Complex anticorrosion coating for ZK30 magnesium alloy

This work aims at developing a new complex anticorrosion protection system for ZK30 magnesium alloy. This protective coating is based on an anodic oxide layer loaded with corrosion inhibitors in its pores, which is then sealed with a sol–gel hybrid polymer. The porous oxide layer is produced by spark anodizing. The sol–gel film shows good adhesion to the oxide layer as it penetrates through the pores of the anodized layer forming an additional transient oxide–sol–gel interlayer.The thickness of this complex protective coating is about 3.7–7.0 μm. A blank oxide–sol–gel coating system or one doped with Ce³⁺ ions proved to be effective corrosion protection for the magnesium alloy preventing corrosion attack after exposure for a relatively long duration in an aggressive NaCl solution.The structure and the thickness of the anodized layer and the sol–gel film were characterized by scanning electron microscopy (SEM). The corrosion behaviour of the ZK30 substrates pre-treated with the complex coating was tested by electrochemical impedance spectroscopy (EIS), scanning vibrating electrode technique (SVET), and scanning ion-selective electrode techniques (SIET).     

The electrochemical behaviour of sol–gel hybrid coatings applied on AA2024-T3 alloy: Effect of the metallic surface treatment

The effect of surface pre-treatments on the electrochemical behaviour of sol–gel coated AA2024-T3 alloys was studied in this work. Three different cleaning procedures were employed: degreasing, mechanical polishing, and chemical etching. The surface morphology was different depending on the pre-treatment. The smoothest surface corresponded to polished samples while the chemically etched ones had the highest roughness, even though the sol–gel film covered all the cavities. The hybrid sol–gel film was prepared by copolymerisation of two different sols, tetra-n-propoxyzirconium (TPOZ) as inorganic precursor and 3-glycydoxypropyltrimethoxysilane (GPTMS) as organic precursor. The corrosion resistance was evaluated using accelerated test (salt spray) and electrochemical measurements under continuous immersion. Both experiments indicated that degreased samples had better anticorrosive properties.     

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.     

Corrosion protection behaviour of sol–gel derived N,N-dimethylthiourea doped 3-glycidoxypropyltrimethoxysilane on aluminium

The present work describes the anticorrosion features of inhibitor doped sol–gel coating on Al metal. Sol–gel coatings were prepared by using 3-glycidoxypropyltrimethoxysilane (GPTMS) as parent precursor. In order to improve the corrosion resistance property of coating, N,N-dimethylthiourea was added into the sol–gel matrix. The corrosion inhibitor doped sol–gel coating on metal was characterized by Fourier transform infrared analysis (FTIR) and scanning electron microscope (SEM). Inhibition effect of N,N-dimethylthiourea doped GPTMS coating on Al substrates in 1% NaCl solution was investigated using electrochemical impedance (EIS) and polarization studies. EIS results showed that the corrosion resistance of sol–gel coating significantly improved upon addition of N,N-dimethylthiourea. The study had outlined the nuances of doping an organic inhibitor to enhance the protection ability of sol–gel coating on Al metal.     

The corrosion resistance of 316L stainless steel coated with a silane hybrid nanocomposite coating

The present work aims at evaluating the corrosion resistance of 316L stainless steel pre-treated with an organic–inorganic silane hybrid coating. The latter was prepared via a sol–gel process using 3-glycidoxypropyl-trimethoxysilane as a precursor and bisphenol A as a cross-linking agent. The corrosion resistance of the pre-treated substrates was evaluated by neutral salt spray tests, linear sweep voltammetry and electrochemical impedance spectroscopy techniques during immersion in a 3.5% NaCl solution. In addition, the effect of the drying method as an effective parameter on the microscopic features of the hybrid coatings was studied using Fourier transform infrared spectroscopy and scanning electron microscopy. Results show that the silane hybrid coatings provide a good coverage and an additional corrosion protection of the 316L substrate.     

Corrosion resistance performance of cerium doped silica sol–gel coatings on 304L stainless steel

The aim of this work is the synthesis and investigation of silane based organic–inorganic hybrid coatings, which can be used to improve the corrosion performance of steel structures subjected to a marine environment. The silane based sol–gel coatings were prepared by dip coating 304L stainless steel in a solution of organically modified silica sol made through hydrolysis and condensation of 3-glycidoxypropyl-trimethoxysilane (GPTMS) as precursor and bisphenol A (BPA) as a cross-linking agent in an acid catalyzed condition. The influence of the addition of cerium and the use of bisphenol A as a cross-linking agent on the microscopic features and morphology as well as on the corrosion resistance of the coatings were examined using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), neutral salt spray tests, potentiodynamic polarization and electrochemical impedance techniques. Results show that cerium modified nano-hybrid coatings exhibit a superior corrosion inhibition performance to that displayed by silica hybrid coatings. Additionally, data showed that the bisphenol A as a cross-linking agent has a significant effect on the morphology and corrosion resistance of the cerium doped silica coating. Omitting the use of bisphenol A causes the creation of defects/cracks in the coating, thereby promoting diffusion of the aggressive electrolyte toward the substrate and decreasing the corrosion resistance of the coating.     

