Improved mechanical and anticorrosion properties of metal oxide-loaded hybrid sol–gel coatings for mild steel in a saline medium

Abstract

A hybrid organic–inorganic sol–gel coating was successfully prepared and subsequently functionalized individually with five different metal oxide additives. The effect of the incorporated oxides on the corrosion protection performance and scratch-resistance properties of the hybrid base coating on mild steel substrates was investigated using electrochemical techniques, namely electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) as well as mechanical testing. The steel-coated specimens were immersed in 3.5?wt.% NaCl corrosive medium for two weeks and the results reveal an excellent corrosion protection performance by all coating formulations with a significant high corrosion-resistance property for the sample loaded with molybdenum oxide. Scanning electron microscopy (SEM) images proved the absence of corrosion signs, defects, micro cracks, or delamination on the surface of the coated samples. Compared with the pure hybrid coating, all the metal oxide-embedded coatings (except for the sample loaded with yttrium(III) oxide) show comparable aqueous contact angle values as well as enhanced hardness and adherence properties. No noticeable dependence was observed for the surface roughness parameters as a function of the type of incorporated metal oxide within the sol–gel matrix. Overall, the results of this study demonstrate that metal oxides can be advantageous to the desired properties of hybrid sol–gel coatings applied to steel surfaces.     

Ultra thin layers as new concepts for corrosion inhibition and adhesion promotion

The adsorption and self-organization process as well as the surface reactions of several bifunctional adhesion promoters on different oxide surfaces have been investigated. The aim was to improve the adhesion between metal oxides and different organic coatings. We developed a large number of bifunctional compounds, which are able to adsorb spontaneously on different pre-treated metal (oxide) surfaces. The second group can be designed for grafting different otherwise incompatible layers.

Therefore, a special two-step procedure has been developed: (1) adsorption of the designed bifunctional molecules on the substrate and (2) surface reaction of the terminal reactive group with a polymeric top coating or with further monomers resulting in a strongly bond composite.

For this purpose substances were chosen having a surface reactive group, an aliphatic spacer and a reactive group for a suitable top layer. Phosphonic acids forming strong bonds with several metal surfaces were chosen as surface-active groups on metal oxide substrates.

The termination of these compounds with further reactive groups opened a wide range of possible applications. Functionalities like amino or carboxylic groups allowed reactions with commercial lacquers (e.g., polyurethane) for improving adhesion promotion and corrosion inhibition of the metal substrates. By using polymerizable groups like thiophene and pyrrole an in situ surface polymerization with further monomers is possible directly on the substrate.

The adsorbed films of bifunctional phosphonic acids on metal (oxide) substrates were characterized by contact angle measurements, X-ray photoelectron spectroscopy (XPS), the surface polymerized films were investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), cyclovoltammetry (CV), and electrochemical impedance spectroscopy (EIS). The results showed that monolayers were formed, which were correctly oriented on the surfaces: the phosphonic acid group was attached to the substrate whereas the terminal group was free standing for further reactions. Surface polymerization with additional monomer was possible either chemically or electrochemically resulting in smooth polymer layers of adjustable thickness. The conducting polymers were found to be p-conductive with a doping level of about 30%. Conductivity measurements revealed a conductivity of about 0.13 S/cm for the best films.

Based on this principle two possible applications are given: firstly, a corrosion protecting system for steel, and secondly, a model release system for protecting steel after damage of the coating.     

Tuning the pH-responsiveness capability of poly(acrylic acid-co-itaconic acid)/NaOH hydrogel: Design,swelling, and rust removal evaluation

The swelling behavior of poly(acrylic acid-co-itaconic acid)/NaOH hydrogel as well as its ability for iron and copper rust removal was studied and established for the first time. Through an experimental design, the influence of the synthesis parameters on hydrogel response was determined. It was found that pH-responsiveness dependence of hydrogel determines its application. In alkaline media, the hydrogel acted as superabsorbent, while in acidic, the most outstanding property was its pickling capability that allowed to clean carbon steel and copper metallic surfaces. Infrared, thermogravimetry, and scanning electron microscopy were performed to determine copolymer formation, thermal properties, and morphology. Metallic crystallographic phases formed during the corrosion processes were determined by X-ray diffractometer. Hydrogel adhesiveness followed by diffusion and dissolution of metal oxides species was identified as the main steps in the rust removal mechanism. This method offers a new, environmentally friendly perspective to eliminate corrosion from metallic surfaces compared with traditional strategies. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48403.     

