Corrosion protection properties of vanadium films formed on zinc surfaces

Vanadium films were prepared on zinc surfaces by using a solution containing vanadate. Corrosion protection properties of vanadium-treated (V-treated), chromium-treated (Cr-treated), and untreated zinc surfaces in contact with a 3.5 wt.% NaCl solution were studied using potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and neutral salt spray (NSS) tests. According to these results, the V-treated layer significantly improved the corrosion resistance of zinc surfaces. In comparison with the Cr-treated layer, the V-treated layer exhibited a better corrosion resistance. The composition of the V-treated layer was studied using X-ray photoelectron spectroscopy (XPS). XPS measurements indicated that the vanadium layer formed on zinc surfaces and the vanadium-rich coating was a hydrated oxide with a composition of V₂O₅, VO₂, and its hydrates such as V₂O_5·nH₂O and VO(OH)₂.     

Surface Characteristics and Electrochemical Impedance Investigation of Spark-Anodized Ti-6Al-4V Alloy

In this study, the surface characteristic of oxide films on Ti-6Al-4V alloy formed by an anodic oxidation treatment in H₂SO₄/H₃PO₄ electrolyte at potentials higher than the breakdown voltage was evaluated. Morphology of the surface layers was studied by scanning electron microscope. The results indicated that the diameter of pores and porosity of oxide layer increase by increasing the anodizing voltage. The thickness measurement of the oxide layers showed a linear increase of thickness with increasing the anodizing voltage. The EDS analysis of oxide films formed in H₂SO₄/H₃PO₄ at potentials higher than breakdown voltage demonstrated precipitation of sulfur and phosphor elements from electrolyte into the oxide layer. X-ray diffraction was employed to exhibit the effect of anodizing voltage on the oxide layer structure. Roughness measurements of oxide layer showed that in spark anodizing, the Ra and Rz parameters would increase by increasing the anodizing voltage. The structure and Corrosion properties of oxide layers were studied using electrochemical impedance spectroscopy (EIS) techniques, in 0.9 wt.% NaCl solution. The obtained EIS spectra and their interpretation in terms of an equivalent circuit with the circuit elements indicated that the detailed impedance behavior is affected by three regions of the interface: the space charge region, the inner compact layer, and outer porous layer.     

Effect of treatment time on a PEO-coated AZ31 magnesium alloy

Plasma electrolytic oxidation (PEO) coatings were formed in a phosphate–silicate-based electrolyte containing K₂ZrF₆ on an AZ31 Mg alloy. The physical and chemical properties of the coatings were investigated using scanning electron microscopy, atomic force microscopy, X-ray diffraction (XRD), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). The results showed that the thickness of the PEO coatings increased linearly with increased treatment times. Additionally, the micropores on the coating surfaces increased in size, but decreased in porosity with increased PEO treatment time. The XRD results showed that the coatings were mainly composed of MgO, MgF₂, MgSiO₃, and ZrO₂, and the electrochemical tests revealed that the corrosion resistance of the coatings increased with increased treatment time. Besides, the EIS results correlated well with the potentiodynamic polarization test results.     

Effect of voltage on microstructure and corrosion resistance of microarc oxidation coatings on CP-Ti

The surface modification of commercially pure titanium (CP-Ti) by microarc oxidation (MAO) under different voltages was investigated using 1%H₃PO₄ solution as an electrolyte. The microstructure, phase composition and elemental distribution of ceramic coatings were investigated using scanning electron microscopy (SEM) and X-ray diffraction. The corrosion behaviour of the coating was also examined by potentiodynamic polarisation testing in a 3·5 wt-%NaCl solution. Micropore oxide films were formed on all the sample groups by MAO. The thickness and micropore size of the MAO coating increased with the increasing voltage. Energy dispersive X-ray spectroscopy results indicate that Ti, O and P became incorporated into the MAO coatings. At a low voltage of 250 V, the MAO coatings were composed of amorphous, P₂O₅, TiP₂O₇ and titania phases (rutile and anatase). Variation of treatment voltages increased the ceramic coatings from an amorphous structure to a phase structure, and the P₂O₅ phase disappeared. The corrosion potential Φ_corr of the MAO sample shifted towards nobler directions, and the corrosion density I_corr fell significantly compared with that of the bare CP-Ti. Corrosion testing showed that the sizes of the micropore of the MAO samples obviously decrease, and the MAO surface becomes smooth.     

