Zinc modified polypyrrole coating on mild steel

Polypyrrole (PPy) films (∼ 1.7 μm thick) have been electrodeposited on mild steel (MS) substrates from 0.1 M pyrrole containing aqueous oxalic acid solution, by using cyclic voltammetry technique. Then, the polymer coatings were modified with deposition of zinc particles (∼ 1 mg/cm²), at a constant potential value of − 1.20 V in 0.2 M ZnSO₄ solution. The corrosion performance of zinc modified PPy coating has been investigated in 3.5% NaCl solution, by using electrochemical impedance spectroscopy and anodic polarisation curves. Also, the corrosion behaviours of zinc modified PPy coated platinum and single PPy coated MS samples have been investigated, for comparison. It was shown that zinc modified coating exhibited very low permeability and provided important cathodic protection to MS for considerably long immersion period. The voluminous zinc corrosion products are formed during exposure time in aggressive solution, giving rise to a blocking effect on the porous structure and led to effective barrier behaviour of zinc modified PPy coating, even after 96 h of exposure time to corrosive solution.     

Electrochemical synthesis of polypyrrole films over each of well-aligned carbon nanotubes

Polypyrrole (PPy) films were uniformly electropolymerized over each carbon nanotube of the well-aligned carbon nanotube arrays. For comparison, PPy films were also coated on flat metallic titanium (Ti) and platinum (Pt) substrates by the same technique. The synthesis and the redox performance of the PPy films were conducted by cyclic voltammetry (CV). The structural characterization including the thickness and uniformity of the PPy films was carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is observed that the coating of the PPy film over carbon nanotubes is much faster than that on flat Ti/Pt surface. Furthermore, the redox performance of the PPy-coated carbon nanotube electrodes over flat Ti/Pt electrodes was significantly improved due to the high accessible surface area of the carbon nanotubes in the aligned arrays, especially in large film formation charge (Q_film). It is very promising that the electrode developed in this study could be used as high performance electrode in rechargeable batteries.     

Anticorrosive properties with catalytic behaviour of primer PANI film and top PPy coating synthesised in presence of novel norephedrine based amino alcohol compound

Polyaniline film as primer coating was deposited on stainless steel (SS) in aniline containing aqueous oxalic acid solution, and subsequent synthesis of top PPy film with 2-((5-ethylthiophen-2-yl) methylamino)-1-phenylpropan-1-ol (AAN) compound of different concentrations was achieved in acetonitrile-LiClO₄ successfully. The corrosion performances of coated and uncoated electrodes in 3·5%NaCl solution were evaluated with the help of AC impedance spectroscopy, anodic polarisation plots and open circuit potential time curves. The protective effect of bilayer coatings with AAN compound on SS electrode grew in parallel with extended exposure time. The regular increase in the charge transfer resistance of SS/PANI/PPy-AAN17 electrode was attributed to allowing the limited ion diffusion of top PPy coating with AAN17 compound. The high 21764 Ω s^?1/2 value of the Warburg coefficient showed that PPy-AAN17 film on the SS/PANI coating led to the formation of protective oxide layers due to the catalytic behaviour of PANI film.     

Poly(N-methyl pyrrole) and its copolymer with pyrrole for mild steel protection

In this study, it was aimed to overcome disadvantages of polypyrrole films like water up taking and resulting low stability, via copolymerization of pyrrole and N-methyl pyrrole and preparing their bilayer films. Poly(N-methyl pyrrole) coating was synthesized electrochemically on mild steel and its corrosion performance has been investigated in 3.5% NaCl aqueous solution, by using electrochemical impedance spectroscopy (EIS), anodic polarization curves and open circuit potential-time relation. This coating was found to have lower protection efficiency than single poly(pyrrole) coating. This case was related to weak adsorption behavior of poly(N-methyl pyrrole) film on mild steel. On the other hand, when this film was applied as a top coat on polypyrrole coated sample; the obtained coating system hindered the attack of corrosive environment and protected mild steel efficiently. The underlying poly(pyrrole) film was strongly adherent and the top poly(N-methyl pyrrole) coat improved barrier property by decreasing permeability and water mobility. Poly(pyrrole-co-N-methyl pyrrole) has also been synthesized from aqueous oxalic acid solutions containing various ratios of these two monomers. The copolymerization rate and protective behavior of copolymer were strongly affected by the monomer concentration ratio. The ratio of 1:3 (N-methyl pyrrole/pyrrole) gave the most protective copolymer coating and it exhibited better barrier property than single PPy. This was related to introduction of -CH₃ group which creates hydrophobic effect, when compared to -NH group of pyrrole unit. However, the solution with the ratio of 1:1 yielded a coating which had lower protection efficiency than single poly(pyrrole) film. This behavior was attributed to significant decrease in adsorption strength as the ratio of n-methyl pyrrole increased.     

