Multilayered polypyrrole–SiO2 composite coatings for functionalization of stainless steel: Characterization and corrosion protection behavior

Two different multilayered composite polypyrrole/SiO₂ coatings were synthesized on 304 stainless steel. Electrochemical and electrophoretic depositions were used to grow polypyrrole and SiO₂ layers, respectively. Coatings were characterized by glow discharge optical emission spectroscopy to observe repartition of elements within different layers, by scanning electron microscopy to observe surface morphology and by electrochemistry to investigate corrosion protection behavior. The electrophoretic approach enables good incorporation of SiO₂ particles. This incorporation was more extensive and more homogeneous than for coatings obtained with the mixing method related in previous works. Moreover, incorporation and repartition of SiO₂ particles are greatly enhanced when the silica layer is grown directly on the steel surface. Corrosion protection of the stainless steel substrate was improved when multilayered composite polypyrrole/SiO₂ coatings were used.     

Fabrication of ZnO–GO hybrid for enhancement of chemically bonded phosphate ceramic coatings corrosion protection performance on AISI304L stainless steel

In this paper, a solvothermal method is used to prepare nano-sized zinc oxide-graphene oxide (ZnO–GO) hybrid, and the ZnO–GO hybrid is characterized by X-ray diffraction analysis, Raman, Fourier transform infrared spectroscopy, and scanning electron microscopy. In addition, chemically bonded phosphate ceramics coatings with different content of ZnO–GO hybrid are prepared on the stainless steel through the sol-gel method. The corrosion performance of the coatings is evaluated by electrochemical properties and the analysis of the surface and cross morphology of the coating. Results indicate ZnO–GO hybrid significantly enhances the compactness and corrosive behavior of the coating because the overlapping structure of GO flake improves the barrier performance of the coating. Besides, ZnO nanoparticles on the surface of GO can react with aluminum dihydrogen phosphate binder, in that case the adhesion between GO and the coating is improved.     

Electrophoretic deposition of bioactive wollastonite and porcelain–wollastonite coatings on 316L stainless steel

Wollastonite and porcelain–wollastonite coatings on stainless steel were obtained by electrophoretic deposition using acetone as dispersive medium. A direct electric current of 800 V for 3 min was used for obtaining the single wollastonite coating. A well-sintered layer was observed after heat treatment at 1050 °C for 1 h in air. The two-layer coating was obtained by depositing dental porcelain at 400 V for 30 s followed by the deposition of wollastonite at 400 V for 3 min. After forming the two layers, this complex coating was heat treated at 800 °C for 5 min. Under these conditions, strong bonds of both the interface wollastonite–porcelain and that of porcelain–metallic substrate were observed. The in vitro bioactivity assessment of the coatings was performed by immersing the deposited substrates in simulated body fluid (SBF) for 21 days. All the materials showed to be highly bioactive through the formation of a homogeneous apatite layer.     

High efficient corrosion inhibitor of water-soluble polypyrrole–sulfonated melamine formaldehyde nanocomposites for 316 L stainless steel

In this work, new water-soluble polypyrrole–sulfonated melamine formaldehyde nanocomposites (PPy–SMF NCs) were first synthesized by one-step in-situ polymerization of pyrrole with FeCl₃ in the presence of various mole ratios of sulfonated melamine formaldehyde (SMF). The characterization of the PPy–SMF NCs was investigated via ultraviolet–visible spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray, dynamic light scattering, transmission electron microscopy, X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, and conductivity measurements. The resulting PPy–SMF NCs were proved to improve the solubility, electrical properties, and thermal stability. The anti-corrosion performance of PPy-SMF NCs on 316 L stainless steel (316 L SS) was examined using electrochemical impedance spectroscopy, potentiodynamic polarization, and weight-loss method. The result showed that the PPy–SMF NCs acts as a mixed-type inhibitor, as well as a protective layer to 316 L SS against corrosion in 3.5% NaCl solution. The Langmuir adsorption isotherm was well fitted and suitable to explain the adsorption behavior of the PPy-SMF NCs on 316 L SS surface. The inhibition efficiency of PPy-SMF NCs is 99% by the weight-loss method which could be attributed to the protective layer formed on 316 L SS surface by the adsorption of PPy-SMF NCs.     

