Production and characterization of the anodic film on Al-6%Zn-1%Mg alloy

The newly developed high strength aluminum alloy Al-6wt.%Zn- 1wt.%Mg was anodized in acids by direct current to produce a protective anodic coating against abrasion and corrosion. Mechanical properties of the anodized finish, including microhardness, abrasion resistance and fatigue strength were evaluated in terms of the composition and temperature of the electrolyte, applied voltage, current density, duration of anodizing and sealing temperature. Pitting is destructive of the anodized finish particularly in a marine environment and thus can also serve as a direct measure of the efficacy of sealing procedures.The optimum conditions for anodizing Al-6wt.%Zn-1wt.%Mg are a bath of 5 wt.% oxalic acid with 3 wt.% formic acid as an additive, a current density of 4 A dm^-2 for 30 min at 20 °C and sealing in boiling deionized water for 30 min. Results show that this anodized finish possesses good mechanical properties and corrosion resistance.     

Effect of equal-channel angular pressing on pitting corrosion resistance of anodized aluminum-copper alloy

The effect of equal-channel angular pressing(ECAP) on the pitting corrosion resistance of anodized Al-Cu alloy was investigated by electrochemical techniques in a solution containing 0.2 mol/L AlCl₃ and also by surface analysis. Anodizing was conducted for 20 min at 200 and 400 A/m² in a solution containing 1.53 mol/L H₂SO₄ and 0.018 5 mol/L Al₂(SO₄)₃·16H₂O at 20 °C. Anodized Al-Cu alloy was immediately dipped in boiling water for 20 min to seal the micro pores present in anodic oxide films. The time required before initiating pitting corrosion of anodized Al-Cu alloy is longer with ECAP than without, indicating that ECAP process improves the pitting corrosion resistance of anodized Al-Cu alloy. Second phase precipitates such as Si, Al-Cu-Mg and Al-Cu-Si-Fe-Mn intermetallic compounds are present in Al-Cu alloy and the size of these precipitates is greatly decreased by application of ECAP. Al-Cu-Mg intermetallic compounds are dissolved during anodization, whereas the precipitates composed of Si and Al-Cu-Si-Fe-Mn remain in anodic oxide films due to their more noble corrosion potential than Al. FE-SEM and EPMA observation reveal that the pitting corrosion of anodized Al-Cu alloy occurs preferentially around Al-Cu-Si-Fe-Mn intermetallic compounds, since the anodic oxide films are absent at the boundary between the normal oxide films and these impurity precipitates. The improvement of pitting corrosion resistance of anodized Al-Cu alloy processed by ECAP appears to be attributed to a decrease in the size of precipitates, which act as origins of pitting corrosion.     

A comparative study of corrosion performance of sealed anodized layers of conventionally colored and interference-colored aluminium

The nanostructure of aluminium anodized layer was modified in phosphoric acid to induce light-interference colors through electrocoloring using nickel pigments. After a mixed sealing procedure, the corrosion performance of the interference-colored anodized layer was evaluated using Tafel polarization and electrochemical impedance spectroscopy. The results were compared with those obtained for conventionally colored anodized layer. The modification made the mixed sealing of subsequent colored layer less effective. The steam-sealing was found to be more effective for sealing the interference-colored anodized layer. For effectively sealed layers, the porous layer plays an additional role besides the barrier layer in determining the corrosion performance.     

Vanadia-based coatings of self-repairing functionality for advanced magnesium Elektron ZE41 Mg–Zn–rare earth alloy

A smart vanadia protective coating of self-repairing functionality that has proven to provide superior corrosion resistance for several magnesium and aluminum alloys has successfully been designed by our group. A newly developed series of magnesium alloys, namely ZE41 alloy, has recently been proposed for automotive, electronics and aerospace applications. The advanced ZE41 alloy possesses very low density, high specific strength, and good castability and weldability characteristics compared to aluminum and steel based alloys. However, the corrosion resistance of ZE41 alloy in the presence of corrosive chloride environment is relatively low. The possibility of utilizing such coatings to add self-repairing functionalities to ZE41 alloy was discussed in this paper. The electrochemical corrosion behavior of the vanadia coatings over ZE41 alloy was investigated in 3.5% NaCl solution using EIS, linear polarization and cyclic voltammetry techniques. The optimum conditions for obtaining protective vanadia coatings of self-repairing abilities and improved localized corrosion resistance were determined. Surface examination of the coatings was investigated using SEM-EDS and macroscopic imaging.     

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.     