Corrosion protection of steel by bi-layered polypyrrole doped with molybdophosphate and naphthalenedisulfonate anions

Bi-layered polypyrrole (PPy) coating was investigated for corrosion prevention of steels. The PPy coating consisted of the inner layer doped with PMo₁₂ (PMo₁₂) and ions and the outer layer doped with naphthalenedisulfonate (NDS) ions. PMo₁₂ ions doped in the inner layer make the passive oxide film on steel stabilized and the outer layer doped with large organic ions of NDS restricts decomposition and release of PMo₁₂ ions in the inner layer. Corrosion tests were made in 3.5% NaCl aqueous solution for the steels covered by the single PPy-PMo₁₂, single PPy-NDS, and bi-layered PPy-PMo₁₂/PPy-NDS coatings. The single PPy-PMo₁₂ and PPy-NDS coatings maintained the steel in passive state for 34 h and 24 h, respectively. The bi-layered coating kept the steel in passive state and prevented corrosion of the steels for the longer time period.     

Electrochemical study on hot corrosion of Si-modified aluminide coated In-738LC in Na2SO4-20 wt.% NaCl melt at 750 °C

The corrosion performance of the slurry Si-modified aluminide coating on the nickel base superalloy In-738LC exposed to low temperature hot corrosion condition has been investigated in Na₂SO₄-20 wt.% NaCl melt at 750 °C by combined use of the anodic polarization and characterization techniques.The coated specimen showed a passive behavior up to −0.460 V vs. Ag/AgCl (0.1 mol fraction) reference electrode, followed by a rapid increase in anodic current due to localized attack in the higher potential region. In the passive region, the anodic dissolution of constituents of the coating occurred through the passive film, probably SiO₂, at slow rate of 20-30 μA/cm². The passive current for the Si-modified coating was two orders of magnitude smaller than that for bare In-738LC, which is known as Cr₂O₃ former in this melt. This indicates that the SiO₂ film is chemically more stable than Cr₂O₃ film under this condition. However, pitting-like corrosion commenced around −0.460 V and proceeded at the high rate of 100 mA/cm² in the higher potential region than +0.400 V. The corrosion products formed on the coating polarized in different anodic potentials were characterized by SEM, EDS and XRD. It was found from the characterization that oxidation was dominant attack mode and no considerable sulfidation occurred at 750 °C. The SiO₂ oxide was not characterized in the passive region because the thickness of the passive film was extremely thin, but was detected as the primary oxide in the localized corrosion region, where the selective oxidation of Al was observed by further progress of the corrosion attack front into the inner layer of coating.     

Investigation of corrosion behavior of silver coated brass in acidic solutions

The corrosion behavior of uncoated brass and electrolytically silver coated brass in cyanide solutions such as 0.1 M HCl, 0.1 M H₂SO₄ and 0.1 M H₃PO₄ was investigated by Tafel polarization curves and cyclic voltammetry curves at 1., 24., 48., 72., 96. and 168. hours. The coating efficiency was calculated by current density of corrosion determined from Tafel polarization curves. Surface analysis of coated brass immersed into acidic solution by 168 hours was done. It is concluded that silver coating is very effective to protect the corrosion of brass for a long time.     

Emeraldine base as corrosion protective layer on aluminium alloy AA5182, effect of the surface microstructure

AA5182 aluminium alloy cold rolled samples were coated by thin films of emeraldine base (EB) obtained from a 5% solution in N-methylpyrrolidinone. Accelerated corrosion tests prove this coating very effective for corrosion protection of aluminium alloys in neutral environment. This study underlines the prominent role of surface cathodic intermetallic particles in pit initiation and coating break down in enhanced corrosion conditions and suggest that, beside the EB barrier properties, the enhanced corrosion resistance observed on the EB coated samples could partly arise from two other mains factors:

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a weak redox activity of the polymer which passivate the metal,

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a proton involving self-healing process taking place at the polymer-metal interface, which contributes to delay local acidification in first steps of corrosion on EB coated aluminium surfaces.

