Protection of passivated iron against corrosion in a 0.1 M NaNO3 solution by coverage with an ultrathin polymer coating of carboxylate SAM

An ultrathin, ordered and two-dimensional polymer coating was prepared on passivated iron by modification of 16-hydroxyhexadecanoate ion HO(CH₂)₁₅CO₂⁻ self-assembled monolayer (SAM) with 1,2-bis(triethoxysilyl)ethane (C₂H₅O)₃Si(CH₂)₂Si(OC₂H₅)₃ and octadecyltriethoxysilane C₁₈H₃₇Si(OC₂H₅)₃. Protection of passivated iron against passive film breakdown and corrosion of iron was examined by monitoring of the open-circuit potential and repeated polarization measurements in an aerated 0.1 M NaNO₃ solution during immersion for many hours. Passive film breakdown on the polymer-coated electrode in the solution was not observed during immersion for 480 h, whereas that of the passivated one occurred at 18.1 h, indicating protection of the passive film from breakdown by coverage with the polymer coating. The protective efficiencies of the passive film covered with the coating were extremely high, around 99.9% in the initial region of the immersion time up to 72 h and more than 98.3% thereafter, indicating prominent cooperative suppression of iron corrosion in 0.1 M NaNO₃ by coverage with the passive film and polymer coating. The polymer-coated surface was characterized by contact angle measurement and electron-probe microanalysis.     

Enhanced corrosion protection of MgO coatings on magnesium alloy deposited by an anodic electrodeposition process

MgO coatings were prepared on magnesium alloy surface by an anodic electrodeposition process in concentrated KOH solution followed by heat treatment in air. The phase composition and microstructure of the as-formed MgO coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion behavior of the MgO-coated samples was evaluated by electrochemical measurements and immersion tests in Hanks’ solution. The results showed that the MgO-coated Mg alloy exhibited a much superior stability and lower corrosion rate, and thus enabled to improve the corrosion resistance, whereas the bare Mg alloy suffered from severely localized corrosion attack.     

Oscillational corrosion potential of HastelloyC coatings fabricated by GS-HVOF spraying

Coatings of HastelloyC fabricated by HVOF spraying with a gas shroud (GS) have shown the superior barrier characteristic and corrosion resistance in seawater environment. During immersion of these coatings in artificial seawater, however, oscillation phenomenon of the corrosion potential was observed. In order to reveal and control the oscillation behaviour, some types of surface modification of the sprayed coatings and changing of the spray condition were carried out and their effect on the corrosion potential was investigated. The oscillation was caused by repetition of seawater penetration through surface oxides of the sprayed particles to their metal face and subsequent passivation of the metal. Such surface oxides were formed upon the stacking process of the sprayed particles on the substrate. Surface modification of the sprayed coatings and changes of the spray condition could reduce the oscillation effectively.     

Improvement of healing treatment with NaNO3 for a passivated iron electrode covered with an ultrathin polymer coating to prevent iron corrosion in some solutions containing aggressive anions

An ultrathin and ordered polymer coating was prepared on a passivated iron electrode by modification of a 16-hydroxyhexadecanoate ion self-assembled monolayer with 1,2-bis(triethoxysilyl)ethane (C₂H₅O)₃Si(CH₂)₂Si(OC₂H₅)₃ and octyltriethoxysilane C₈H₁₇Si(OC₂H₅)₃. Further, the passivated and polymer-coated electrode was healed by treatment in 1.0 M NaNO₃ for 4 h. Prevention of passive film breakdown and iron corrosion for the passivated, polymer-coated and healed electrode was examined by monitoring of the open-circuit potential and repeated polarization measurements in oxygenated 0.1 M KClO₄, 0.1 M Na₂SO₄ and 0.1 M NaCl for many hours. The values of the time for passive film breakdown, t_bd were >240, 22.2 and 9.5 h in these solutions, respectively. The protective efficiencies for the electrode were extremely high, more than 99.9% before t_bd, indicating complete protection of substrate iron against corrosion in these solutions, unless passive film breakdown occurred. The presence of on the passive surface by treatment in 1.0 M NaNO₃ was detected by X-ray photoelectron and FTIR reflection spectroscopies. The self-healing activity of adsorbed to suppress passive film breakdown was discussed.     

Effects of grain size on the electrochemical corrosion behaviour of electrodeposited nanocrystalline Fe coatings in alkaline solution

Effect of grain size reduction on the electrochemical corrosion behaviour of nanocrystalline Fe was investigated using Tafel polarization curves and electrochemical impedance spectroscopy (EIS) measurements. Nanocrystalline iron was fabricated by pulse electrodeposition using citric acid bath. The grain size of a nanocrystalline surface was analyzed by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The corrosion resistance of Fe in alkaline solution considerably increased as the grain size decreased from microcrystalline to nanocrystalline. The behaviour of passive film growth and corrosion was discussed in terms of excess of free energy caused by nanocrystalline surface.     

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.     