Electrochemical study of corrosion behaviour of carbon steel A106 and stainless steel 304 in aqueous monoethanolamine

Abstract

Corrosion behaviour of carbon steel A106 and stainless steel 304 (SS304) in aqueous monoethanolamine was studied by performing electrochemical polarisation experiments. Potentiodynamic curves were studied and compared under conditions with different temperatures, carbon loading and O₂ percentage in purging gases. It was found that corrosion of A106 and SS304 was promoted under conditions with higher temperature. While the presence of O₂ speeds the corrosion of A106, it has a negligible impact on SS304 at 80°C and lowers the corrosion rate at 40°C. Corrosion rates and other important parameters were calculated based on the electrochemical curves for A106. Sample surfaces after tests were examined by scanning electron microscopy and energy dispersive spectroscopy. Mechanisms involved in iron dissolution and passivation from oxide films were discussed.     

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.     

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.     

Improved corrosion behavior of nanocrystalline zinc produced by pulse-current electrodeposition

Pulse electrodeposition was used to produce nanocrystalline (nc) zinc from zinc chloride electrolyte with polyacrylamide and thiourea as additives. Field emission scanning electron microscopy (FESEM) was used to study the grain size and surface morphology of the deposits and X-ray diffraction was used to examine their preferred orientation. Corrosion behavior of the electrodeposited nc zinc in comparison with electrogalvanized (EG) steel in de-aerated 0.5 N NaOH solution was studied using potentiodynamic polarization and impedance measurements. A scanning electron microscope (SEM) was used to characterize the surface morphology of the EG steel before corrosion testing. Surface morphologies of nc zinc deposits and EG steel were also studied after potentiondynamic polarization by SEM. Nanocrystalline zinc (56 nm) with random orientation was produced. The estimated corrosion rate of nc zinc was found to be about 60% lower than that of EG steel, 90 and 229 μA/cm², respectively. The surface morphology of corroded nc zinc was characterized by discrete etch pits, however, uniform corrosion was obtained after potentiodynamic polarization of EG steel. The passive film formed on the nc zinc surface seems to be a dominating factor for the corrosion behavior observed.     

Oxide formation on aluminium alloys in boiling deionised water and NaCl, CeCl3 and CrCl3 solutions

Rutherford backscattering spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy have been employed to provide depth, surface chemical and morphological information respectively for a range of Al-alloys treated for 2 h at 90-100 °C in either deionised water (DIW), 30 mM NaCl, 10 mM CeCl₃ or 10 mM CrCl₃ solutions. Alloys included Al sheet 1100-O, 2024-T3, 3004-H19, 5005-O, 6061-T6 and 7075-T6. In DIW all alloys developed the same oxide thickness with a boehmite-like appearance. In NaCl and CeCl₃ solutions the oxide thickness varied with alloy composition but was considerably thicker than the deionised case and maintained a boehmite-like appearance. Ce was distributed throughout the oxide but at low levels. In all cases some Ce^IV was detected on the surface of the oxide, the remainder being Ce^III. In CrCl₃ solution, the oxide thickness was extremely thin with the appearance of a dense oxide rather than a boehmite-type structure.     

Hardness and electrochemical behavior of ceramic coatings on CP titanium by pulsed laser deposition

Thin films of alumina and silicon carbide are deposited on titanium substrate by the pulsed laser deposition technique using Nd: YAG laser. Deposited films are characterized using x-ray diffraction, scanning electron microscopy, energy-dispersive x-ray spectroscopy, absorption spectroscopy and nanoindentation. Film hardness of the ceramic coating is found to be high compared to that of the substrates. Corrosion behavior of substrates after ceramic coating is studied in 3.5% NaCl solution by potentiodynamic polarization and electrochemical impedance spectroscopy measurements. Experimental results show an increase in corrosion resistance of titanium after being coated with a ceramic material.     

Plasma anodized ZE41 magnesium alloy sealed with hybrid epoxy-silane coating

Anodic coatings on magnesium ZE41 alloy were formed by DC plasma electrolytic oxidation (PEO) in spark regime in solution composed of NaOH, Na₂SiO₃ and KF. The positive effect of poly(ethylene oxide) addition into the anodizing electrolyte on PEO process, anodic film porosity and its protective performance was described. Anodic films were sealed with hybrid epoxy-silane formulation. The corrosion behavior of the coated ZE41 was studied through electrochemical impedance spectroscopy (EIS) in 0.6 M NaCl solution. Resulting duplex PEO/epoxy-silane coating provides good protective performance without significant signs of corrosion during 1 month of immersion test.     