Hybrid coating on steel: ZnNi electrodeposition and surface modification with organothiols and diazonium salts

Sacrificial electrodeposited ZnNi is currently studied for replacing chromate conversion coatings (CCC) in anticorrosion applications. The present-day performances of ZnNi are still away from those of CCCs and the additional organic layers such as polymers and paints are still permeable and cannot prevent the corrosive species to reach the metal. Suitable adhesion primers could improve the situation by minimizing the access of the corrosive species to the polymer/metal interface.As a contribution to this interface problem, the present work provides a comparison of the protective properties of two structurally related molecules (4-nitrothiophenol and 4-nitrobenzenediazonium) grafted on a ZnNi coating electrodeposited on steel. Films of 4-nitrophenyl have been prepared according to the self-assembly process while films of 4-nitrobenzene have been obtained by electrochemical grafting, n-dodecanethiol being used as model system.The adsorption of these molecules as well as the resulting organic film is characterized by X-ray photoelectron spectroscopy (XPS) and polarization modulation-infrared reflection absorption spectroscopy (PM-IRRAS). The protective properties of the organic films against corrosion are investigated by linear sweep voltammetry (LSV), cyclic voltammetry (CV) and scanning vibrating electrode technique (SVET).     

Corrosion of carbon steel influenced by anaerobic biofilm in natural seawater

The bacteria in the anaerobic biofilm on rusted carbon steel immersed in natural seawater were characterized by culturing and molecular biology techniques. Two types of anaerobic bacterium, sulfate-reducing bacteria (SRB) Desulfovibrio caledoniensis and iron-reducing bacteria Clostridium sp. uncultured were found. The compositions of the rust layer were also analyzed and we found that iron oxide and sulfate green rust were the major components. To investigate the corrosion mechanisms, electrochemical impedance spectra was obtained based on the isolated sulfate-reducing bacteria and mixed bacteria cultured from rust layer in laboratory culture conditions. We found that single species produced iron sulfide and accelerated corrosion, but mixed species produced sulfate green rust and inhibited corrosion. The anaerobic corrosion mechanism of steel was proposed and its environmental significance was discussed.     

Hybrid organosiloxane material/metal oxide composite as efficient anticorrosive coatings for mild steel in a saline medium

Hybrid sol–gel materials have been found very promising anticorrosive coatings for metal substrates. In this article, the synthesis of novel hybrid organic‐inorganic sol–gel polymer; starting from tetraethyl orthosilicate, (3‐aminopropyl) trimethoxysilane, dimethoxy‐methyl‐octadecylsilane and polydimethylsiloxane, silanol terminated precursors, is reported. The hybrid polymer has been further loaded individually as well with five different metal oxides, then deposited on mild steel panels. All cured coating formulations have been characterized using thermogravimetric analysis, contact angle measurements, electrochemical impedance spectroscopy, vicker‐microhardness, surface roughness, and critical load analyses. Results have revealed that the parent coating exhibits excellent thermal stability and hydrophobic nature with minor observed changes on the two properties for the metal oxide‐loaded coatings. Electrochemical impedance and visual inspection results indicated excellent corrosion protection performance for all metal oxide composite coatings (except magnesium oxide) on steel when immersed in 3.5% NaCl solution for a prolonged time. Furthermore, the coating containing molybdenum oxide exhibited a maximum hardness, homogeneity, and adherence to the steel surface. The developed coating formulations in this study can be considered as a promising alternative to the currently‐used toxic chromate and phosphate coatings. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 134, 46718     