几种封闭方法处理后铝阳极氧化膜的耐蚀性比较

通过极化曲线、开路电位监测、交流阻抗技术等电化学方法研究了工业纯铝L3阳极氧化膜试样在1mol/LNaCl溶液中的电化学行为,比较了经过沸水封闭、重铬酸钾封闭和镍盐冷封闭处理后的氧化膜试样的耐蚀性。结果表明:封闭处理改善了氧化膜试样在氯化钠溶液中的耐蚀性,耐蚀性从弱到强的顺序是:未封闭<沸水封闭<重铬酸钾封闭相似文献   

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．     

Characterization of anodized and sealed aluminium by EIS

Anodized and sealed aluminium samples exposed to different atmospheres for up to three years have been studied by means of electrochemical impedance spectroscopy (EIS). EIS was used to obtain detailed information concerning the electrochemical properties of the porous and barrier layer of anodized aluminium. An equivalent circuit that reproduces the a.c. impedance results of porous aluminium oxide films is proposed. The results reveal that the EIS technique is a good tool for obtaining detailed information on the influence of autosealing and the ageing process on anodized aluminium. This research shows that porous layer sealing quality increases over months and years (especially in the first 24 months) as the ageing proceeds when exposed to the natural atmospheric conditions. The analysis was completed with the aid of the SEM technique.     

Sol-gel silica coatings on ZE41 magnesium alloy for corrosion protection

Silica coatings have been applied on the surface of ZE41 magnesium alloy following the organic sol-gel route and the dip-coating technique. Three different concentrations of sol solution and two densification temperatures of the coating (400 °C and 500 °C) were used to optimize the compaction of the coatings and as a result reach the corrosion protection of the metallic substrate tests in 3.5 wt.% NaCl aqueous solution. Crack-free coatings with thickness in the 2-3 μm were obtained on the ZE41 magnesium alloy. The combination of high alkoxide concentration in the sol-gel formulation, and the high sintering temperature (500 °C) leads to coating (D500) with the optimal physical barrier against the corrosion process. This coating was capable of resisting more than 7 days in contact with the aggressive electrolyte suffering minor corrosion degradation. A corrosion mechanism for each of the tested specimens has been proposed.     

硅烷处理对 EW75 M 稀土镁合金阴极电泳涂层性能的影响

目的探究硅烷处理对阴极电泳涂层的耐腐蚀性能及其与基体间结合力的影响。方法对稀土镁合金表面进行硅烷前处理,再沉积阴极电泳涂层,评价涂层的耐腐蚀性能、抗溶胀性能,分析涂层的腐蚀微观形貌、组织结构及界面结合。结果硅烷膜层具有一定的防护性能,能够减少阴极电泳涂层针孔、橘皮的出现,增强阴极电泳涂层的致密性。硅烷改性能够提高阴极电泳涂层与基体的结合力,硅烷预处理的电泳试样在NMP溶液中浸泡102 h,依然结合完好;而未经硅烷预处理的电泳试样浸泡7 min后,涂层就完全剥落。此外,硅烷处理能够极大地改善阴极电泳涂层的阻抗性能,使涂层在3.5%(质量分数)NaCl溶液中浸泡227 h的极化电阻Rp仍在108数量级以上。结论硅烷阴极电泳复合涂层具有良好的耐蚀性能和抗溶胀性能,值得推广应用。     

Evaluation of the corrosion resistance of anodized aluminum 6061 using electrochemical impedance spectroscopy (EIS)

The corrosion resistance of anodized Al 6061 produced by two different anodizing and sealing processes was evaluated using electrochemical impedance spectroscopy (EIS). The scanning electron microscope (SEM) was employed to determine the surface structure and the thickness of the anodized layers. The EIS data revealed that there was very little change of the properties of the anodized layers for samples that were hard anodized in a mixed acid solution and sealed in hot water over a 365 day exposure period in a 3.5 wt% NaCl solution. The specific admittance A_s and the breakpoint frequency f_b remained constant with exposure time confirming that the hard anodizing process used in this study was very effective in providing excellent corrosion resistance of anodized Al 6061 over extended exposure periods. Some minor degradation of the protective properties of the anodized layers was observed for samples that were hard anodized in H₂SO₄ and exposed to the NaCl solution for 14 days.     

Understanding the corrosion behaviour of the magnesium alloys EV31A,WE43B,and ZE41A

The corrosion rates of the Mg alloys immersed in 3.5 wt% NaCl solution saturated with Mg(OH)₂ were in the following increasing order: WE43B (0.23 mm/y) < EV31A (0.88 mm/y) < pure‐Mg (1.6 mm/y) < ZE41A (8.5 mm/y). The average corrosion rate for WE43B was somewhat lower than the intrinsic corrosion rate of Mg as shown by high‐purity Mg, attributed to (a) no corrosion acceleration by the small second phase particles, and (b) a more protective surface film. The high corrosion rates of ZE41A were attributed to the presence of a coarse semicontinuous T‐phase, which served as strong cathodic sites.     

