Sol-gel coatings for chemical protection of stainless steel

Sol-gel thin coatings of ZrO₂, SiO₂, 70SiO₂-30TiO₂ and 88SiO₂-12Al₂O₃ compositions (mole %) have been prepared from sonocatalyzed sols and deposited by dip-coating technique on 316L stainless steel foils. The influence of the coatings on the chemical corrosion of the substrate has been measured through potentiodynamic polarization curves in aqueous 15% H₂SO₄ solution between 25 and 50°C. The values of the corrosion potential, polarization resistance and corrosion rate have been determined. Analysis of the data combined with scanning electron microscopy studies indicate that the films act as a geometric blocking against exposure to the corrosive media and increase the lifetime of the substrate up to a factor 8.5.     

Sol-gel alumina coatings on stainless steel for wear protection

The aluminium oxide films on austenitic steel are prepared from sols of re-dispersed boehmite nano powders in water. After dip-coating of the sol, a heat treatment including drying, calcination and annealing in vacuum at temperatures up to 1100°C is performed to obtain crack-free coatings of a thickness up to 6 μm. XRD measurements detect α- and γ-alumina, a TiO_x-phase at the metal/coating interface and a gradient of phase formation in the coating. The strong adhesion on the substrates is due to the layered assembly and gradient composition of the coating caused by an inter-diffusion of metal cations and oxygen in the metal/oxide interface during heat treatment. Residual stress measurements by X-rays result in compressive stresses of 2–4 GPa in the alumina coatings. The pin-on-disc test shows a remarkable improvement of wear resistance obtained by sol-gel coatings. The α-alumina content and the compressive stress of the coatings correlate with wear resistance of the coatings.     

Sol-gel ZrO2 coatings for chemical protection of stainless steel

ZrO₂ coatings deposited on 316 L stainless steel sheets were synthesized by sol-gel method using Zr(OC₃H₇)₄ as precursor and isopropanol, glacial acetic acid, and water as solvents for application with ultrasounds. Different solutions for dip-coating were prepared with compositions varying between 0.025 and 0.9 mol/dm³ of ZrO₂. X-ray diffraction shows that the films densified at 800°C are crystalline with a tetragonal structure. The thickness of the coatings varied from 0.35–0.75 m. The influence of the ZrO₂ coatings on the corrosion behavior of stainless steel substrates in aqueous NaCl was studied through potentiodynamic polarization curves at 1 mV/s. The values of the electrochemical parameters allow for an explanation of the role of the films in the increased resistance of steel against corrosion in moderately aggressive environments.     

Polydopamine supported preparation method for solid-phase microextraction coatings on stainless steel wire

In this paper, we introduced a novel and versatile route to prepare solid-phase microextraction coatings on the chemically inert stainless steel wire. Polydopamine films can be created on metallic substrates by an oxidant-induced polymerization and subsequently support various secondary reactions to prepare functional surfaces. In the present work, polydopamine-bioactivated stainless steel wire was successfully modified by nanostructured hydroxyapatite. Extraction performance of the fiber was assessed on several polycyclic aromatic hydrocarbons in water solutions. Extraction mechanism was suggested based on the correlation of partition coefficients and LogPs. Both aqueous and solid real life samples were used to test the reliability of the solid-phase microextraction-gas chromatography method; some analytes were detected and quantified.     

Antiicing performance of superhydrophobic coatings on aluminum and stainless steel

The mechanisms determining the antiicing protection imparted by superhydrophobic coatings to the aluminum and steel surfaces of structural parts and control equipment of aircrafts are discussed. The results of testing at negative surface temperatures and high velocities of air—vapor flow demonstrated that application of superhydrophobic coatings produced by various methods provides a substantial mitigation of ice accretion compared to uncoated metal surfaces. The superhydrophobic coatings on aluminum surfaces completely prevent the formation of ice at moderate flow velocities of up to 40 m s^?1. At higher flow rates under conditions of ice formation on the test desk, the effect of periodic self-cleaning of the superhydrophobic surface was detected. Superhydrophobic coatings on stainless-steel parts of air pressure sensor also demonstrate explicit antiicing effect associated with the reduced heating power required to completely prevent ice formation on the sensor surface.     