镁合金表面聚苯胺膜层防腐性能的研究

用循环伏安法在镁合金表面制备了聚苯胺膜层,并用扫描电子显微镜(SEM)和电极电位研究了聚苯胺膜的表面形貌和腐蚀防护性能.研究表明:聚苯胺膜层并不是均匀覆盖在基体表面,而且膜层表面存在很多的微孔;聚苯胺改变了膜层的电极电位;在腐蚀介质中聚苯胺膜以点蚀或剥离的方式进行腐蚀.     

Corrosion behaviour of Sn-containing oxide layer on AZ91D alloy formed by plasma electrolytic oxidation

Corrosion phenomenon of magnesium alloys is one of the limits for using magnesium alloys in automotive and aerospace industries. The aim of this study is the development of Sn-containing protective oxide coating by a simple plasma electrolytic oxidation in KOH/KF/Na₃PO₄ electrolyte on AZ91D magnesium alloy in galvanostatic mode. The film morphology and composition were analysed by SEM coupled with EDS, XRD and Raman spectroscopy. In the oxide, tin is mainly incorporated as crystallised MgSn(OH)₆ compound in the layer. The main properties of Sn-containing oxide coating on AZ91D are both keeping the corrosion rate at open-circuit conditions at an acceptable value, and providing a sufficient passivation plateau to reduce the pitting sensibility. The lather characteristic, revealed by pitting tests, addresses the major drawback of magnesium alloys which often undergo important galvanic coupling in service. Consequently, the addition of low stannate concentration in the electrolyte to form Sn-rich anodic oxide on magnesium alloys represents an interesting way to synthesize protective coatings by PEO in a short time of anodization.     

Determination of corrosion potential of coated hollow spheres

Copper hollow spheres were created on porous iron particles by electro-less deposition. The consequent Ni plating was applied to improve the mechanical properties of copper hollow micro-particles. Corrosion properties of coated hollow spheres were investigated using potentiodynamic polarisation method in 1 mol dm⁻³ NaCl solution. Surface morphology and composition were studied by scanning electron microscopy (SEM), light microscopy (LM) and energy-dispersive X-ray spectroscopy (EDX). Original iron particles, uncoated copper spheres and iron particles coated with nickel were studied as the reference materials. The effect of particle composition, particularly Ni content on the corrosion potential value was investigated. The results indicated that an increase in the amount of Ni coating layer deteriorated corrosion resistivity of coated copper spheres. Amount of Ni coating layer depended on conditions of Ni electrolysis, mainly on electrolysis time and current intensity. Corrosion behaviour of sintered particles was also explored by potentiodynamic polarisation experiments for the sake of comparison. Formation of iron rich micro-volumes on the particle surface during sintering caused the corrosion potential shift towards more negative values. A detailed study of the morphological changes between non-sintered and sintered micro-particles provided explanation of differences in corrosion potential (E_corr).     

The effect of phosphate on MAO of AZ91D magnesium using AC power source

A rapid and convenient anodization technology with AC power source to obtain the MAO films formed on magnesium alloy AZ91D in phosphate bath (base electrolyte + Na₃PO₄) with or without aluminate and silicate was studied. The corrosion resistance of the anodic films was studied by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques and the microstructure and composition of films were examined by SEM and XRD. The results show that Na₃PO₄can promote the occurrence of sparking during the MAO process, while abundant heat generated by sparking might enhance the formation of the glassy phase of the compound when the electrolyte contains the additives of NaAlO₂and Na₂SiO₃simultaneously. The optimized MAO film is ivory‐white smooth by naked eye, while presents porous and microcracks in microscopic scale. The anodic film formed in the alkaline solution with optimized parameters possesses superior corrosion resistance by electrochemical test. The XRD pattern shows that the components of the anodized film consist of MgO, MgAlO₂, and MgSiO₃. No oxide crystal with P element can be found.     

Corrosion of friction stir welded magnesium alloy AM50

The microstructure of a friction stir welded magnesium alloy AM50 was examined by means of optical light microscopy. The chemical composition, particularly the iron content, and morphology of the oxide film were analyzed and discerned via auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Corrosion behaviour of the welds and base materials were investigated by virtue of neutral salt spray tests and potentiodynamic polarization measurements in conventional cells and in a mini cell. The results demonstrate that minor increases in iron concentration as might be speculated to occur as a consequence of tool/work piece interaction during the welding process on the corrosion resistance of the weld can be ignored. The corrosion morphology was predominantly influenced by the distribution of the Mg₁₇Al₁₂ phase. Here, it was also found that the corrosion resistance of the friction stir weld varied in response to changes in the joint microstructure.     