Characterization of sol–gel coated 316L stainless steel for biomedical applications

Silica based organic–inorganic hybrid coatings were deposited on 316L stainless steel by sol–gel technique. The hybrid sols were prepared by hydrolysis and condensation of 3-methacryloxypropyltrimethoxysilane (TMSM) and tetraethylorthosilicate (TEOS) at different molar ratios. Electrochemical experiments were performed to evaluate the corrosion resistance properties of the coatings. Structural characterization of the coatings was performed using scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. Contact angle measurement and cell morphology assay were performed to investigate the hydrophilicity and in vitro cytotoxicity of the coatings, respectively. The results indicate formation of a crack-free and highly adherent film acting as a protective barrier against the physiological medium. Corrosion resistance of hybrid coatings was influenced by the molar ratios of TMSM:TEOS. The best corrosion protection was obtained at TMSM:TEOS molar ratio of 1:1. Sol–gel coatings enhanced the hydrophilicity of 316L steel surfaces. Also, these coatings showed non-toxicity for L929 cells.     

Hybrid sol–gel coatings for corrosion protection of galvanized steel in simulated concrete pore solution

The aim of this experimental research was to study the electrochemical behavior of organic–inorganic hybrid (OIH) coatings for corrosion protection of hot-dip galvanized steel (HDGS) in the first instants of immersion in simulated concrete pore solution (SCPS) (pH > 12.5). The electrochemical performance of the OIH coatings was assessed by electrochemical impedance spectroscopy, potentiodynamic polarization curves, macrocell current density, and polarization resistance. The OIH coatings were prepared via the sol–gel method and were deposited on HDGS surfaces by dip-coating using one or three dip steps. The electrochemical results obtained for HDGS samples coated with OIH matrices in SCPS showed higher corrosion resistance than bare HDGS; as the molecular weight (MW) of Jeffamine^® increased the barrier protection of the coating decreased. The lowest protection efficiency was found for HDGS samples synthesized with oligopolymers with an MW of 2000. Coatings produced with an oligopolymer of 230 MW conferred the highest protection. The surface morphology of the OIH coatings deposited on HDGS surfaces was studied by atomic force microscopy. The results show that the roughness of the OIH films depends on the MW of Jeffamine^® and on the number of dip-coating steps used. Thermogravimetry results show that the Jeffamine^® MW affected the thermal properties of the prepared OIH samples. The prepared OIH materials are thermally stable within the range of 20–80°C.     

Zinc–magnesium-pigment rich coatings for corrosion protection of aluminum alloys

Chromate (Cr(VI))-based pigments have been widely used for corrosion protective coatings because of their outstanding protection efficiency especially for aluminum alloy products. However, due to environmental issues associated with Cr VI, more and more requests are being made for alternative solutions. In the presented work zinc was modified by alloying with magnesium to achieve a combination of properties – cathodic protection and less reactivity during production, storage and application of the pigments. The magnesium content leads to a lowering of the electrochemical potential which allows the cathodic protection of aluminum alloys. zinc–magnesium pigments were prepared in different compositions with special attention to the intermetallic phases MgZn, Mg₂Zn₃, and MgZn₂. Pigments were produced and a zinc–magnesium rich coating was formulated and compounded. Pickled samples of AA 2024 unclad were coated and the corrosion behavior investigated. A durability of more than 10,000 h in salt spray test could be achieved.     

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.     

Electrosynthesis and protection role of polyaniline–polvinylalcohol composite on stainless steel

Polyaniline–polyvinyl alcohol (PANI–PVA) composite has been electrodeposited on stainless steel surface from aqueous sulfuric acid solution of aniline monomer in presence of soluble PVA at different concentrations. The PVA increased the rate of electropolymerization where 4 g/L PVA formed a composite of 37 wt% PANI and 63 wt% PVA composition. The composite layer exhibited more adhesion to the steel surface in comparison with PANI layer but with less thermal stability. It has higher protection role for the stainless steel (SS) against general and pitting corrosion. It enhanced the passivation of the SS surface by increasing the thickness of oxide film and improving the composition.     