Electrochemical behaviour of SiO2 sol-gel coatings on stainless steel

SiO₂ coatings onto stainless steel substrates have been prepared by sol-gel in order to study the performance and mechanism of attack in different corrosive solutions. The electrochemical behaviour of the samples has been evaluated by Electrochemical Impedance Spectroscopy using NaCl and HCl as electrolytes. Comparative tests have been performed on samples with one and two silica layers as well as on uncoated ones. SiO₂ coatings produce no important protection of stainless steels subjected to electrochemical corrosion. This behaviour may be explained by micropores and microcracks produced during the coating sintering.     

Cerium hybrid silica coatings on stainless steel AISI 304 substrate

AISI 304 Stainless Steel is widely used in different industrial fields because of its mechanical and corrosion properties. However, its tendency to corrosion in presence of halide ions limits the applications. One strategy to improve the corrosion resistance is the use of coatings barriers containing corrosion inhibitors in their formulation. The lanthanides present attractive green and corrosion properties for the substitution of chromates, which are the most common substances used as corrosion protection. However, these compounds are highly toxic, and an intense effort is being undertaken to replace them. Cerium is a good alternative because of its relatively low cost and abundance. It fulfils the basics requirements for being considered an alternative inhibitor: the ions form insoluble hydroxides and they present low toxicity. Inorganic and hybrid sol-gel coatings have been developed to increase the corrosion resistance of metals and they provide an excellent vehicle for the incorporation of secondary phases including particles and metal ions as cerium ions. The aim of this work was to study the influence of the incorporation of cerium ions in hybrid silica sol-gel coatings deposited on AISI 304 stainless steel as substrate as a potential replacement of chromate treatments. This system should combine the barrier protection effect of silica coating with the corrosion inhibitor effect of the cerium ions inside the coatings. After 7 days of immersion in NaCl, coated substrates showed lower current densities than the bare steel, although the coatings produced from Ce (III) salts experience a slight weakening in time and those obtained from Ce (IV) chemicals evidence an enhance in the coating performance, probably due to the plugging of corrosion products in the defective areas of the film.     

Protective properties of electrochemical polyaniline coatings on low-carbon steel

Protective properties of polyaniline coatings electrochemically deposited in the galvanostatic mode from oxalic acid solutions onto the surface of low-carbon steel were studied by measuring polarization curves.     

Protective properties of redox polymer film deposited on stainless steel

We propose a novel composite organic-inorganic coating in the form of a redox polymer film for protection of stainless steel against general corrosion in strong acid medium (2 M H₂SO₄). We utilize an anion exchange polymer, protonated poly(4-vinylpyridine), into which hexacyanoferrate anions have been introduced. Owing to the presence of Fe(CN)₆^3–/4– at the interface formed by the film and the steel, a sparingly soluble metal hexacyanoferrate (mostly Prussian blue, PB) is formed as an overcoating on the steels surface, presumably on the passive (metal oxide) layer. The redox polymer film on the steel seems to act as a composite three-dimensional bilayer-type coating in which hexacyanoferrate(III,II) anions (that are capable of effective charge storage) exist in the outer portions of the film, whereas the inner PB layer improves the systems overall adherence and stability. By analogy to a conducting polymer (e.g. polyaniline, polypyrrole), introduction of the redox polymer composite film leads to stabilization of the steel substrates potential within the passive range.Contribution to the 3rd Baltic Conference on Electrochemistry, Gdansk-Sobieszewo, Poland, 23–26 April 2003Dedicated to the memory of Harry B. Mark, Jr. (28 February 1934–3 March 2003)     

Fabrication of CuO-ZnO nanowires on a stainless steel mesh for highly efficient photocatalytic applications

We report the photocatalytic activity of flower-like CuO-ZnO heterostructured nanowires (NWs) fabricated on a stainless steel mesh. The mesh provided an extensive surface area and facilitated efficient mass transfer. The composed NWs exhibited excellent photocatalytic activity and showed additional enhanced properties due to multilayered, dual light source effects during the photodecomposition of a non-biodegradable azo dye.     