常温镍盐封闭处理对铝阳极氧化膜性能的影响

为进一步提高铝合金阳极氧化膜的性能,采用以镍盐为主的常温封闭剂对其进行封闭处理。与沸水封闭相比较,分别采用滴碱实验和落砂实验研究了氧化膜封闭前后的耐蚀性能和耐磨性能,结果表明：镍盐封闭处理具有能耗低、污染小、封闭快速等优点,而且耐蚀性和耐磨性明显由于传统的沸水封闭处理,是一种较为理想的封闭处理工艺。     

Enhancement of resistance to galvanic corrosion of ZE41 Mg alloy by equal channel angular pressing

The galvanic corrosion behavior of as-received and ECAPed ZE41 Mg alloy coupled with Al7075 alloy is investigated using zero resistance ammeter in three different corrosive environments, 0, 0.1, and 1 M NaCl, to mimic the conditions experienced in engineering applications. The mechanism of galvanic corrosion for the ZE41 Mg–Al7075 aluminum alloy is explained. It is observed that a robust surface film containing a composite layer of oxide/hydroxide of magnesium and aluminum is established in deionized water (0 M). However, only a single layer of magnesium oxide/hydroxide is detected in chloride-containing environments. Equal channel angular pressing (ECAP) improved the resistance to galvanic corrosion by 58% and 54% when compared with the as-cast counterparts in 0 and 1 M NaCl solution, respectively. In contrast, galvanic corrosion resistance decreased by 26% in 0.1 M NaCl after ECAP while the as-received samples evinced pits unfavorable to be used in engineering applications. ECAP is a promising method to combat galvanic corrosion encountered by ZE41 magnesium alloy used in automobiles and components of military vehicles.     

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.     

铝合金型材阳极氧化膜的滴碱与落砂试验

通过试验，研究了合金成分、常温封孔、阳极氧化工艺条件对氧化膜耐蚀性，耐磨性的影响；讨论了这些因素对氧化膜耐蚀性、耐磨性的影响机理，指出了为保证氧化膜的耐蚀性、耐磨性而采取的工艺措施。     

Improvement in corrosion resistance of commercial pure titanium for the enhancement of its biocompatibility

A new surface modification protocol encompassing an anodization treatment has been developed to improve the surface properties of biomedical titanium. Anodization in presence of nickel sulfate is one of a good way to improve the resistance of commercial pure titanium (cp‐Ti) in phosphate saline buffer solution (PSB). The potentiostatic behavior of anodized titanium in sulfuric acid with nickel sulfate differed markedly from that of titanium anodized in sulfuric acid free from nickel sulfate. Polarization and electrochemical impedance spectroscopy (EIS) recorded an increase in the corrosion resistance of the passive film. Scanning electron microscopy (SEM) and energy diffraction X‐ray (EDX) analysis were used to investigate the morphology and structure of the anodized film in absence and in presence of nickel sulfate. On the other hand, sealed anodized titanium samples exposed to PSB for up to 3 days have been studied by EIS to obtain detailed information concerning the electrochemical properties of sealed anodized titanium. An equivalent circuit that reproduces the impedance results of porous cp‐Ti oxide (TiO₂) films is proposed. These observations indicate that anodization of cp‐Ti in presence of nickel sulfate and sealing the anodized film can serve as a simple low‐temperature method to enhance the corrosion resistance of cp‐Ti when used as an implant material.     

Electrochemical investigation of the influence of laser surface melting on the microstructure and corrosion behaviour of ZE41 magnesium alloy – An EIS based study

Surface melting of a magnesium alloy, ZE41 (4%-Zn, 1%-RE) was performed to achieve electrochemical homogeneity at the surface by microstructure refinement. Large secondary precipitates are particularly known to cause severe pitting in magnesium alloys. The corrosion resistance of the laser treated and untreated alloy was investigated by potentiodynamic polarisation and electrochemical impedance spectroscopy. Contrary to the reported behaviour of other magnesium alloys (such as AZ series alloys), laser surface melting did not significantly improve the corrosion resistance of ZE41. This observation is attributed to the absence of beneficial alloying elements such as Al in ZE41 alloy.     

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.     

Effects of titanium carbide (TiC) and anodizing voltages on discoloration resistance of colored-titanium sheets

Discoloration resistance of anodized titanium sheets was investigated by accelerated discoloration tests and atmospheric exposure tests. Anodized oxide layers were analyzed by various surface analytical methods. TiC beneath the titanium oxide layer accelerated discoloration in cases where thinner oxide layers existed. Increase of their thickness improved resistance even though TiC existed. On the other hand, in case of reducing TiC, the specimen with the thinnest oxide layer showed the most resistant to discoloration. The top-surface region of the oxide layer is considered to play an important role in discoloration resistance.