Corrosion resistance properties of Ormosil coatings on 2024-T3 aluminum alloy

The corrosion resistance behavior of organically modified silane (Ormosil) thin films on 2024-T3 aluminum alloy substrates was investigated using electrochemical impedance spectroscopy (EIS) and accelerated salt spray analysis techniques. Coatings were prepared containing 0–16.6 vol.% alkyl-modified silane, X_n---Si(OR)_4−n, where X=methyl, dimethyl, n-propyl, n-butyl, i-butyl, n-hexyl, n-octyl, or i-octyl. Coating thicknesses were measured to be in the 6–16 μm range, with the thickest coatings being observed for the highest concentrations of alkyl-modified silane. Contact angle measurements showed an enhancement in hydrophobicity of the Ormosil film imparted by increasing size and concentration of the alkyl-modifiers in the coating. In general, corrosion resistance characteristics, as determined using EIS and salt spray techniques, were found to increase with increasing alkyl-modified silane concentration and alkyl chain length. The best overall corrosion resistance was observed for coating systems containing ≥10.4 vol.% alkyl-modified silane; the hexyl-modified films exhibited corrosion resistance properties superior to the other Ormosil coatings. Immersion studies conducted in 0.5 M K₂SO₄ indicated that coating degradation occurs via hydration of the dense linear chain silicate network leading to the formation of porous cyclic structures.     

Electrochemical impedance spectroscopy investigations of epoxy zinc rich coatings: Role of Zn content on corrosion protection mechanism

Effect of zinc content in the epoxy zinc rich coating on the mechanism of corrosion protection was investigated using electrochemical impedance spectroscopy (EIS). The zinc content in the coating (on dry film) was varied from 40% to 90% in steps of 10%. Open circuit potential (OCP) measurements and salt spray tests were also carried out to generate supporting evidences and to deepen the understanding in the area of zinc rich coatings. The healing or bridging ability of these coatings was studied by making a scribe on the coating and monitoring the evolution of OCP. EIS was also utilized to screen the amount of zinc particles required to provide efficient galvanic protection and to study the effect of purity of Zn on corrosion protection performance. Both EIS and OCP measurements indicate that coatings containing 40% Zn (on dry film) provides very good barrier protection coming mainly from polymer whereas the one containing >80% Zn provides excellent galvanic protection to the steel substrate. When the zinc loading is between 60% and 70%, coatings neither show barrier protection nor galvanic protection.     

Influence of the alloying component on the protective ability of some zinc galvanic coatings

The composition of the corrosion products of pure Zn galvanic coatings as well as of some zinc alloys (Zn-Mn and Zn-Co) after treatment in selected free aerated model media (5% NaCl and 1N Na₂SO₄) is studied and discussed. X-ray diffraction and X-ray photoelectron spectroscopy investigations are used for this purpose. It is concluded that the corrosion products (zinc hydroxide chloride hydrate in 5% NaCl and zinc hydroxide sulfates hydrates in 1N Na₂SO₄) play a very important role for the improved protective ability of the zinc alloys toward the iron substrate, compared to the pure Zn coatings. Another result is that, for a given medium, the corrosion products are one and the same for both alloys independently of the fact that the alloying component is electrically more positive or negative than the zinc. Some suggestions about the models of the appearance of these products and their protective influence are also discussed.     

Characterization of active pigments in damage of organic coatings on steel by means of electrochemical impedance spectroscopy

Active anticorrosive pigments are solid additives for primers which can give further protection for areas with coating damage in addition to their barrier effect. These pigments are expected to prevent corrosion of metal substrate in coating damage by build-up of permanently passive conditions at the metal surface (electrochemical protection) and/or by build-up of solid compounds which plug the coating damage (chemical protection). Electrochemical Impedance Spectroscopy (EIS) was applied to characterize the corrosion protection behaviour of alkyd primers containing different pigments. Impedance spectra were recorded in the frequency range 50 mHz f 50 kHz at the open-circuit potential as a function of the type of pigment and the exposure time in different corrosive media. In general, two different parts can be distinguished in the impedance diagrams. The higher frequency part is related to the insulating properties of the primer and the lower frequency part can be attributed to electrochemical processes taking place within the coating defects. The parameters derived from EIS results show that the low frequency data can be used for characterization of the protective properties of anticorrosive pigments in the presence of defects in organic coatings.     