Photocatalytic coatings for environmental applications

A series of nano- and micronparticle-grade anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. Their photocatalytic activites have been determined in a series of paint films by FTIR, chalking, color, gloss change and weight loss after artifical weathering. The pigments have also been examined by rapid assessment methodologies using photodielectric microwave spectroscopy, 2-propanol oxidation and hydroxyl analysis. The microwave response under light and dark cycles provides an extended timescale probe of charge-carrier dynamics in the pigments. Pigment particle size, surface area and properties clearly play an important role in dispersion and any polymer-pigment interactions. Photooxidation studies on several types of paint films show a clear demarcation between nanoparticle- and pigmentary-grade titanium dioxide, with the former being more active because of their greater degree of catalytic surface activity. The photosensitivity of titanium dioxide is considered to arise from localized sites on the crystal surface (i.e. acidic OH), and occupation of these sites by surface treatments inhibits photoreduction of the pigment by ultraviolet radiation; hence, the destructive oxidation of the binder is inhibited. Coatings containing 2-5% by weight alumina or alumina and silica are satisfactory for general-purpose paints. If greater resistance to weathering is desired, the pigments are coated more heavily to about 7-10% weight. The coating can consist of a combination of several materials, e.g. alumina, silica, zirconia, aluminum phosphates of other metals. For example, the presence of hydrous alumina particles lowers van der Waals forces between pigments particles by several orders of magnitude, decreasing particle-particle attractions. Hydrous aluminum oxide phases appear to improve dispersibility more effectively than most of the other hydroxides and oxides. Coated nanoparticles are shown to exhibit effective light stabilization in various water- and oil-based paint media in comparison with conventional organic stabilizers. Hindered piperidine stabilizers are shown to provide no additional benefits in this regard, often exhibiting strong antagonism. The use of photocatalytic titania nanoparticles in the development of self-cleaning paints and microbiological surfaces is also demonstrated in this study. In the former case, surface erosion is shown to be controlled by varying the ratio of admixture of durable pigmentary-grade rutile (heavily coated) and a catalytic-grade anatase nanoparticle. For environmental applications in the development of coatings for destroying atmospheric pollutants such as nitrogen oxide gases (NO(X)), stable substrates are developed with photocatalytic nanoparticle-grade anatase. In this study, porosity of the coatings through calcium carbonate doping is shown to be crucial in the control of the effective destruction of atmospheric NO(X) gases. For the development of microbiological substrates for the destruction of harmful bacteria, effective nanoparticle anatase titania is shown to be important, with hydrated high surface area particles giving the greatest activity.     

Preparation of temperature sensitive molecularly imprinted polymer for solid-phase microextraction coatings on stainless steel fiber to measure ofloxacin

A kind of new temperature sensitive molecularly imprinted polymer (MIP) with ofloxacin (OFL) as template was prepared for the coating of solid phase microextraction (SPME). Dopamine was self-polymerized on stainless steel fiber (SSF) as the SPME support followed by silanization. Then MIP was synthesized as SPME coating on the modified SSF in a capillary, with N-isopropyl acrylamide as temperature sensitive monomer and methacrylic acid as functional monomer. The synthesis could be well repeated with multiple capillaries putting in the same reaction solution. The obtained MIP fiber was evaluated in detail with different techniques and various adsorption experiments. At last the MIP fiber was used to extract the OFL in milk. Satisfied recoveries between 89.7 and 103.4% were obtained with the limit of quantification (LOQ_LC) of 0.04 μg mL⁻¹ by the method of SPME coupled with high performance of liquid chromatography (HPLC).     