Oxygen evolution on semiconducting oxides

The oxygen evolution reaction is of particular interest to secondary metal air batteries and water electrolysis plants. However, most of the earlier work has been on precious metals and there are no guidelines for the choice of semiconducting oxides as oxygen evolving electrodes. In this study, the role of the metal/metal oxide or the lower metal oxide/higher metal oxide couple in determining the minimum voltage required for the evolution of oxygen is emphasized, together with other essential requirements such as electrical resistivity, electrode microstructure, corrosion resistance and catalytic properties. A survey of various metal oxides based on the above criterion suggested that NiCo₂O₄ is of particular interest and Teflon bonded electrodes based on this material gave over 13000 A/m² at 1.63 V vs dhe, 70°C, 5 N KOH.     

Electronic properties of thermally formed thin iron oxide films

The oxide layer, present between an organic coating and the substrate, guarantees adhesion of the coating and plays a determinating role in the delamination rate of the organic coating. The purpose of this study is to compare the resistive and semiconducting properties of thermal oxides formed on steel in two different atmospheres at 250 °C: an oxygen rich atmosphere, air, and an oxygen deficient atmosphere, N₂. In N₂, a magnetite layer grows while in air a duplex oxide film forms composed by an inner magnetite layer and a thin outer hematite scale. The heat treatment for different amounts of time at high temperature was used as method to sample the thickness variation and change in electronic and semiconducting properties of the thermal oxide layers. Firstly, linear voltammetric measurements were performed to have a first insight in the electrochemical behavior of the thermal oxides in a borate buffer solution. Electrochemical impedance spectroscopy in the same buffer combined with the Mott-Schottky analysis were used to determine the semiconducting properties of the thermal oxides. By spectroscopic ellipsometry (SE) and atomic force microscopy (AFM), respectively, the thickness and roughness of the oxide layers were determined supporting the physical interpretation of the voltammetric and EIS data. These measurements clearly showed that oxide layers with different constitution, oxide resistance, flatband potential and doping concentration can be grown by changing the atmosphere.     

The modification of indium tin oxide with phosphonic acids: mechanism of binding, tuning of surface properties, and potential for use in organic electronic applications

Transparent metal oxides, in particular, indium tin oxide (ITO), are critical transparent contact materials for applications in next-generation organic electronics, including organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). Understanding and controlling the surface properties of ITO allows for the molecular engineering of the ITO-organic interface, resulting in fine control of the interfacial chemistries and electronics. In particular, both surface energy matching and work function compatibility at material interfaces can result in marked improvement in OLED and OPV performance. Although there are numerous ways to change the surface properties of ITO, one of the more successful surface modifications is the use of monolayers based on organic molecules with widely variable end functional groups. Phosphonic acids (PAs) are known to bind strongly to metal oxides and form robust monolayers on many different metal oxide materials. They also demonstrate several advantages over other functionalizing moieties such as silanes or carboxylic acids. Most notably, PAs can be stored in ambient conditions without degradation, and the surface modification procedures are typically robust and easy to employ. This Account focuses on our research studying PA binding to ITO, the tunable properties of the resulting surfaces, and subsequent effects on the performance of organic electronic devices. We have used surface characterization techniques such as X-ray photoelectron spectroscopy (XPS) and infrared reflection adsorption spectroscopy (IRRAS) to determine that PAs bind to ITO in a predominantly bidentate fashion (where two of three oxygen atoms from the PA are involved in surface binding). Modification of the functional R-groups on PAs allows us to control and tune the surface energy and work function of the ITO surface. In one study using fluorinated benzyl PAs, we can keep the surface energy of ITO relatively low and constant but tune the surface work function. PA modification of ITO has resulted in materials that are more stable and more compatible with subsequently deposited organic materials, an effective work function that can be tuned by over 1 eV, and energy barriers to hole injection (OLED) or hole-harvesting (OPV) that can be well matched to the frontier orbital energies of the organic active layers, leading to better overall device properties.     

