05 mol·L-1的NaCl溶液中的腐蚀行为进行研究.探讨加入LDH-V后溶胶-凝胶涂层的耐蚀机理.结果表明,一定量LDH-V的加入不仅可以提高溶胶-凝胶涂层的耐蚀性能,还可对涂层被破坏区域进行自修复,起到延缓铝合金基体腐蚀的作用.然而,当LDH-V的添加溶度超过一定值时,会破坏涂层的完整性并降低涂层的腐蚀防护性能.实验结果表明LDH-V最佳的添加浓度为1.5×10-3mol·L-1.     

Corrosion protection of aluminium alloy AA2024 with cerium doped methacrylate-silica coatings

The aim of this work is the synthesis and characterization of hybrid coatings doped with cerium salts for corrosion protection of AA2024. The control of the inorganic and organic polymerization process allows the preparation of coatings with an open structure and a hydrophilic character. These facts facilitate the incorporation and mobility of cerium ions through the structure, enhancing its ability to promote a self-healing mechanism. The thermal treatment of the coatings has been limited to 120 °C to preserve the mechanical properties of the alloy. The electrochemical behaviour of the coatings has been evaluated in 0.3 wt% NaCl solution by means of EIS technique. Electrochemical measurements evidence good barrier properties at initial immersion time, and signals of corrosion inhibition from cerium ions at long immersion times could be assigned to the increasing of the impedance modulus at low frequencies and the presence of cerium oxide/hydroxide precipitates.     

Investigation of Glass-Like Sol-Gel Coatings for Corrosion Protection of Stainless Steel Against Liquid and Gaseous Attack

Glass-like sol-gel coatings have been investigated as corrosion protective coatings on stainless steel. Magnesium- and borosilicate coatings with thickness of about 100–700 nm and methyl-modified SiO2 coatings with a thickness of about 2 m were deposited on stainless steel plates by dip-coating. The coatings were densified between 400°C and 500°C in different atmospheres (N2, air) for 1 h. The corrosion protection against gaseous attack was investigated by accelerated corrosion tests, at 800°C in air for 1 h. A corrosion protection factor was calculated from the relation Fe/Fe2O3, determined by XRD on the surface of coated and uncoated samples. Methyl-modified SiO2 coatings showed a protection factor, which was 2 orders of magnitude higher than for the other coatings. Electrochemical investigations were performed on samples submerged in a NaCl solution for 200 h. The corrosion propagation, polarization resistance and impedance vector were measured. For accelerated corrosion tests, polarization intensity curves were determined for high potentials of up to 1 V. Again excellent results were obtained for the methyl-modified SiO2 coatings, which remained passive for 200 h. Results of the salt spray corrosion test, however, showed no corrosion protection by the sol-gel coatings. After 2000 h in the salt spray chamber the steel was corroded and the coatings peeled off. It is concluded that for the further development of these coatings an improved interfacial passivation will be required.     

Corrosion resistance of zinc electrodeposited from acidic and alkaline electrolytes using pulse current

Corrosion resistance of zinc coatings was investigated in an accelerated corrosion test in a condensation chamber. Zinc was electrodeposited from alkaline and acidic electrolytes using direct current (DC) or pulse current (PC). The zinc coating was subsequently protected against corrosion with a chrome (III) layer. Morphology and structure of the coatings was investigated using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction analysis (XRD) before and after the corrosion test. Corrosion resistance of alkaline zinc coatings electrodeposited with DC and PC under test conditions was found to be comparable. The corrosion resistance of zinc coatings deposited from acidic electrolytes by PC was lower in comparison with corrosion resistance of zinc coatings deposited using DC. Presented at the 8th Conference on Solid State Chemistry, Bratislava, Slovakia, 6–11 July 2008.     

Sol–Gel-Derived Corrosion-Protection Coatings

There is a current need for alternative coatings that can provide corrosion resistance to metals or alloy surfaces due to the environmental hazards posed by conventional coatings. Herein, we report on novel organically-modified sol–gel coatings for the protection of metal and alloy surfaces. The basic concept of chemical conversion of metal surfaces is based on deposition of a hydrophobic, nonporous sol–gel barrier layer for surface protection and corrosion prevention. The properties of these organosilica coatings can be tuned by varying the composition of precursors. The evaluation of hydrophobicity, adhesive strength, and anticorrosion properties of organically-modified sol–gel derived coatings suggests their potential utility as technologically-compatible alternatives to conventional coatings.