Antibacterial polyelectrolyte micelles for coating stainless steel

In this study, we report on the original synthesis and characterization of novel antimicrobial coatings for stainless steel by alternating the deposition of aqueous solutions of positively charged polyelectrolyte micelles doped with silver-based nanoparticles with a polyanion. The micelles are formed by electrostatic interaction between two oppositely charged polymers: a polycation bearing 3,4-dihydroxyphenylalanine units (DOPA, a major component of natural adhesives) and a polyanion (poly(styrene sulfonate), PSS) without using any block copolymer. DOPA units are exploited for their well-known ability to anchor to stainless steel and to form and stabilize biocidal silver nanoparticles (Ag(0)). The chlorine counteranion of the polycation forms and stabilizes biocidal silver chloride nanoparticles (AgCl). We demonstrate that two layers of micelles (alternated by PSS) doped with silver particles are enough to impart to the surface strong antibacterial activity against gram-negative E. coli. Moreover, micelles that are reservoirs of biocidal Ag(+) can be easily reactivated after depletion. This novel water-based approach is convenient, simple, and attractive for industrial applications.     

Poly(3-octylthiophene) and polystyrene blends thermally treated as coatings for corrosion protection of stainless steel 304

The effect of thermal annealing of poly(3-octylthiophene) (P3OT) and polystyrene (PS) blend coatings on the corrosion inhibition of stainless steel in a 0.5 M NaCl solution was investigated. P3OT was synthesized by direct oxidation of the 3-octylthiophene monomer with ferric chloride (FeCl₃) as oxidant. Stainless steel electrodes with mirror finish were coated with P3OT/PS blend by drop-casting technique. In order to study the temperature effect on the function like physical barrier against the corrosive species of P3OT/PS polymeric blend, the coatings were thermally annealed at three different temperatures (55?°C, 80?°C, and 100?°C). The corrosion behavior of P3OT/PS-coated stainless steel was investigated in 0.5 M NaCl at room temperature, by using potentiodynamic polarization curves, linear polarization resistance (LPR), and electrochemical impedance spectroscopy. The LPR values indicated that, at 100?°C, P3OT/PS coatings showed a better protection of the 304 stainless steel in 0.5 M NaCl; the corrosion rate diminished in two orders of magnitude with regard to the bare stainless steel. The superficial morphology of the coatings before and after the corrosive environment was researched by atomic force microscopy, optic microscopy, and scanning electronic microscopy. Morphological study showed that the increased temperature benefited the integration of the two polymeric phases, which improved the barrier properties of the coatings. The coating/metal adhesion and the coating thickness were evaluated. The temperature increases the adhesion degree coating/substrate; thus, the coating annealed at 100?°C showed the best adhesion.     

Preparation of thin layer materials with macroporous microstructure for SOFC applications

A facile and versatile method using polymethyl methacrylate (PMMA) microspheres as pore formers has been developed to prepare thin layer oxide materials with controlled macroporous microstructure. Several mixed oxides with fluorite and perovskite-type structures, i.e. doped zirconia, ceria, ferrites, manganites, and NiO-YSZ composites have been prepared and characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption and mercury porosimetry. The synthesised materials are nanocrystalline and present a homogeneous pore distribution and relatively high specific surface area, which makes them interesting for SOFC and catalysis applications in the intermediate temperature range.     

Micro-electrospray with stainless steel emitters

The physical processes underlying micro-electrospray (micro-ES) performance were investigated using a stainless steel (SS) emitter with a blunt tip. Sheathless micro-ES could be generated at a blunt SS tip without any tapering or sanding if ESI conditions were optimized. The Taylor cone was found to shrink around the inner diameter of the SS tubing, which permitted a low flow rate of 150 nL/min for sheathless microspray on the blunt tip (100 microm i.d. x 400 microm o.d.). It is believed that the wettability and/or hydrophobicity of SS tips are responsible for their micro-ES performance. The outlet orifice was further nipped to reduce the size of the spray cone and limit the flow rate to 50-150 nL/min, resulting in peptide detection down to attomole quantities consumed per spectrum. The SS emitter was also integrated into a polymethylmethacrylate microchip and demonstrated satisfactory performance in the analysis and identification of a myoglobin digest.     