Electrochemical corrosion behaviour of microcrystalline aluminium in acidic solutions

The electrochemical corrosion behaviour of microcrystalline pure aluminium coating, fabricated by a magnetron sputtering technique, has been investigated in both 0.5 mol/l NaCl and 0.5 mol/l Na₂SO₄ acidic (pH = 2) aqueous solutions. The corrosion resistance of the microcrystalline Al coating has deteriorated more compared with that of the cast pure Al in Na₂SO₄ acidic solution. However, its oxide film has a higher pitting resistance in the NaCl acidic solution. Chloride ions play a big role in the formation of the oxide film on the microcrystalline Al coating. The higher pitting resistance was attributed to the more acidic isoelectric point which the oxide film achieved.     

Corrosion protection of Zn-phosphate containing water borne dispersion coatings on steel: Part 1: Design and analysis of model water based latex films on iron substrates

Model thin film coated substrates were developed to investigate the functionality of Zn-phosphates, the polymeric binder and emulsifiers during the interface formation on iron substrates. By means of grazing incidence FTIR-spectroscopy, X-ray photoelectron spectroscopy and atomic force microscopy, it was observed that ultra-thin films of zinc phosphates and hydroxides, with a thickness of less than 2 nm, adsorb on the iron surface during the formation of the latex coating/metal interface. Moreover, film formation conditions were developed for the deposition of hexagonal closed packed defect-free-monolayers of latex particles over large areas. This allowed the application of time of flight SIMS for the analysis of the monolayer-metal interface and the study of its morphology before and after film annealing. After annealing, the monolayer film showed an enrichment of surfactants at the particle boundaries and at the interface to the phosphate and hydroxide covered metal substrate.     

Evaluation of the anti-corrosive effect of acid pickling and sol-gel coating on magnesium AZ31 alloy

The effect of different acid pre-treatment procedures on the corrosion of magnesium AZ31 alloy was compared by measuring the amount of hydrogen gas formed when the surface was in contact with aqueous 5% sodium chloride solution. A 4-7 μm thick sol-gel coating prepared by phosphoric acid catalyzed sol-gel processing of a methyltriethoxysilane/tetraethoxysilane mixture was applied to the differently pre-treated magnesium surfaces. The corrosion rate of the alloy decreased by a factor of up to 60 by combination of acid pickling and sol-gel coating. The addition of triethylphosphate or 1,2,4-triazole as corrosion inhibitors led to further improvements. Composition and texture of the films was investigated by scanning electron microscopy and energy dispersive X-ray analysis.     

Structure and water-barrier properties of vanadate-based corrosion inhibitor films

Uniform, crack-free vanadium-based corrosion inhibitor films were grown on aluminum alloy 2024. Neutron and X-ray reflectivity were utilized to investigate the structure, water-barrier properties and speciation of the inhibitor film. The top part of the alloy participates in the formation of vanadate film. The similarity of films prepared from H₂O and D₂O implies the vanadate film is not hydrated. The film has a layered structure with vanadium enriched at the alloy interface. The film behaves as an effective water barrier when the thickness is greater than 800 Å. Films grown without K₃Fe[CN]₆ accelerator are found to be extremely thin.     

Kinetic modelling of the attenuation of carbon steel canister corrosion due to diffusive transport through corrosion product layers

This work models the attenuation of the corrosion of carbon steel canisters due to the influence of diffusive transport through the corrosion product - a growing film of magnetite. The model makes use of an algorithm based on the analytical solution of 1D diffusion equations for the movement of Fe²⁺, H₂, H₂O and OH⁻ through this magnetite film.     

Direct modification of a passivated iron electrode with alkyltriethoxysilanes for preventing passive film breakdown in a borate buffer containing 0.1 M of chloride ion

A passive film on an iron electrode was modified with alkyltriethoxysilanes directly. In order to examine the protective ability of the modified passive film against breakdown, the pitting potential, E_pit was measured by anodic polarization of the modified electrode in a borate buffer solution (pH 8.49) containing 0.1 M of Cl⁻. The value of E_pit for the modified electrode shifted in the positive direction from that of the unmodified electrode, indicating prevention of passive film breakdown. The modified passive film was not broken down in the passive and transpassive regions of polarization curve in some cases. However, many current spikes appeared in the all curves of the modified electrodes. The modified surface of passivated electrode was characterized by X-ray photoelectron and FTIR reflection spectroscopies and contact angle measurement. There were defects and clusters of associated water within the modified film and hence, Cl⁻ could permeate through the defects, leading to appearance of current spikes and occurrence of breakdown.     

EIS study of a self-repairing microarc oxidation coating

The self-repairing microarc oxidation (MAO) coating consisting of a bottom nanocrystalline layer covered by a top conversion ceramic coating was fabricated on 2024 Al alloy by a duplex process with surface mechanical attrition (SMAT) prior to microarc MAO treatment. A 20 μm thick nanocrystalline layer with average grain size of 52.8 nm was fabricated by SMAT, and on which covered by a top MAO coating of 5 μm. The self-repairing property caused by the formation of a dense passive film at the damaged regions contacting the bottom nanocrystalline layer enhances the corrosion reisitance of the SMAT-MAO coating.     

