Evaluation of the properties of anodized aluminum 6061 subjected to thermal cycling treatment using electrochemical impedance spectroscopy (EIS)

The corrosion resistance of anodized Al 6061 produced by two different anodizing and sealing processes was evaluated for 30 days during exposure to 3.5 wt% NaCl using EIS. Thermal cycling treatments at 120, 160 and 200 °C have been applied for the two types of samples. The degradation of the properties of the anodized layers has been determined by thorough analysis of the EIS data for control samples and samples that had undergone thermal cycling. Scanning electron microscopy has been used to evaluate the damage to the anodized aluminum layers due to thermal cycling. It was found that the thermal treatment produced considerable damage of both the porous layer and the barrier layer. The EIS data suggest that some cracks extended into the bare metal. The damage of the oxide layers increased with increasing thermal cycling temperature for both types of samples. Self-sealing of the porous layer and the barrier layer occurred during immersion in NaCl.     

Anodizing and corrosion behaviour of aluminium

In this study, aluminum was anodized in 0.4 M H₂SO₄ + 0.145 M H₃BO₃ solution by means of potentiostatic method. In first step, the most appropriate conditions (anodizing voltage and period) were determined. The voltage-current behavior was investigated between 0 to 30 V and oxide formation potential (15 V) was determined. Then chorono-amperometric measurement was obtained at this potential during 2 hours. Therefore, anodizing process was applied at 15 V between aluminium anode and mild steel cathode. In the second step, the corrosion performance of non-anodized (Al) and anodized samples (Al₂O₃) have been investigated in 3.5% NaCl solution by using electrochemical impedance spectroscopy and polarization techniques. The interface between metal and solution was modeled with the equivalent circuit successfully. The circuit’s elements were calculated with Ivium Soft fitting program. The polarization curves were obtained between −1.8 V and 0.5 V potential ranges. The experimental results proved that the thickness and quality of oxide film was improved with help of anodizing process. Thus the corrosion resistance of anodized aluminum is significantly increased.     

Corrosion Behavior of Anodized Al Coated by Physical Vapor Deposition Method on Cu–10Al–13Mn Shape Memory Alloy

Physical vapor deposition method was utilized to apply Al coating onto Cu–10Al–13Mn alloy, then coated layer was anodized in different temperatures: 5 and 10°C as well as several potentials: 20, 30, 40, 50 V in order to achieve best anodizing parameters. The effects of anodizing parameters on alumina nanotube formation and corrosion resistance were investigated. Phase analysis on surface was conducted by X-ray diffraction method and nanotube characteristics was studied by scanning electron microscopy (SEM) and surface topology was investigated by atomic force microscopy (AFM). Additionally, the corrosion resistance of coatings was studied by potentiodynamic test in 1M NaCl solution. The results depicted that whole deposited Al layer was anodized and FCC alumina was formed merely. Polarization test results was illustrated that Al anodized layer significantly improved Cu–10Al–13Mn corrosion resistance. Uncoated specimen had highest corrosion rate and anodized layer in lower temperature and voltage had minimum alumina nanotube dimension; as a result, it had best corrosion behavior in NaCl corrosive solution.     

Evaluation of different sealing methods for anodized aluminum‐silicon carbide (Al/SiC) composites using EIS and SEM techniques

Electrochemical impedance spectroscopy (EIS) and the scanning electron microscope (SEM) have been used in an investigation of the effectiveness of various sealing methods that can be used to improve the corrosion resistance of an anodized aluminum‐silicon carbide (Al/SiC) composite. Anodic oxide films were grown on Al7075‐T6 and the Al/SiC composite by sulfuric acid anodizing and sealing in a cold saturated solution of nickel acetate. Other samples were sealed using the traditional method of boiling water or hot nickel acetate for comparison. The results revealed a uniform anodized layer on Al7075‐T6 that resisted pitting corrosion for more than 2 weeks exposure to NaCl, whereas a cracked oxide film with variations in thickness was observed on the composite material. Pit initiation occurred in less than 5 days on the anodized Al/SiC that was sealed in the hot solutions. This study suggests that the traditional hot sealing methods did not provide sufficient corrosion protection for aluminum metal–matrix composites (MMCs) because the reinforcing SiC particles deteriorated the surface film structure. However, this defective film can be repaired by nickel hydrate precipitation during cold sealing or by applying a thick polyurethane coating.     

工艺参数对Al合金阳极氧化膜腐蚀行为的影响

用电化学极化和扫描电镜(SEM)观察法,研究了电流密 度、氧化时间和氧化温度等工艺参数对LC4铝合金阳极氧化膜的厚度、显微形貌和在NaCl溶 液中耐蚀性的影响，讨论了影响氧化膜耐蚀性的机制，提出了提高耐蚀性的最佳阳极氧化工 艺参数.     