The influence of Ce-based coatings as pretreatments on corrosion stability of top powder polyester coating on AA6060

Cerium-based conversion coatings (CeCCs) are one of the most prospective alternatives to the widely used chromate conversion coatings (CCCs) due to their anticorrosion efficiency, environmentally friendly nature and low cost. In this work, the CeCCs on AA6060 were prepared by immersion into aqueous cerium salt solutions at room temperature, and subsequently post-treated in heated phosphate solution. The effect of counter ion (nitrate and chloride) on the coating properties was studied testing CeCCs as sole or conversion layers for the top polyester coating. Since the 60 μm thick polyester coating was applied, an artificial defect of 0.8 mm hole was introduced to faster assess the differences between pretreatments. The system with CCC pretreatment was used as reference. Corrosion properties were investigated in 0.5 M NaCl solution by electrochemical impedance spectroscopy while the adhesion strength was measured by NMPR (N-methyl-2-pyrrolidone) and pull-off tests. As shown, the post-treated chloride-based CeCC offered better protection than crack-free thin nitrate-based CeCC, when used as sole coatings. On the other hand, it was brought to evidence that in combination with top powder polyester coating, the CeCC deposited from nitrate solution exhibited better protection compared to protective system pretreated with chloride-based one. Excellent polyester coating adhesion was found independently on aluminium surface pretreatment.     

Electrochemical and sorption characteristics and thermal stability of epoxy coatings electrodeposited on steel modified by Zn–Co alloy

The corrosion behaviour, transport properties and thermal stability of epoxy coatings electrodeposited on steel and steel modified by Zn–Co alloys were investigated during exposure to 3% NaCl solution. The electrochemical impedance spectroscopy (EIS), gravimetric liquid sorption measurements and thermogravimetric analysis (TGA) were used. Zn–Co alloys were electrodeposited on steel from chloride and sulphate baths, by different current densities. From the time dependence of pore resistance and coating capacitance of epoxy coating, diffusion coefficient of water through epoxy coating and thermal stability it was shown that Zn–Co sublayer obtained from chloride solution significantly improves the corrosion stability of the protective system based on epoxy coating. Almost unchanged values of pore resistance were obtained over the long period of exposure time, indicating the great stability of this protective system, due to the existence of a passive layer consisting of basic salts.     

Corrosion electrochemical behavior of epoxy anticorrosive paints based on zinc molybdenum phosphate and zinc oxide

Electrochemical impedance spectroscopy (EIS) was applied as a principal tool to describe the efficiency of anticorrosive epoxy paints (primers) based on zinc molybdenum phosphate (ZMP) pigment. Steel-coated samples were exposed to a 0.5 M NaCl solution. During the study the corrosion potential (E_oc) and R_p values also were monitored every 24 h. It is discussed the incorporation of micronized ZnO (1 μm) pigment to the base mixture and its positive, reinforcement effect on the protective properties of ZMP primer. The explanation is related to the izoelectric point (IEP) of ZnO particles (pH < 9), which determines their positive surface charge and electrostatic attraction with the molybdate anion. In this case the charge of the formed double layer capacitor is very high. Moreover, the mentioned attraction inhibits and saves ZnO particles from their rapid dissolution to hydroxide.     

Acceleration and quantitative evaluation of degradation for corrosion protective coatings on buried pipeline: Part I. Development of electrochemical test methods

The electrochemical degradation of polyethylene coated onto SS400 was examined in synthetic groundwater. Electrochemical techniques (electrochemical impedance spectroscopy, potentiodynamic and potentiostatic polarization tests) and surface analysis (scanning electron microscopy) were used to accelerate and evaluate the coating degradation. The pulsed potentiostatic polarization test accelerating both the cathodic reduction and anodic oxidation reactions was applied to reproduce the coating degradation mechanisms of cathodic disbondment and oxide lifting. The applied potentials were determined to be ±300 mV_SCE versus open-circuit potential from the analysis of the anodic and cathodic polarization data. Results from the EIS confirmed that coating degradation is accelerated effectively by the pulsed potentiostatic polarization testing.     