     

Copper(II) 8-hydroxyquinolinate 3D network film with corrosion inhibitor embedded for self-healing corrosion protection

This work presents a self-healing protective coating for copper. The complex coating is composed of copper(II) 8-hydroxyquinolinate (Cu(8HQ)₂) 3D network film storing corrosion inhibitor as base layer and epoxy as top layer covering on the base layer. The results of EIS and polarization curves reveal the self-healing effect of Cu(8HQ)₂ 3D network film with corrosion inhibitor embedded. After being scratched with a knife-edge, the complex coating rapidly releases inhibitor embedded in 3D network structure to suppress the corrosion process. The scratch on the coating is covered with inhibitor adsorption film and the anodic activity is restrained.     

Determination of the long term corrosion behavior of coated steel with A.C. impedance measurements

A previous study of the a.c. impedance of thin polymer films on aluminum alloys, steel, and phosphated steel, determined over a wide frequency range, has resulted in a general model for the electrochemical impedance across the coated surfaces. According to this model, penetration of the coating by the electrolyte results in an impedance behavior which is typical of a dielectric film short-circuited by conducting electrolytic paths perpendicular and parallel to the polymer/metal interface. Comparison of the time dependence of the impedance of coated steel substrates with the impedance across a free film has demonstrated that the corrosion of the substrate enhances the development of parallel paths of ionic conduction in the coating. A.c. impedance measurements carried out on polybutadiene-coated steels pretreated by different procedures determine quantitatively the resistance R_p of the conducting paths. The time dependence of R_p for the coated substrates has been determined for exposure periods of up to three weeks. Visual observation of the corroded and delaminated areas according to ASTM D610 has been carried out simultaneously, and correlations have been established with R_p. The results are discussed in terms of the possibility to predict the corrosion behavior of a coated metal, based on the measurement of R_p.     

Long-term anticorrosion behaviour of polyaniline on mild Steel

Anticorrosion performances of polyaniline emeraldine base/epoxy resin (EB/ER) coating on mild steel in 3.5% NaCl solutions of various pH values were investigated by electrochemical impedance spectroscopy (EIS) for 150 days. In neutral solution (pH 6.1), EB/ER coating offered very efficient corrosion protection with respect to pure ER coating, especially when EB content was 5-10%. The impedance at 0.1 Hz of the coating increased in the first 1-40 immersion days and then remained constant above 10⁹ Ω·cm² until 150 days, which in combination with the observation of a Fe₂O₃/Fe₃O₄ passive film formed on steel confirmed that the protection of EB was mainly anodic. In acidic or basic solution (pH 1 or 13), EB/ER coating also performed much better than pure ER coating. However, these media weakened the corrosion resistance due to breakdown of the passive film or deterioration of the ER binder.     

Electrolytic MgO/ZrO2 duplex-layer coating on AZ91D magnesium alloy for corrosion resistance

By a two-step fabrication process of electrolytic deposition and annealing treatment, an MgO/ZrO₂ duplex-layer coating has been prepared on AZ91D magnesium alloy as a protective film against corrosion. Owing to the chemical bonding formed after the condensation of precursory hydroxides, the adhesion strength, thickness and compactness of MgO coating on the substrate are significantly enhanced by the intermediate ZrO₂ layer which prevents the formation of corrosion product Mg₂(OH)₃Cl·4H₂O. As a result, the MgO/ZrO₂ duplex-layer coated specimen reveals relatively high corrosion resistance and superior stability in 3.5 wt% NaCl solution with respect to the MgO single-layer coated specimen.     

High temperature corrosion behavior of a multilayer CrAlN coating prepared by magnetron sputtering method on a K38G alloy

A multilayer CrAlN coating of Cr_0.58Al_0.42N/Cr_0.84Al_0.16N/Cr_0.51Al_0.49N has been fabricated by a reactive magnetron sputtering method. It consists of a bonding layer, a Cr-rich intermediate layer and an Al-rich outer layer. The multilayer structure provides the coating with good protection against different types of high temperature corrosion, i.e., high temperature oxidation and hot corrosion. The outer Al-rich layer gives the coating good oxidation resistance at 1000 and 1100 °C due to the formation of a continuous alumina scale. The parabolic rate constants of the coated samples decrease by about 2 orders of magnitude compared with that of the bare alloy samples. The intermediate Cr-rich layer can form a Cr₂O₃ scale to provide good protection under the hot corrosion conditions in the Na₂SO₄ salt fluxing at 900, 950 and 1000 °C. The incubation period of the hot corrosion extends several times longer when the alloy was coated by the multilayer coating at the three selected temperatures.     