Effect of Micro Arc Oxidation Coatings on Corrosion Resistance of 6061-Al Alloy

In the present study, the corrosion behavior of micro arc oxidation (MAO) coatings deposited at two current densities on 6061-Al alloy has been investigated. Corrosion in particular, simple immersion, and potentiodynamic polarization tests have been carried out in 3.5% NaCl in order to evaluate the corrosion resistance of MAO coatings. The long duration (up to 600 h) immersion tests of coated samples illustrated negligible change in weight as compared to uncoated alloy. The anodic polarization curves were found to exhibit substantially lower corrosion current and more positive corrosion potential for MAO-coated specimens as compared to the uncoated alloy. The electrochemical response was also compared with SS-316 and the hard anodized coatings. The results indicate that the overall corrosion resistance of the MAO coatings is significantly superior as compared to SS316 and comparable to hard anodized coating deposited on 6061 Al alloy.     

Corrosion Resistance and Color Properties of Anodized Ti-6Al-4V

In this research, color anodizing of Ti-6Al-4V alloy was performed in phosphoric acid solution of 0.4 M concentration and within 30 s in different voltages (10-120 V) of a DC power supply. The effect of anodizing voltages on the color and thickness of anodized layers on Ti-6Al-4V alloy surface was surveyed. Thickness and refractive index of layers were measured by spectrophotometery and reflectance curves. According to the results, thickness of layers increased with increasing anodizing voltage and was in the range of 38-167 nm. Also the refractive index of anodic film was approximately constant at about 2 and increased inconsiderably with increasing anodizing voltage. Corrosion resistance of the anodized samples in 20 and 50 V was surveyed in physiological solutions of Ringer’s solution, Artificial Saliva solution, and Ringer’s + 150 mM H₂O₂ solution at the temperature of 37 °C by potentiodynamic polarization method. The anodized sample in 50 V indicated lower corrosion rate than the non-anodized sample as well as the sample which was anodized in 20 V in all solutions. The non-anodized sample indicated the highest corrosion rate of about 0.25 μA cm⁻².     

Baking treatment effect on materials characteristics and electrochemical behavior of anodic film formed on AZ91D magnesium alloy

The composition and microstructure of the anodic films formed on AZ91D Mg alloy, with or without baking, were investigated. The associated corrosion behavior of the anodized alloy in 3.5 wt% NaCl solution was also examined using electrochemical impedance spectroscopy (EIS). The results show that MgO was the main component in the anodic film which also contained some Mg(OH)₂, Al₂O₃, Al(OH)₃, and MgAl₂O₄. Both the amorphous and crystalline forms of anodic film were identified. The degree of crystallinity depended on baking temperature, which increased with increasing temperature in the range of 50-250 °C. The amounts of MgO and Al₂O₃ increased as a result of a dehydration reaction. The polarization resistance of anodized Mg alloy was improved significantly by increasing the oxide content in the anodic film. An optimum value of polarization resistance of anodic film was obtained for the alloy baked at 150 °C for 2 h followed by air cooling.     

Electrochemical Impedance Spectroscopy of protective coatings

Electrochemical impedance spectroscopy (EIS) carried out over a wide frequency range (10⁵ to 10^?2 Hz) allows us to obtain mechanistic information concerning corrosion protection by coatings. Examples are given for polymer coatings on an Al alloy as a function of surface petreatment and for anodized Al alloys. For polymer coatings on metals, information concerning the coating's properties and its changes with exposure time can be obtained from the high and medium frequency regions of the impedance spectrum, while the corrosion reaction at the metal/coating interface can be evaluated from the low frequency part. The pore resistance of the coating and its changes with exposure time have been used to rank different pretreatment procedures for a given metal/coating combination. For Al alloys, pronounced differences in corrosion resistance between a conversion coating and an anodized layer under a polyurethane coating have been observed. The use of a segmented electrode allows measurements of the impedance across as well as under a coating. From these data, information concerning delamination can be obtained. An example is given for an epoxy coating on steel. The use of EIS as a quality control and corrosion test for anodized Al alloys is discussed. The effects of the anodization procedure (sulfuric acid and chromic acid), sealing procedure (hot water and dichromate) and alloy chemistry (Al 2024, 6061 and 7075) have been studied during exposure to aerated 0.5 N NaCl. All these parameters play an important role in the corrosion resistance of the alloys. The sealing process can be evaluated from the high frequency part of the spectrum. Pronounced differences in the spectra for the two sealing procedures are observed. The corrosion behavior is reflected in the low frequency part, which is essentially dominated by barrier layer properties.     