The passivity of AISI 316L stainless steel in 0.05 M H2SO4

The passivity of AISI 316L stainless steel (AISI 316L) in 0.05 M H₂SO₄, in the steady-state condition, has been explored using various electrochemical techniques, including potentiostatic polarization, electrochemical impedance spectroscopy (EIS), and Mott–Schottky analysis. Based on the Mott–Schottky analysis in conjunction with the point defect model (PDM), it was shown that the calculated donor density decreases exponentially with increasing passive film formation potential. The thickness of the barrier layer was increased linearly with the film formation potential. These observations were consistent with the predictions of the PDM, noting that the point defects within the barrier layer of the passive film are metal interstitials, oxygen vacancies, or both. Also no evidence for p-type behavior was obtained, indicating that cation vacancies do not have any significant population density in the passive film.     

Electrochemical behavior of hydrophobic silane–zeolite coatings for corrosion protection of aluminum substrate

The anticorrosive properties of a silane–zeolite composite coating applied on a 6061 aluminum substrate was investigated. The composite film, deposited by dip-coating technique, was prepared with different contents of SAPO34 powder (60–90 wt%) with the purpose of evaluating the protective action offered by the zeolite-filled silane matrix. Corrosion protection performance, during immersion in 3.5% NaCl solution, was evaluated by means of a potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS). The coating evidenced good barrier properties and high hydrophobicity. The addition of zeolite in the silane matrix induced, as expected, a reduction of cathodic and anodic current. The zeolite improved the barrier properties of the hybrid sol–gel films, enhancing the resistance to localized corrosion attacks. Better results were observed for the sample with 80 wt% of zeolite filler that evidenced still acceptable protective action after 3 days of immersion in the sodium chloride solution.     

Influence of inhibitor addition on the corrosion protection performance of sol–gel coatings on AA2024

Sol–gel films are actively investigated during the last decade as possible candidates for environmentally friendly pre-treatments. However, the important drawback in this case is the lack of active corrosion protection and self-healing ability.     

Effect of cerium concentration on corrosion resistance and polymerization of hybrid sol–gel coating on martensitic stainless steel

Stainless steels are increasingly used in the aeronautics field for the manufacture of structural parts. One of them, the X13VD martensitic stainless steel (X12CrNiMoV12-3), known for its good mechanical properties, has a poor corrosion resistance in confined or severe environments. In the past years, Cr(VI) based pre-treatments have been currently used for corrosion protection of different metals, however, they are toxic and due to environmental regulations, they will be definitely banned in a near future. Alternatives to replace Cr(VI) show advantages and drawbacks considering key properties such as: corrosion resistance, adhesion of coatings, fatigue resistance, durability and reliability. However, some of their possible alternatives show high potential.     

Characterization and adhesion strength study of detonation-sprayed MoB–CoCr alloy coatings on 2Cr13 stainless steel substrate

A novel MoB–CoCr alloy coating was deposited onto stainless steel (2Cr13) substrate using a detonation gun (D-gun) spraying technique. Microstructures of the powder and coating were investigated by X-ray diffraction (XRD), scanning election microscopy (SEM), and transmission electron microscopy (TEM), and a quantitative determination of the adhesion strength of the coating was calculated by combination of modified four-point bending (4PB) test and finite element analysis (FEA) simulation. The results show that the coating mainly consists of ternary transition metal boride matrix phases (CoMo₂B₂, MoCoB) and binary borides (MoB and CrB). Nanocrystalline grains with a size of 50–100 nm were observed in the coating. The average energy release rate and phase angle are 191.2 J m⁻² and 41.7o, respectively, which show strong bond strength compared to other reported values.     

Synthesis and characterization of epoxy–silicone–polythiophene interpenetrating polymer network for corrosion protection of steel

Polymer alloys, particularly interpenetrating polymer networks (IPNs) exhibit excellent coating properties. Often combination of polymers result in IPNs with controlled morphologies and synergistic behavior. In this study, corrosion-resistant IPNs were prepared from immiscible resins (epoxy, silicone and thiophene) using a cross-linking agent and a catalyst. GPC, FTIR, NMR, TG, DTA and SEM studies used to fix the best performing IPN. Surface morphology studies using SEM confirm the incorporation of silicone and polythiophene in to the epoxy polymer to form homogeneously micro structured IPN. The heat-resistance of the IPN was determined as per ASTM 2485. The improved corrosion resistance of the IPN was evaluated by AC impedance measurements.     

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.     