A comparison between the corrosion behavior of nanostructured copper thin films deposited on oxidized silicon and copper sheet in alkaline media

Copper thin films were deposited on oxidized silicon at a substrate temperature of 70 °C and 150 °C using EB-PVD technique. The morphology and crystal orientation of the deposited film were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Corrosion behavior of films was studied through electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, immersion test, and cathodic chronopotentiography. Additionally, the crystalline structure of corroded samples immediately after polarization was examined by XRD. Corrosion current density for copper deposits was higher than copper sheet by polarization tests, while the data obtained by the EIS technique emphasized higher corrosion current density for copper sheet. However there was a conflict between polarization and EIS data, the other results obtained by immersion and cathodic chronopotentiography tests proved that the corrosion resistance of copper deposits was higher than copper sheet in the same alkaline media, which can be attributed to chemical composition and higher thickness of the passive layer formed on copper deposits. On the other hand, breakdown potential (E_bp) for copper sheet was about 0.3 V_SCE, while a distinct E_bp was not found for copper deposits. This was a sign of higher stability of the passive layer formed on copper deposits. The XRD patterns of samples immediately after polarization showed a higher content of Cu(OH)₂ on copper deposits in comparison with copper sheet. The stable morphology formed on the surface of copper after polarization was monoclinic CuO, which is assumed to have a significant effect on copper protection in alkaline media. This morphology was more compact on copper deposits in comparison with copper sheet. This was due to higher ability of deposits to react with hydroxyl ions.     

Characterization of calcium-modified zinc phosphate conversion coatings and their influences on corrosion resistance of AZ31 alloy

Two kinds of phosphate conversion coatings, including zinc phosphate coating and zinc-calcium phosphate coating, were prepared on the surface of AZ31 alloy in phosphate baths. The morphologies of these coatings were observed using scanning electron microscopy. Their chemical compositions and structures were characterized using energy-dispersive X-ray spectrum, X-ray photoelectron spectroscopy and X-ray diffraction. The corrosion resistance of the coatings was evaluated by potentiodynamic polarization technique. The results show that the flowerlike Zn-Ca phosphate conversion coatings are mainly composed of hopeite (Zn₃(PO₄)₂·4H₂O). They have a quite different morphology from the dry-riverbed-like Zn phosphate coatings that consist of MgO, MgF₂, Zn or ZnO and hopeite. Both of the zinc and zinc-calcium phosphate coatings can remarkably reduce the corrosion current density of the substrates. The Zn-Ca coating exhibits better corrosion resistance than the Zn coating. Introduction of calcium into the phosphate baths leads to the full crystallinity of the Zn-Ca coating.     

Corrosion behaviour of AZ31B magnesium alloy in NaCl solutions saturated with CO2

Corrosion behaviour of AZ31B magnesium alloy in different concentrations of NaCl solution saturated with CO₂ was studied by electrochemical techniques, Fourier transform infrared spectroscopy, scanning electron microscopy and energy dispersive X-ray. The corrosion rate increases with increasing NaCl concentration both in the presence and absence of CO₂. The corrosion rate in NaCl solution saturated with CO₂ is bigger than that in single NaCl solution. The inhibitive effect of CO₂ is also observed with immersion time increased in NaCl solution saturated with CO₂, showing that CO₂ reduces the average corrosion rate due to the formation of insoluble products.     

Influence of Zn on oxide films on Alloy 690 in borated and lithiated high temperature water

To clarify the corrosion control effect of Zn injection into hydrothermal environments, the oxide films on Alloy 690 in the deaerated borated and lithiated water have been investigated using potentiodynamic polarization curves, electrochemical impedance spectra at 300 °C and ex-situ X-ray photoelectron spectroscopy. With Zn injection in the solution, ZnCr₂O₄ and ZnFe₂O₄ were formed in the inner and outer layers of the oxide films on Alloy 690, respectively, through exchange reactions between Zn²⁺ and Fe²⁺/Ni²⁺. A simple model for oxide film structure change and the mechanism of corrosion inhibition by Zn injection is proposed and discussed.