Corrosion protection by zirconia-based thin films deposited by a sol–gel spin coating method

This paper focuses on the structure and corrosion behavior of 316L stainless steel coated by inorganic ZrO₂, hybrid ZrO₂–PMMA, and combined inorganic–hybrid films. The coatings were deposited by a particulate sol–gel spin-coating route, using carboxymethyl cellulose as a nanoparticle dispersant. The electrochemical evaluations were conducted in a simulated body fluid, via potentiodynamic polarization and impedance spectroscopic experiments. According to the results, the hybrid coating presented a better corrosion protection compared to the inorganic coating, due to a lesser density of structural defects. However, the best corrosion resistance was found for a combined coating which consists of an inorganic bottom layer and a hybrid top layer, due to a desirable compromise of good adhesion and low defect density.     

Sol–gel coatings on metals for corrosion protection

Sol–gel protective coatings have shown excellent chemical stability, oxidation control and enhanced corrosion resistance for metal substrates. Further, the sol–gel method is an environmentally friendly technique of surface protection and had showed the potential for the replacement of toxic pretreatments and coatings which have traditionally been used for increasing corrosion resistance of metals. This review covers the recent developments and applications of sol–gel protective coatings on different metal substrates, such as steel, aluminum, copper, magnesium and their alloys. The challenges for industrial productions and future research on sol–gel corrosion protective coatings are also briefly discussed.     

Electrochemical investigation of high-performance silane sol–gel films containing clay nanoparticles

Silane sol–gel coatings are widely used as adhesion promoters between inorganic substrates, such as metals, and organic coatings. The aim of these pre-treatments is to enhance the corrosion protection performance of the organic coating improving the adhesion to the substrate and acting as a barrier against water and aggressive ions diffusion. It is a matter of fact that the silane sol–gel pre-treatments do not provide an active protection against corrosion processes except for the partial inhibition of the cathodic reaction. Inorganic pigments can improve the barrier properties of the silane sol–gel film, enhancing the resistance against corrosion. In this study, different amounts of montmorillonite nanoparticles were added to a water based silanes mixture in order to improve the barrier properties of the sol–gel coating. Hot dip galvanized steel was used as substrate. The sol–gel film consists of a combination of three different silanes, GPS, TEOS and MTES. The clay nanoparticles used in this study were mainly neat montmorillonite. The proper concentration of filler inside the sol–gel films was determined comparing the corrosion resistance of silane layers with different nanoparticles contents. Additionally, the effect of CeO₂ and Ce₂O₃ enriched montmorillonite particles. The EIS analysis and the polarization measurements demonstrated that the optimal amount of neat montmorillonite nanoparticles is about 1000 ppm. The same electrochemical techniques highlighted the limited effect of the cerium oxides grafted to the clay nanoparticles on the corrosion resistance of the silane sol–gel film. The TEM analysis proved the presence of a nano-crystalline structure inside the silane sol–gel film due to the formation of crystalline silica domains.     

Preparation of hybrid silica sol–gel coatings on mild steel surfaces and evaluation of their corrosion resistance

Hybrid silica sol–gel coatings were prepared on mild steel substrate by dip coating technique. The coatings were subsequently heat treated at 200 °C in order to improve their corrosion properties. The coating sols were synthesized using Glycidoxypropyltrimethoxysilane (glymo) and Aminopropylethoxysilane (ameo) as precursor materials. Potentiodynamic polarization curves were derived and Electrochemical Impedance Spectroscopy (EIS) measurements were made in NaCl solution. The surface and cross-section morphology of coated specimens were characterized by scanning electron microscopy (SEM). Fourier transformed infrared (FTIR) analysis was used to identify the presence of various functional groups in the coating solutions. A comparison of the corrosion resistance of the coated and uncoated mild steel was presented. The results indicated that the corrosion resistance of the coated mild steel was improved considerably.     

Synthesis of fluorinated/methacrylated epoxy based oligomers and investigation of its performance in the UV curable hybrid coatings

Organic–inorganic hybrid coatings based on fluorinated/methacrylated soybean oil and bisphenol A/F epoxy methacrylate were obtained by combining photopolymerization and sol–gel process. Hard and transparent hybrid coatings were prepared on polycarbonate panels and their physical and mechanical properties such as gel content, hardness, adhesion, gloss, contact angle as well as tensile strength were measured. Results from the mechanical measurements showed that the properties of hybrid coatings improved with the increase in fluorine and sol–gel precursor contents. Thermo gravimetric analysis results demonstrated that fluorine and silica incorporations significantly enhanced the thermal oxidative stability of the hybrid coating materials. The surface morphology was also characterized by scanning electron microscopy (SEM). SEM studies indicated that inorganic particles were dispersed homogenously throughout the organic matrix.     