Polypyrrole coatings as a treatment for zinc-coated steel surfaces against corrosion

A single-step electropolymerization of pyrrole on zinc-coated steel electrodes in aqueous medium was investigated under several techniques (potentiodynamic, galvanostatic and potentiostatic modes) by using carboxylate salts, such as the citrate, the succinate, the oxalate, the malate and the tartrate. Adherent and homogeneous PPy coatings were obtained only with tartrate counter-ions.The corrosion behaviour of zinc-coated steel electrodes electrochemical modified by PPy films were estimated by open circuit potential and DC polarization in NaCl, HCl and H₂SO₄ aggressive solutions. The obtained results show that the PPy coatings increase the corrosion potential and reduce the corrosion current. The salt spray test results show that the PPy coating prepared under ultrasonic waves is more resistant than those obtained in normal conditions.     

Effects of Al and Ni doping on oxidation and corrosion resistance of electrophoretic deposited YSZ coatings

Yttria-stablized zirconia (YSZ)/(Ni,Al) coatings were deposited on Inconel 600 alloy substrate by the electrophoretic deposition combined with vacuum sintering technique. The effects of isothermal oxidation at 1100 °C on the composition of the coatings and the crack healing were investigated, and the corrosion resistance of the coatings in 3.5% NaCl (mass fraction) solution was also studied. The results showed that the cracks on the coating gradually healed up with the increase of the isothermal oxidation time. During isothermal oxidation process, the coating composed of Ni₃Al was transformed to α-Al₂O₃ particulates. The α-Al₂O₃ particulates can seal the defects such as pores and cracks, and meanwhile prevent the oxygen diffusion into the coatings. The polarization curves and EIS results indicated that the coatings oxidized for 40 h had a more positive corrosion potential, higher breakdown potential, higher impedance module at low frequency and much lower corrosion current density compared with YSZ coated and uncoated Inconel 600 alloys.     

Improvement of the Electrochemical Behavior of Steel Surfaces Using a [Ti-Al/Ti-Al-N] n Multilayer System

The aim of this work is to improve the corrosion resistance of AISI D3 steel surfaces using a [Ti-Al/Ti-Al-N] _n multilayer system deposited with different periods (Λ) and bilayer numbers (n), via magnetron co-sputtering pulsed d.c. procedure, from a metallic (Ti-Al) binary target. The multilayer coatings were characterized by cross-sectional scanning electron microscopy that showed the modulation and microstructure of the [Ti-Al/Ti-Al-N] _n multilayer system. The composition of the single Ti-Al and Ti-Al-N layer films was studied via x-ray photoelectron spectroscopy, where typical signals for Ti_2p1/2, Ti_2p, N_1s, and Al_2p3/2 were detected. The electrochemical properties were evaluated by electrochemical impedance spectroscopy and Tafel polarization curves. The optimal electrochemical behavior was obtained for the [Ti-Al/Ti-Al-N] _n multilayered period of Λ = 25 nm (100 bilayers). At these conditions, the maximum polarization resistance (1719.32 kΩ cm²) and corrosion rate (0.7 μmy) were 300 and 35 times higher than that of uncoated AISI D3 steel substrate (5.61 kΩ cm² and 25 μmy, respectively). Finally, scanning electron microscopy was used to analyze the [Ti-Al/Ti-Al-N] _n multilayered surface after the corrosive attack. The improvement effects in the electrochemical behavior of the AISI D3 coated with the [Ti-Al/Ti-Al-N] _n multilayered coatings could be attributed to the number of interfaces that act as obstacles for the inward and outward diffusions of Cl^? ions, generating an increment in the energy or potential required for translating the corrosive ions across the coating/substrate interface.     

Preparation of Fe/Cr doped SiO2 thin film on 304 stainless steel substrate originated from corrosion

Abstract

A Fe/Cr doped SiO₂ thin film which can improve the anticorrosion characteristic of materials was prepared on SUS304 stainless steel substrates by a modified hydrothermal method that constitutes a two part preparation process. The corrosion of stainless steel and the dissolution of silica from Na–Ca glass are simultaneously stimulated by an aqueous lithium bromide solution, and then a Fe/Cr doped SiO₂ thin film is coated onto the substrate. The Fe/Cr doped SiO₂ film was characterised by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and electron probe X-ray microanalyser. The results showed that the corrosion products and the silica are combined to form a Fe/Cr doped SiO₂ thin film. Furthermore, a chemical bond is formed between the silicon coating and the metal that improves the compactness and the adhesive property of the film. The anticorrosion characteristics of the film were measured by the methods of weight loss and electrochemical potentiodynamic polarisation, indicating that the Fe/Cr doped SiO₂ thin film can improve the anticorrosion performance of stainless steels effectively.     