Evaluation of the corrosion protection performance of epoxy coatings containing Mg nanoparticle on carbon steel in 0.1 M NaCl solution by SECM and EIS techniques

The effect of corrosion protection performance of epoxy coatings containing magnesium (Mg) nanoparticles on carbon steel was analyzed using scanning electrochemical microscopy (SECM) and electrochemical impedance spectroscopy (EIS). Localized measurements such as oxygen consumption and iron dissolution were observed using SECM in 0.1 M NaCl in the epoxy-coated sample. Line profile and topographic image analysis were measured by applying ?0.70 and +0.60 V vs the Ag/AgCl/saturated KCl reference electrode as the tip potential for the cathodic and anodic reactions, respectively. The tip current at ?0.70 V for the epoxy-coated sample with Mg nanoparticles decreased rapidly, which is due to cathodic reduction in dissolved oxygen. The EIS measurements were conducted in 0.1 M NaCl after wet and dry cyclic corrosion test. The increase in the film resistance (R _f) and charge transfer resistance (R _ct) values was confirmed by the addition of Mg nanoparticles in the epoxy coating. Scanning electron microscope/energy-dispersive X-ray spectroscope analysis showed that Mg was enriched in corrosion products at a scratched area of the coated steel after corrosion testing. Focused ion beam–transmission electron microscope analysis confirmed the presence of the nanoscale oxide layer of Mg in the rust of the steel, which had a beneficial effect on the corrosion resistance of coated steel by forming protective corrosion products in the wet/dry cyclic test.     

Comprehensive study on corrosion protection properties of Al2O3, Cr2O3 and Al2O3–Cr2O3 ceramic coatings deposited by plasma spraying on carbon steel

In this study, individual Al₂O₃ and Cr₂O₃ coatings and Cr₂O₃-25, 50, 75 wt% Al₂O₃ composite coatings were applied on carbon steel by atmospheric plasma spraying method. Corrosion experiments were performed on as-sprayed and epoxy resin sealed coatings including potentiodynamic polarization, electrochemical impedance spectroscopy and long-term immersion in 3.5 wt% NaCl solution. Phase composition and microstructure of the coatings were investigated by x-ray diffraction, optical microscopy and scanning electron microscopy, before and after the corrosion experiment. The results showed that the Cr₂O₃ coating exhibited the best corrosion resistance, due to the densest microstructure and highest adhesion strength. The Cr₂O₃-25 wt% Al₂O₃ coating had the highest interconnected porosities and thus had the least corrosion resistance compared to other coatings. In general, the as-sprayed coatings induced a maximum increase of 3.93 times the polarization resistance (R_p) in the polarization experiment and a 3.5 times increase in the charge transfer resistance (R_ct) in the EIS experiment, which was not significant. Stresses caused by increased volume of corrosion products in the coating-substrate interface resulted in the spallation of Cr₂O₃-25, 50 wt% Al₂O₃ coatings from the substrate over long-term of immersion. The adhesion strength of the coatings was a determining criterion for the long-term durability of the coatings. The sealing treatment resulted in a significant increase in R_p and R_ct.     

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 stability of polyester coatings on steel pretreated with different iron–phosphate coatings

The influence of steel surface pretreatment with different types of iron–phosphate coatings on the corrosion stability and adhesion characteristics of polyester coatings on steel was investigated. The phosphate coating was chemically deposited either from the simple novel plating bath, or with the addition of NaNO₂, as an accelerator in the plating bath. The morphology of phosphate coatings was investigated using atomic force microscopy (AFM). The corrosion stability of polyester coatings on steel pretreated by iron–phosphate coatings was investigated by electrochemical impedance spectroscopy (EIS) in 3% NaCl solution, while “dry” and “wet” adhesion were measured by a direct pull-off standardized procedure. It was shown that greater values of pore resistance, R_p, and smaller values of coating capacitance of polyester coating, C_c, on steel pretreated with iron–phosphate coating were obtained, as compared to polyester coating on steel phosphated with accelerator, and on the bare steel. The surface roughness of phosphate coating deposited on steel from the bath without accelerator is favorable in forming stronger bonds with polyester coating. Namely, the dry and wet adhesion measurements are in accordance with EIS measurements in 3% NaCl solution, i.e. lower adhesion values were obtained for polyester coating on steel phosphated with accelerator and on the bare steel, while the iron–phosphate pretreatment from the novel bath enhanced the adhesion of polyester coating on steel.