Electrochemical behaviour and structure of rust formed on Si- and Al-bearing steel after atmospheric exposure

The corrosion resistance of Si- and Al-bearing steel was estimated by atmospheric exposure test, and the structure of the rust was examined by EPMA (electroprobe X-ray microanalysis) and TEM (transmission electron microscopy) analysis. Moreover, the electrochemical behaviour of rust was investigated by EIS (electrochemical impedance spectroscopy).The Si- and Al-bearing steel exhibited excellent corrosion resistance in the exposure test as compared with carbon steel (SM). EPMA and TEM analysis showed that Si and Al mainly existed in nanoscale iron complex oxides in the inner rust formed on this steel. The Al K spectrum of the rust exhibited a peak that was the same as that of Al₂O₃ in the EPMA and TEM-EELS (electron energy loss spectroscopy) analysis. This result suggests that Al was present in the complex oxides as Al³⁺. In the same way, Si was identified as being in an intermediate state in the complex oxides of the inner rust.EIS measurement of the exposure test samples revealed much higher rust resistance (R_rust) and corrosion reaction resistance (R_t) of Si- and Al-bearing steel compared to that of SM. Finally, it was found that nanoscale complex iron oxides formed in the inner rust of Si- and Al-bearing steel, resulting in increased R_rust and R_t, and corrosion suppression.     

Corrosion behaviour of bulk ultra-fine grained AZ91D magnesium alloy fabricated by equal-channel angular pressing

Effect of microstructure change on corrosion behaviour of equal-channel angular pressed (ECAPed) AZ91D Mg alloy was investigated. The ECAPed alloy with ultra-fine grained (UFG) α-phase matrix and refined β-phase particles displays a significantly lower corrosion resistance, resulting in more pits after in-situ corrosion, higher mass-loss rate immersed in NaCl solution, larger I_corr values in polarization curves and lower fitted R_t values in EIS plots. Two factors weaken the corrosion resistance: the first is the strains-induced crystalline defects providing the α-phase matrix more corrosion activation, the second is the refined β-phase particles losing barrier to the corrosion propagation in α-phase matrix.     

Effect of anodizing on galvanic corrosion behavior of T300 CFRP/5083P-O Al bolted joints

T300 carbon fiber reinforced polymer (CFRP) and 5083P-O aluminum (5083P-O Al) alloy bolted joints have been used in high-speed trains due to the advantages of light weight and high strength. However, high potential difference between the CFRP and 5083P-O Al will induce galvanic corrosion and result in accelerating corrosion rate of 5083P-O Al, which is a potential risk for its engineering applications. In this work, combination with the electrochemical analysis, surface and cross-section corrosive morphologies analysis, the galvanic corrosion behavior between CFRP/5083P-O Al bolted joints with and without anodizing in 3.5 wt.% NaCl spray was investigated. Results indicated that severe corrosion occurred on unanodized 5083P-O Al in the coupled regions of the CFRP/5083P-O Al bolted joint due to galvanic corrosion. With the content of sulfuric acid increasing, the thickness of each Al₂O₃ layer and atomic oxygen content increases significantly. 5083P-O Al anodized by the 135 g/L H₂SO₄ + 8 g/L H₃BO₃ mixed solution had the favorable Al₂O₃ film, which increased the resistance of 5083P-O Al by roughly three orders of magnitude, effectively improving the corrosion resistance of 5083P-O Al.     

Electrochemical impedance spectroscopy as a monitoring tool for passivation and localized Corrosion of aluminum alloys

Pitting and crevice corrosion of Al alloys and Al-based metal matrix composites can be detected by characteristic changes of the impedance spectra in the low frequency region. A pitting model has been developed which is in agreement with the experimental data. A fitting procedure has been used to analyze a large number of data which have been obtained for as-received samples and samples which had been passivated in CeCl₃ solutions. This chemical passivation process produces surfaces which are very resistant to localized corrosion. Al 6061, Al/SiC and Al/graphite which had been passivated in CeCl₃ for one week did not pit in 0.5 N NaCl for at least one month. Electrochemical impedance spectroscopy (EIS) is a convenient tool for monitoring of the passivation and the corrosion processes.     