Influence of substrate composition on corrosion protection of sol–gel thin films on magnesium alloys in 0.6 M NaCl aqueous solution

The corrosion protection behaviour of organic–inorganic hybrid thin films on AZ31 and AZ61 magnesium alloy substrates has been studied. These films were prepared by a sol–gel dip-coating method. The organopolysiloxane precursors were γ-methacryloxypropyltrimethoxysilane (MAPTMS) and tetramethoxysilane (TMOS). An attempt was made to determine the possible relationships between the degradation of the sol–gel film and composition of the metal substrate during the exposure of the metal/coating system to 0.6 M NaCl aqueous solutions. For this purpose electrochemical impedance spectroscopy (EIS) and hydrogen evolution measurements were applied. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) analyses revealed that the sol–gel films formed on the surface of AZ61 alloy were far more perfect and uniform than those formed on the AZ31 alloy. This behaviour was attributed to the effect of the native oxide film initially present on the surface of the AZ61 alloy, which inhibited the attack of magnesium. Results indicated that the sol–gel coated AZ61 substrate tended to develop corrosion products slower than the sol–gel coated AZ31 substrate, trend that could change by prolonging exposure time. After 6 days of immersion, a clear inhibitive effect of the corrosion products formed during the test was observed in the case of the sol–gel coated AZ31, but not with the coated AZ61 alloy substrate, a phenomenon explained by the carbonate enrichment observed by XPS.     

Effect of cerium concentration on microstructure,morphology and corrosion resistance of cerium–silica hybrid coatings on magnesium alloy AZ91D

The aim of this work was to investigate the effect of cerium concentration on microstructure, morphology and anticorrosion performance of cerium–silica hybrid coatings on magnesium alloy AZ91D. Vinyltriethoxysilane (VETO) and γ-glycidoxypropyltrimethoxysilane (GPTMS) were employed as precursors to prepare sol–gel based silica coating. Cerium nitrate hexahydrate as dopant in five different concentrations was added into the silica coatings. Fourier transform infrared (FT-IR) spectrum analysis, viscosity measurements and scanning electron microscopy (SEM) were employed to characterize the microstructure and morphology of these coatings. It was found that with the increase of cerium concentration, the degree of decomposition of silane chains in the coating network increased. The corrosion resistance of the cerium–silica hybrid coatings was estimated by electrochemical impedance spectroscopy (EIS) measurements and potentiodynamic polarization tests. The results demonstrated that corrosion resistance of coatings initially increases and then decreases as cerium concentration goes up. When the cerium concentration is 0.01 M, corrosion resistance reaches its maximum.     

Characterization of hybrid sol–gel coatings doped with hydrotalcite-like compounds to improve corrosion resistance of AA2024-T3 alloys

The inhibition effect of hydrotalcite addition to hybrid sol–gel coatings applied on AA2024-T3 alloy was evaluated. Hydrotalcite belongs to the anionic clay family with wide applications, most of them based on its anion exchange capacity due to its double layered structure. In this work hydrotalcite (HT) powder was prepared by the classical co-precipitation method using magnesium and aluminum nitrates as precursors. Different weight percentages (1, 5 and 10%, w/w) of hydrotalcite with Mg/Al ratio of 2.5 were added to hybrid sols prepared by copolymerization of 3-Glycydoxypropyltrimethoxysilane (GPTMS) and tetra-n-propoxyzirconium (TPOZ). The sol–gel coatings were deposited by dip-coating method on AA2024-T3 substrate. Scanning electron microscopy (SEM) and mechanical profilometry measurements revealed the heterogeneous particle sizes and the distribution of the agglomerates. Hydrotalcite additions significantly increased the bond strength between metal and coating, according to pull-off test results.The corrosion performance was evaluated by the salt spray fog chamber test and by Electrochemical Impedance Spectroscopy (EIS). The results showed a marked improvement of the corrosion resistance on the aluminum alloy when HT was added to the hybrid sol–gel coating. This positive effect was more evident at higher weight percentages of hydrotalcite.     

Corrosion protection of AZ91D magnesium alloy with sol–gel coating containing 2-methyl piperidine

Sol–gel coatings were prepared using 3-glycidoxypropyltrimethoxysilane (GPTMS) and tetraethoxysilane (TMOS) as precursors, diethylentriamine as curing agent. Inhibition effect of 2-methyl piperidine on AZ91D in 0.005%, 0.05% and 0.5% (wt.%) NaCl solution is investigated. Potentiodynamic polarization tests revealed that 2-methyl piperidine significantly decreased the anodic activity of AZ91D especially at high concentration. Corrosion behaviors of sol–gel coatings incorporated with 2-methyl piperidine on AZ91D in the Harrison's solution were analyzed using electrochemical impedance spectroscopy (EIS). EIS results showed the corrosion resistance of sol–gel coatings and sol–gel sealed phosphate conversion coating on AZ91D were significantly improved through addition of 2-methyl piperidine. Fourier transform infrared spectra (FT-IR) confirmed that the 2-methyl piperidine was compatible with sol–gel matrix.     