多孔复合金属氧化物的制备及其在消除挥发性有机物中的应用

挥发性有机物(Volatile Organic Compounds,VOCs)是一类大气环境污染物,危害人身健康,必须严格控制其排放。在消除VOCs的方法中,催化氧化被认为是最有效的方法之一,其消除效果取决于催化剂的性能。简要综述近年来多孔复合金属氧化物(包括钙钛矿型氧化物、类钙钛矿型氧化物、尖晶石型氧化物和六铝酸盐)及其负载过渡金属氧化物和贵金属(合金)催化剂的制备及其氧化消除VOCs的催化性能。比表面积、孔结构、粒子分散度、粒径、吸附氧物种浓度、低温还原性、反应物活化能力或过渡金属氧化物或贵金属(合金)与载体之间的相互作用等是影响催化剂活性的主要因素。此外,就此类研究工作的未来发展趋势还进行了展望。     

乙氧基化咪唑啉阻锈剂的制备及性能研究

以油酸和羟乙基乙二胺为原料，经酰胺化、环化反应生成咪唑啉中间体，再加成聚合环氧乙烷，得到乙氧基化咪唑啉阻锈剂。通过失重法、盐水浸渍法、盐水浸烘实验、电化学综合实验和扫描电镜等方法评价该阻锈剂对Q235碳钢在3.5%NaCl的饱和Ca(OH)2溶液的阻锈性能，并通过水泥净浆流动性实验，测试该阻锈剂对混凝土流动性的影响。研究结果表明，该阻锈剂在3.5%NaCl的饱和Ca(OH)2腐蚀介质中能有效的保护Q235碳钢免遭腐蚀，通过电化学实验，证明该阻锈剂是以抑制阳极反应为主的混合型阻锈剂，当质量浓度为4%时，钢筋的阻锈效率达到99.13%，扫描电镜显示碳钢表面光滑平整。水泥净浆流动性实验结果表明该阻锈剂对混凝土流动性无明显不利影响。     

Water-Resistant Oxide Layers at the Interface of Zinc/Epoxy Bonded Joints

The durability properties of bonded lap shear joints made from an epoxy/dicyandiamide adhesive and zinc metal coupons have been investigated. The metal coupons were anodized in sodium hydroxide solutions before bonding. The influence of the anodizing conditions on the morphology and composition of the oxide layers has been studied using SEM and TEM imaging analyses as well as X-ray photoelectron spectroscopy. The hydrolytic stability of the bonded joints has been assessed by storing the joints in water at 70 °C or 90 °C for periods of time up to 5 weeks. Polypropylene has been used as a model adhesive to study the influence of mechanical interlocking effects on the performance of the bonded joints. Depending on the anodizing conditions, the improved durability properties have been attributed either to „mechanical interlocking effects“ or to the higher hydrolytic stability of the oxide layers generated during the anodizing treatment.

Some of the results gained from the anodization of zinc have been extrapolated to hot-dipped galvanized steel. Bonded joints made from hot-dipped galvanized coupons anodized under smooth conditions (2% NaOH) displayed residual shear strengths of up to 70% higher than specimens simply degreased after immersion test. The generation of stable oxide layers as well as the suppression of intergranular corrosion phenomena at the metal/adhesive interface can explain the improved durability properties.     

Optimization of hybrid sol–gel coatings by combination of layers with complementary properties for corrosion protection of AA2024

The coating system usually employed for corrosion protection of metal substrates consists of different layers which can be constituted of a chemical conversion coating applied on the metal surface followed by a number of organic layers. Hybrid films prepared by the sol–gel method provide a good approach as protective layers on metallic surfaces, although it is necessary to combine the barrier functionality with an active protection mechanism to avoid corrosion when the coating is damaged. Previous works have shown that it is very difficult to reach this result in a mono-layer sol–gel because the amount of inhibitors incorporated tends to increase significantly the porosity of the coating and reduces drastically the barrier properties. This work presents the characterization of a multi-layer sol–gel hybrid inorganic–organic coating system with a structure composed of an intermediate cerium inhibited layer deposited between two un-doped layers on AA2024 alloy. The comparison between the inhibited system and a bi-layer non-inhibited one has allowed to assess the migration of the cerium ions into the hybrid coating towards the substrate corresponding to the improvement of the corrosion properties. The combination of the physical barrier and the active protection enables to obtain an effective protective system.     