Inhibition of iron corrosion in acid solutions by Cefatrexyl: Behaviour near and at the corrosion potential

The corrosion inhibition of iron in HCl, HClO₄, H₂SO₄ and H₃PO₄ solutions (1M for each) by cefatrexyl has been studied by polarization resistance (R_p) and electrochemical impedance spectroscopy (EIS) at the corrosion potential. The results obtained at 30 °C revealed that cefatrexyl acts as a weak inhibitor in HCl solution while it shows excellent inhibition performance in the remaining acids. Adsorption of cefatrexyl in HCl solution obeys Langmuir’s isotherm with a very low value of the free energy of adsorption (physisorption) while its adsorption in the other acids follows Temkin’s isotherm with very high negative values of (chemisorption). Data obtained from EIS measurements were analyzed to model the corrosion inhibition process through appropriate equivalent circuit models. The calculated values of the apparent activation energy (E_a) and the pre-exponential factor (λ) indicate that cefatrexyl blocks nearly the whole active centers of iron surface in H₃PO₄ solution even at elevated temperatures. The inhibition mechanism of cefatrexyl was discussed.     

Corrosion protection of aluminum pigments by sol-gel coatings

New sol-gel coatings for the corrosion protection of aluminum pigments were developed. To this end, the pigment particles are first coated with a silica layer by phosphoric acid-catalyzed sol-gel processing of Si(OEt)₄, to which either hexadecyltrimethoxysilane or dimethyldimethoxysilane is condensed. The coated pigments have excellent anticorrosive properties in alkaline solutions as well as under boiling water conditions.     

Electrochemical study of growth behaviour of plasma electrolytic oxidation coating on Ti6Al4V: Effects of the additive

The growth behaviour of plasma electrolytic oxidation (PEO) coating on Ti6Al4V was studied by scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization test, focusing on effects of the electrolyte additive - calcium hypophosphite. The EIS analysis of the outer layer of the PEO coating provided insight into the structure of the coating, which was confirmed by SEM results. The EIS analysis of the inner layer of the PEO coating provided information matching well with the results of the potentiodynamic polarization test.     

Study of iron benzoate as a novel steel corrosion inhibitor pigment for protective paint films

The aim of this work was to study the lab-synthesized iron benzoate as a novel steel corrosion inhibitor pigment to formulate anticorrosive paint films. The iron benzoate anticorrosive properties were studied by electrochemical and spectroscopic essays employing pigment suspensions. In a second stage, it was evaluated the performance of anticorrosive paint films containing iron benzoate by accelerated (salt spray and humidity chambers) and EIS tests. The protective layer nature grown under the paint films in the salt spray chamber was also assessed.Experimental results shown that ferric benzoate was adequate to formulate anticorrosive paint films with improved anticorrosive assessed.     

Evaluation of the corrosion protection behaviour of poly(neutral red) films on passivated copper

The corrosion protection behaviour of poly(neutral red) (PNR) films on copper electrodes has been investigated using open circuit measurements, Tafel plots and electrochemical impedance spectroscopy. Pure copper electrodes were initially passivated in sodium oxalate, salicylate or hydrogen carbonate solution, to inhibit copper dissolution at potentials where neutral red monomer oxidation occurs, before its electropolymerization by potential cycling. The corrosion inhibition by these films was tested in 0.10 M KCl solution. It was found that, after long immersion times (96 h), the best protection efficiency was with PNR films formed on copper passivated in oxalate solution.     

Corrosion studies of stainless steel protected by a TiO2 thin film deposited on a sulfonate-functionalized self-assembled monolayer

Thioacetate hexadecyltrimethoxysilane was deposited on SiO₂-coated stainless steel to form a thioacetate-functionalized monolayer. In situ oxidation of the thioacetate yielded a sulfonate-functionalized monolayer. Solution deposition of TiO₂ on this monolayer covered the stainless steel with a thin layer of the metal oxide (5-10 nm). Cyclic voltammetry (CV) and potentiostatic current transient demonstrated the efficiency of the corrosion protection in sodium chloride media, including protection against pitting corrosion.     

Characterization and corrosion studies of ceria thin film based on fluorinated AZ91D magnesium alloy

The CeO₂ thin film was prepared via sol-gel method on fluorinated AZ91D magnesium alloy surfaces. The surface morphology, composition and the corrosion resistance of the film were investigated in details using scanning electron microscope, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy as well as potentiodynamic polarization tests. It was found that small amount of MgO and MgF₂ were encapsulated in CeO₂ thin film. The electrochemical measurement results demonstrated that the CeO₂ thin film on fluorinated AZ91D magnesium alloy could improve the corrosion resistance approximately by two orders of magnitude compared with that of the bare substrate.