Hydrogen permeation through sol–gel-coated iron during galvanostatic charging

One-layer sol–gel silica–zirconia and two-layer silica–zirconia and zirconia coatings were deposited on one side of iron membranes by spin-coating, densified in air and annealed up to 800 °C in vacuum. Hydrogen permeation through the membranes, coated and uncoated, polarised cathodically under galvanostatic control in 0.1 M NaOH solution was studied using the electrochemical permeation technique. During the initial period, the effect of the sol–gel coatings was insignificant. However, the coatings quite efficiently prevented the iron surface become more active to hydrogen entry during a long-lasting cathodic polarisation. In addition, the electrochemical-corrosion behaviour of the coated iron and the effect of the sol–gel coatings on the effective diffusivity of hydrogen in the coated membranes were studied. On the basis of the polarisation curves and the hydrogen permeation data it was proved that the sol–gel coatings blocked the iron surface for the hydrogen evolution reaction and, consequently, for the hydrogen entry into iron. The effective coating coverage was determined by comparison of the hydrogen fluxes permeating the coated and uncoated membranes. Finally the real concentration of hydrogen beneath the uncoated iron sites and the amount of hydrogen stored in a membrane were evaluated.     

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.     

Corrosion behavior of nanohybrid titania–silica composite coating on phosphated steel sheet

Corrosion resistance behavior of sol–gel-derived organic–inorganic nanotitania–silica composite coatings was studied. Hybrid sol was prepared from Ti-isopropoxide and N-phenyl-3-aminopropyl triethoxy silane. The structure, morphology, and properties of the coating were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and thermo gravimetric analysis. The corrosion performances of the sol–gel-coated samples were investigated by electrochemical impedance spectroscopy (EIS) and standard salt spray tests. The hybrid coatings were found to be dense, more uniform, and defect free. In addition, the coatings also proved its excellent corrosion protection on phosphated steel sheet.     

Self-repairing coating for corrosion protection of aluminum alloys

The development of effective anticorrosion pre-treatments for metallic substrates is an issue of great importance for durability of metal structures and components. In this work, we proposed and demonstrated the concept of self-repairing coating for corrosion protection of aluminum alloys, using cagelike oil core/silica gel shell particles. These micron-scale, cagelike smart microspheres with opened and closed pores were successfully fabricated, and at the same time encapsulated repairing agent (methyl methacrylate) and catalysts (potassium persulfate and sodium thiosulfate) into the microspheres, respectively. Such smart particle composites (SPCs) were prepared based on an interfacial self-assembly process and sol–gel reaction. They were then self-assembled on the AA2024 aluminum alloy surface, followed by the application of a sol–gel film. The hybrid film worked as a primer coating featuring the self-repairing property. Both the EIS and SEM/EDS data demonstrated that the encapsulated repairing agent was released as a response to external stimulus (scratches) and polymerized to repair the coating defects. By comparing the corrosion rate of AA2024 in three coating systems, the self-repairing effect is quantified to be 22% after 2 h immersion in deoxidized 3% NaCl solution.     

Corrosion resistance of phosphate coatings obtained by cathodic electrochemical treatment: Role of anode–graphite versus steel

The formation of phosphate coatings by cathodic electrochemical treatment using graphite and steel anodes and evaluation of their corrosion resistance is addressed in this paper. The type of anode used, graphite/steel, has an obvious influence on the composition of the coating, resulting in zinc–zinc phosphate composite coating with graphite anode and zinc–iron alloy–zinc phosphate–zinc–iron phosphate composite coating with steel anode. The corrosion resistance of the coating is found to be a function of the composition of the coating. The deposition of zinc/zinc–iron alloy along with the zinc phosphate/zinc and zinc–iron phosphate using graphite/steel anodes has caused a cathodic shift in the E_corr compared to uncoated mild steel substrates. The i_corr values of these coatings is very high. EIS studies reveal that zinc/zinc–iron alloy dissolution is the predominant reaction during the initial stages of immersion. Subsequently, the formation of zinc and iron corrosion products imparts resistance to the charge transfer process and increases the corrosion resistance with increase in immersion time. The corrosion products formed might consist of oxides and hydroxychlorides of zinc and iron. The study suggests that cathodic electrochemical treatment could be effectively utilized to impart the desirable characteristics of the coating by choosing appropriate anode materials, bath composition and operating conditions.