Mechanism of enhancement of the corrosion of steel by alternating currents and electrocatalytic properties of cycled steel surfaces

The effect of potential cycling on the growth of oxide layers on steel is analysed. It is shown that the redox reactions in the oxide layers formed are diffusion controlled and it is proposed that the transport of OH^– is the limiting step of growth. The relationship of these results to ac corrosion is discussed and it is shown that alternating currents flowing through structures can accelerate the corrosion rate if the potential excursion encompasses the reduction of an inner oxide layer, not only by the growth of hydrated oxide layers, but also by the observed enhancement of the electrocatalytic properties towards oxygen reduction of the oxide layers formed on cycling.     

Oxide films at the nanoscale: new structures, new functions, and new materials

We all make use of oxide ultrathin films, even if we are unaware of doing so. They are essential components of many common devices, such as mobile phones and laptops. The films in these ubiquitous electronics are composed of silicon dioxide, an unsurpassed material in the design of transistors. But oxide films at the nanoscale (typically just 10 nm or less in thickness) are integral to many other applications. In some cases, they form under normal reactive conditions and confer new properties to a material: one example is the corrosion protection of stainless steel, which is the result of a passive film. A new generation of devices for energy production and communications technology, such as ferroelectric ultrathin film capacitors, tunneling magnetoresistance sensors, solar energy materials, solid oxide fuel cells, and many others, are being specifically designed to exploit the unusual properties afforded by reduced oxide thickness. Oxide ultrathin films also have tremendous potential in chemistry, representing a rich new source of catalytic materials. About 20 years ago, researchers began to prepare model systems of truly heterogeneous catalysts based on thin oxide layers grown on single crystals of metal. Only recently, however, was it realized that these systems may behave quite differently from their corresponding bulk oxides. One of the phenomena uncovered is the occurrence of a spontaneous charge transfer from the metal support to an adsorbed species through the thin insulating layer (or vice versa). The importance of this property is clear: conceptually, the activation and bond breaking of adsorbed molecules begin with precisely the same process, electron transfer into an antibonding orbital. But electron transfer can also be harnessed to make a supported metal particle more chemically active, increase its adhesion energy, or change its shape. Most importantly, the basic principles underlying electron transfer and other phenomena (such as structural flexibility, electronic modifications, and nanoporosity) are now largely understood, thus paving the way for the rational design of new catalytic systems based on oxide ultrathin films. Many of the mechanisms involved (electron tunneling, work function changes, defects engineering, and so forth) are typical of semiconductor physics and allow a direct link between the two fields. A related conceptual framework, the "electronic theory of catalysis", was proposed a long time ago but has been largely neglected by the catalytic community. A renewed appreciation of this catalytic framework, together with spectacular advances in modeling and electronic structure methods, now makes it possible to combine theory with advanced experimental setups and meet the challenge of designing new materials with tailored properties. In this Account, we discuss some of the recent advances with nanoscale oxide films, highlighting contributions from our laboratory. Once mastered, ultrathin oxide films on metals will provide vast and unforeseen opportunities in heterogeneous catalysis as well as in other fields of science and technology.     