Comparison of electrochemical corrosion behaviour of MgO and ZrO2 coatings on AM50 magnesium alloy formed by plasma electrolytic oxidation

Two types of PEO coatings were produced on AM50 magnesium alloy using pulsed DC plasma electrolytic oxidation process in an alkaline phosphate and acidic fluozirconate electrolytes, respectively. The phase composition and microstructure of these PEO coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion behaviour of the coated samples was evaluated by open circuit potential (OCP) measurements, potentiodynamic polarization tests, and electrochemical impedance spectroscopy (EIS) in neutral 0.1 M NaCl solution. The results showed that PEO coating prepared from alkaline phosphate electrolyte consisted of only MgO and on the other hand the one formed in acidic fluozirconate solution was mainly composed of ZrO₂, MgF₂. Electrochemical corrosion tests indicated that the phase composition of PEO coating has a significant effect on the deterioration process of coated magnesium alloy in this corrosive environment. The PEO coating that was composed of only MgO suffered from localized corrosion in the 50 h exposure studies, whereas the PEO coating with ZrO₂ compounds showed a much superior stability during the corrosion tests and provided an efficient corrosion protection. The results showed that the preparation of PEO coating with higher chemical stability compounds offers an opportunity to produce layers that could provide better corrosion protection to magnesium alloys.     

Corrosion-Erosion Effect on TiN/TiAlN Multilayers

The goal of this work is to study electrochemical behavior under corrosion-erosion conditions for [TiN/TiAlN] _n multilayer coatings with bilayer number (n) of 2, 6, 12, and 24 and/or bilayer period (??) of 1500, 500, 250, 150, and 125?nm deposited by a magnetron sputtering technique on Si (100) and AISI 1045 steel substrates. The Ti-N and Ti-Al-N structures for multilayer coatings were evaluated via x-ray diffraction analysis. Silica particles were used as the abrasive material in corrosion-erosion tests within the 0.5?M H₂SO₄ solution at impact angles of 30° and 90° over the surface. The electrochemical characterization was carried out using the polarization resistance technique (Tafel) to observe changes in corrosion rates as a function of the bilayer number (n) or bilayer period (??) and impact angle. Corrosion rate values of 359?mpy of uncoated steel substrate and 103?mpy for substrate coated with n?=?24 (???=?125?nm) under an impact angle of 30° were found. On the other hand, with an impact angle of 90° the corrosion rate exhibited 646?mpy on uncoated steel substrate and 210?mpy for substrate coated with n?=?24 (???=?125?nm). This behavior was related to the curves of mass loss for both coated samples and the surface damage was analyzed via SEM images for the two different impact angles. These results indicate that TiN/TiAlN multilayer coatings deposited on AISI 1045 steel are a practical solution for applications in erosive-corrosive environments.     

Changes in mass and stress during anodic oxidation and cathodic reduction of the Cu/Cu2O multilayer film

The anodic oxidation and cathodic reduction processes of the Cu/Cu₂O multilayer film and pure Cu film in pH 8.4 borate buffer solution were analyzed by electrochemical quartz crystal microbalance (EQCM) for gravimetry and bending beam method (BBM) for stress measurement. The mass loss of the multilayer film during anodic oxidation at 0.8 V (SHE) in the passive region was less than that of the pure Cu film. The comparison between current transients and mass changes during anodic oxidation has succeeded in separating the anodic current density into two partial current densities of oxide film growth, i_O^2-, and of Cu²⁺ dissolution through the passive film, i_Cu²⁺. As a result, in the case of the pure Cu film, the anodic current density was mainly due to i_Cu²⁺, while in the case of the multilayer film, i_Cu²⁺ was almost equal to i_O^2-. The compressive stress for the multilayer film was generated during anodic oxidation, while the tensile stress for the pure Cu film was generated.The mass loss of the multilayer film during cathodic reduction at a constant current density (i_c = −20 μA cm⁻²) was significantly less than that estimated from coulometry, suggesting that H₂O produced by cathodic reduction remained in the multilayer film. The compressive stress was generated during cathodic reduction of the multilayer film, which was ascribed to H₂O remained in the multilayer film.