Advanced sol–gel coatings for practical applications

One of the features of the sol–gel techniques is closeness to the industrial applications. Another feature is the variation of shapes of obtained materials like bulk, fiber, coating film, powder and so on. Among them, the author has focused on research of the sol–gel coatings on various substrates for practical applications as well as the fundamental research under the collaboration with industry. In this review, results of such research will be presented. These include (a) protective coating on metal sheets, (b) micropatterning on glass substrates, (c) water-repellant coating on windshields, (d) colored coating on glass bottles for easy recycling, (e) superhydrophobic and superhydrophilic coating on glass plates, and (f) anti-reflective coating on glass lenses for cameras. Some were highly successful, and some were not, of course. The author also contributed to the foundation of The Japanese Sol–Gel Society in 2003. The activities of The Society in these 8 years are introduced.     

Polymer coatings on magnesium-based implants for orthopedic applications

Nowadays, the need for bio-implants, which can gradually degrade after fulfilling the therapeutic tasks is continuously increasing. Under such situation, magnesium (Mg) and its alloys have been proposed and intensively studied as the new-generation medical implants due to their favorable biodegradability and biocompatibility. However, their swift corrosion in physiological environments can always cause an early fracture and further the surgical failure, greatly hindering their broad applications. Therefore, great efforts have been made to alter the degradation behaviors of Mg-based implants. Biodegradable polymeric surface coatings have been revealed to be a straightforward and effective strategy for retarding the fast degradation and improving the bioactivity of Mg and its alloys. This article reviews the recent progress of polymer-based coatings on Mg substrates, regarding the coating strategies, coating properties, and their performance in corrosive protection and biocompatibility promotion via in vitro as well as some in vivo models. The specific pros and cons of different polymeric coatings are also discussed. Finally, we put forward some perspectives on the future direction of polymeric coatings on biomedical Mg-based implants to better adapt to clinical trials.     

Tailor-made UV-curable powder clear coatings for metal applications

UV-curable powder coatings combine most of the benefits of conventional powder coatings together with the advantages of radiation-curable liquid coatings. This new coating process is not only environmentally friendly. It can also be used to coat substrates like wood, plastic, glass or metal at low temperatures within a short curing time. Several coatings based on binders like urethane vinylethers/unsaturated polyesters or methacrylates are developed for metals, wood or pvc flooring. This paper describes urethane acrylates as a new resin system for UV-curable powder clear coatings. The binders can be amorphous or (semi)crystalline. By combining both types it is possible to get tailor-made binders which exhibit an unique coating performance of excellent adhesion, good flexibility and high hardness on metal substrates. The appearance of the clear coatings can be varied from high to low translucent. In addition, the coatings show a good weatherability.     

Mesoporous and mesostructured TiO2 coatings for photocatalytic applications

The correlation between the textural properties and the photocatalytic activity of nanocrystalline Titanium dioxide (TiO₂)-anatase films obtained by sol–gel has been investigated. Mesoporous and mesostructured TiO₂-anatase films were prepared using different titanium precursors and Pluronic (F127) and polyethylene glycol hexadecyl ether P5884 (Brij58) surfactants via acid catalysis. Ca(NO₃)₂ and WCl₆ were incorporated to TiO₂ sols to investigate the effect of the doping on the photocatalytic behaviour. The microstructure and textural properties were characterised by X-ray diffraction, spectral ellipsometry and transmission electronic microscopy. The photocatalytic properties were evaluated in aqueous solution (methyl orange) and in gas phase (trichloroethylene, sulphide acid and methyl-ethyl-ketone) using multilayer films deposited on glass-slides. TiO₂-B-Brij-58 films exhibited the most efficient photocatalytic activity either in aqueous or gas medium. The Ca doping strongly enhances the photocatalytic activity associated with the reduced recombination of electrons and holes in the catalyst.