环保型除锈剂的研究

传统无机酸酸洗对钢铁基体的腐蚀性强,并且会产生酸雾。制备出一种环保型除锈剂。确定了除锈剂的成分,并分析了各成分的作用。研究了有机酸、缓蚀剂、表面活性剂、渗透剂对除锈时间和除锈效果的影响。     

Evaluation of Graphene/WO<Subscript>3</Subscript> and Graphene/CeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Structures as Electrodes for Supercapacitor Applications

The combination of graphene with transition metal oxides can result in very promising hybrid materials for use in energy storage applications thanks to its intriguing properties, i.e., highly tunable surface area, outstanding electrical conductivity, good chemical stability, and excellent mechanical behavior. In the present work, we evaluate the performance of graphene/metal oxide (WO₃ and CeO _x ) layered structures as potential electrodes in supercapacitor applications. Graphene layers were grown by chemical vapor deposition (CVD) on copper substrates. Single and layer-by-layer graphene stacks were fabricated combining graphene transfer techniques and metal oxides grown by magnetron sputtering. The electrochemical properties of the samples were analyzed and the results suggest an improvement in the performance of the device with the increase in the number of graphene layers. Furthermore, deposition of transition metal oxides within the stack of graphene layers further improves the areal capacitance of the device up to 4.55 mF/cm², for the case of a three-layer stack. Such high values are interpreted as a result of the copper oxide grown between the copper substrate and the graphene layer. The electrodes present good stability for the first 850 cycles before degradation.     

Analysis of iron oxides accumulating at the interface between aggregates and cement paste

Iron oxides formed by the corrosion of a steel reinforcing bar were found to accumulate at interfaces between aggregate and cement paste in Portland cement-based concrete. Microstructural characterization of the zones of oxide was carried out using optical microscopy, scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) spectroscopy. The study showed that the oxide layer has an average thickness of about 1.5 mm, and consists of inner and outer layers with different types of oxides in each layer. A mechanism based on the Schikorr reaction is proposed for the preferential formation of iron oxides in the two layers taking into account local variations in pH of the pore solution during the early ages of the cement paste.     

Study of the properties of a multi-component inhibitor used for water treatment in cooling circuits

This work is devoted to the corrosion inhibition of a carbon steel in a 200 mg l^–1 NaCl solution by an original multicomponent inhibitor: fatty amines in association with phosphonocarboxylic acid salts and a biocide at low dosage. Its principal advantage is the absence of toxicity and its biodegradability. Steady-state current–voltage curves were combined with electrochemical impedance measurements to characterize the inhibitive properties of each compound and to optimize the concentration of the compounds in the mixture. Phosphonocarboxylic acid salts were observed to act as an anodic inhibitor whereas mixed action was shown for fatty amines. The inhibitive efficiency was increased when the phosphonocarboxylic acid salt concentration was increased whereas for fatty amines an increase of the concentration did not improve corrosion inhibition. Optimal concentrations of fatty amines and phosphonocarboxylic acid salts were determined. X-ray photoelectron spectroscopy (XPS) was used to analyse the layer formed on the metal surface by the inhibitive mixture. The film was composed of an iron oxide/hydroxide mixture incorporating the organic compounds. The inhibitive molecules interact with the iron oxides.     

Functional nanoceramics for intermediate temperature solid oxide fuel cells and oxygen separation membranes

This work reviews results of research aimed at design and characterization of mixed ionic–electronic conducting perovskite–fluorite nanocomposite oxide ceramics. Nanocrystalline oxides were prepared via Pechini route, nanocomposites – via ultrasonic dispersion of their mixture in organic solvents with addition of surfactants. Genesis of the real structure of nanocomposites at sintering by conventional as well as advanced (microwave or e-beam treatment) techniques was studied in details by structural methods. Applied preparation procedures ensured nano-sizes of perovskite/fluorite domains even in dense ceramics and a high spatial uniformity of their distribution. Redistribution of elements between perovskite and fluorite domains without formation of new phases was revealed. Characterization of nanocomposite transport properties by oxygen isotope heteroexchange and conductivity or weight relaxation demonstrated that perovskite–fluorite interfaces are paths for fast oxygen diffusion. Best perovskite–fluorite combinations tested as cathode layers or dense oxygen separation layers in asymmetric supported membranes demonstrated performance promising for the practical application.