Acceleration and quantitative evaluation of degradation for corrosion protective coatings on buried pipeline: Part II. Application to the evaluation of polyethylene and coal-tar enamel coatings

The pulsed potentiostatic polarization technique proposed previously has been applied to evaluate the electrochemical degradation of polyethylene and coal-tar enamel coated SS400 for buried pipeline. The degradation of coated systems was accelerated and evaluated using electrochemical techniques (pulsed potentiostatic polarization test, electrochemical impedance spectroscopy) and surface analyses (scanning electron microscopy and energy dispersive X-ray spectroscopy). It was found that polyethylene coating had better protective performance than coal-tar enamel coating due to its low porosity. It was confirmed that the electrochemically accelerated test is an effective technique in the evaluation of coating performance.     

The effects to the structure and electrochemical behavior of zinc phosphate conversion coatings with ethanolamine on magnesium alloy AZ91D

This paper discussed a zinc phosphate conversion coating formed on magnesium alloy AZ91D from the phosphating bath with varying amounts of ethanolamine (MEA). The effects of MEA on the form, structure, phase composition and electrochemical behavior of the phosphate coatings were examined using an scanning electron microscopy (SEM), X-ray diffraction (XRD) potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) measurements. Interpretations of the electrical elements of the equivalent circuit were obtained from the SEM structure of the coatings, assumed to be formed of two layers: an outer porous crystal layer and an inner flat amorphous layer. The result showed that adding MEA refined the microstructure of the crystal layer and that the phosphate coating, derived at the optimal content of 1.2 g/L, with the most uniform and compact outer crystal layer provided the best corrosion protection.     

Plasma treatment of automotive steel for corrosion protection – a dry energetic process for coatings

In the thrust of pursuing new environmentally friendly technology for automotive application, a new corrosion protection coating system for automotive steel has been developed through interface engineering affected by an energetic plasma process. The plasma treated coating system outperformed the current phosphated galvanized steel system in scab corrosion tests. In the plasma process, the steel substrate was subjected to plasma cleaning and in situ plasma polymerization deposition. Plasma of a mixture of argon and hydrogen was created to remove the surface contaminants and the inherent oxide layer. A very thin film (50–100 nm) was then deposited by a plasma generated from alkylsilanes (e.g. trimethylsilane (TMS)). The interface can be so designed that strong corrosion-resistant interfacial bonds such as Fe–Si, Fe–C, and Si–C can be obtained. The interfacial chemistry involved in the plasma process and corrosion reaction are characterized by reflection absorption infrared spectroscopy (RAIR), X-ray photoelectron spectroscopy (XPS), and sputter neutral mass spectroscopy (SNMS).     

Corrosion protective performances of commercial low-VOC epoxy/urethane coatings on hot-rolled 1010 mild steel

The protective properties of low-VOC epoxy/urethane paint systems of commercial grade have been investigated using a variety of techniques such as electrochemical impedance spectroscopy (EIS). One epoxy-polyamide mastic/urethane, three high-solid epoxy-amine/urethane coatings, one solvent-free epoxy-amine/urethane, one water-based epoxy-amine and one high-VOC alkyd paint system (used as paint reference system) were applied on hot-rolled 1010 mild steel panels and exposed for up to 2000 h in the salt spray cabinet (SSC) or for 1 year at an outdoor marine test site. These paints were tested for their barrier properties, corrosion-induced adhesion loss and visual defects, as well as for their flexibility and resistance to direct impact. The barrier properties increased in the following order: alkyd相似文献   

Cathodic disbonding of a thick polyurethane coating from steel in sodium chloride solution

The cathodic disbonding of a thick, pigmented polyurethane coating from steel in 3.5 wt.% NaCl solution was studied by using an electrochemical AC impedance technique. Double-cylinder electrolyte cells were designed to separate the measurements of cathodic disbonding process from the influence of the impedance of an artificial defect. It was found that for a thick, pigmented polyurethane coating, the more important transport pathway of the reactive species is along the coating/steel interface rather than through the coating. There existed a delay time for the cathodic disbonding process, and cathodic polarization was not a predominant factor in determining the cathodic disbonding behavior in the early stages. The thick polyurethane coating, which was applied on a well sand-blasted steel surface, had excellent resistance to cathodic disbonding.     

The effects of zinc ions on the performance of epoxy coated mild steel under cathodic protection (CP)

This work aimed to address the issues that arise when cathodic protection is applied to a coated structure. The use of zinc chloride (ZnCl₂) as a corrosion inhibitor was suggested; the idea was to bridge the weakly cross linked areas on the coating by depositing a film of zinc compounds in order to seal them from the corrosive environment.