Effect of chloride ions on electrochemical properties of anodized <Emphasis Type="Italic">AV</Emphasis> and <Emphasis Type="Italic">D</Emphasis>16 aluminum alloys in aqueous and glycerin-containing aqueous sodium sulfate solutions

The effect of chloride ions (0.01 N NaCl) on the electrochemical properties of anodized (in chromic anhydride or sulfuric acid) AV and D16 aluminum alloys in aqueous sulfate (0.5% Na₂SO₄) and glycerin-containing aqueous sulfate (0.5% Na₂SO₄, 33% glycerin) solutions is studied. Depending on the conditions of anodizing and the composition of the alloy and environment, currents on the anodized alloys in the passive range are shown to be smaller by one to four orders of magnitude compared to those on nonanodized alloys. Anodizing increases the resistance of alloys against pitting corrosion. Alloys anodized in sulfuric acid and then treated in dichromate are not susceptible to pitting corrosion. Alloys anodized in chromic anhydride are less resistant against pitting.     

Improving corrosion resistance of pure Al through ECAP

Abstract

Significant grain refinement and corrosion resistance improvement were achieved in industrial pure Al through equal channel angular pressing (ECAP). The effect of microstructure change on its corrosion resistance was investigated by optical/electron microscopy observation, constant immersion tests, polarisation tests and electrochemical impedance spectroscopy (EIS) in aqueous NaCl solution. The ultrafine grained (UFG) bulk pure Al (with grain sizes of 300–500 nm) has a higher pitting potential E_pit, a lower corrosion current density I_corr in polarisation tests and an increased polarisation resistance R_p from EIS plots, along with reduced corrosion damage in immersion tests, compared with the as cast material. It was found that the improved corrosion resistance resulted from the uniform distribution of fine Si containing impurities and the formation of a denser oxide film. The ECAPed samples with smaller Si containing impurities have lower microgalvanic currents and reduced susceptibilities of pitting corrosion, which is consistent with the classical ‘small cathode, large anode’ mechanism. The strain induced crystalline defects, for example, high angle grain boundaries and dislocations, appear to provide more nucleation sites for the formation of a denser and thicker oxide film, thus enhancing its corrosion resistance.     

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.     

Initial corrosion protection of Zn–Mn alloys electrodeposited from alkaline solution

The effects of a deposition current density (c.d.) on the corrosion behaviour of Zn–Mn alloy coatings, deposited from alkaline pyrophosphate solution, were investigated by atomic absorption spectrophotometry (AAS), X-ray diffraction (XRD), atomic force microscopy (AFM), optical microscopy, electrochemical impedance spectroscopy (EIS) and measurement of corrosion potential (E_corr). XRD analysis disclosed that zinc hydroxide chloride was the main corrosion product on Zn–Mn coatings immersed in 0.5 mol dm⁻³ NaCl solution. EIS investigations revealed that less porous protective layer was produced on the alloy coating deposited at c.d. of 30 mA cm⁻² as compared to that deposited at 80 mA cm⁻².     

SiC_p/2024Al复合材料阳极氧化膜耐蚀性的电化学阻抗研究

采用电化学阻抗技术研究了碳化硅颗粒增强 ２０２４铝基复合材料（ＳｉＣｐ／２０２４Ａｌ）硫酸阳极氧化膜在 ３．５％ＮａＣｌ水 溶液中的耐蚀性;作为比较,对 ２０２４ Ａｌ的阳极氧化膜耐蚀性也进行了研究 结果表明,ＳｉＣｐ／２０２４Ａｌ复合材料的阳极氧化膜 具有良好的耐 ＮａＣｌ溶液腐蚀的能力,而且重铬酸盐封闭比热水封闭的阳极氧化膜耐蚀性更好．由于氧化膜中出ＳｉＣ颗粒的存在破 坏了氧化膜的完整性和均匀性,故复合材料阳极氧化膜的耐蚀性不如 ２０２４ Ａｌ合金．     

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.     

Effect of heat treatment on pitting corrosion resistance of 6061 Al/SiC<Subscript>P</Subscript> composite coated by the cerium oxide film in 3.5 N NaCl solution

One of the main drawbacks of 6061 Al/SiC_P composite is its poor pitting corrosion resistance in the aggressive environment containing chloride ions, such as seawater, for example. The present article deals with the investigations of effects of aging on the corrosion behavior of 6061 Al/SiC_P composite and of the heat treatment on the pitting corrosion resistance of 6061 Al/SiC_P composite coated by cerium oxide prepared by chemical bath technique. Potentiodynamic polarization test was used to study the corrosion behavior of cerium oxide coatings in 3.5N NaCl solution. The microstructure of cerium oxide was examined by scanning electron microscopy (SEM) and the formed phases were identified by X-ray diffraction (XRD). The pitting corrosion resistance of the cerium oxide coating was found to be improved after heat treatment at 300°C for 30 min.