Comparison of susceptibility to pitting corrosion of AA2024-T4, AA7075-T651 and AA7475-T761 aluminium alloys in neutral chloride solutions using electrochemical noise analysis

The susceptibility to pitting corrosion of AA2024-T4, AA7075-T651 and AA7475-T761 aluminium alloys was investigated in aqueous neutral chloride solution for the purpose of comparison using electrochemical noise measurement. The experimentally measured electrochemical noises were analysed based upon the combined stochastic theory and shot-noise theory using the Weibull distribution function. From the occurrence of two linear regions on one Weibull probability plot, it was suggested that there existed two stochastic processes of uniform corrosion and pitting corrosion; pitting corrosion was distinguished from uniform corrosion in terms of the frequency of events in the stochastic analysis. Accordingly, the present analysis method allowed us to investigate pitting corrosion independently. The susceptibility to pitting corrosion was appropriately evaluated by determining pit embryo formation rate in the stochastic analysis. The susceptibility was decreased in the following order: AA2024-T4 (the naturally aged condition), AA7475-T761 (the overaged condition) and AA7075-T651 (the near-peak-aged condition).     

A room temperature cured sol–gel anticorrosion pre-treatment for Al 2024-T3 alloys

The inherent reactivity of the Al–Cu alloys is such that their use for structural, marine, and aerospace components and structures would not be possible without prior application of a corrosion protection system. Historically these corrosion protection systems have been based upon the use of chemicals containing Cr(VI) compounds. Organic–inorganic hybrid silane coatings are of increasing interest in industry due to their potential application for the replacement of current toxic hexavalent chromate based treatments. In the present study, a hybrid epoxy–silica–alumina coating with or without doped cerium nitrate has been prepared using a sol–gel method. The hybrid coatings were applied by a dip-technique to an Al–Cu alloy, Al 2024-T3, and subsequently cured at room temperature. The anticorrosion properties of the coatings within 3.5% NaCl were studied using electrochemical impedance spectroscopy (EIS), and conventional DC polarisation. An exfoliation test method involving immersion in a solution of 4 M NaCl, 0.5 M KNO₃ and 0.1 M HNO₃ was also used. The cerium nitrate doped sol–gel coating exhibited excellent anticorrosion properties providing an adherent protection film on the Al 2024-T3 substrate. The resistance to corrosion of the sol–gel coating was also evaluated by analysing the morphology of the coating before and after corrosion testing using scanning electron microscopy.     

Improving the corrosion protection of aluminium alloys using reactive magnetron sputtering

Most high strength aluminium alloys used in the aircraft industries are susceptible to corrosion. Up to now hexavalent chromium is the conventional corrosion inhibitor. Because chromium in hexavalent state is carcinogenic, it is necessary to develop effective alternative inhibitor systems. We investigated magnetron sputtered substoichiometric and stoichiometric aluminium nitride (AlN_x with x ? 1) coatings for corrosion protection of aluminium alloys 2024-T3, 6061-T4 and 7075-T6. The corrosion behaviour of the treated surfaces was tested by anodic polarization scanning and salt spray testing.From the polarization curves it can be concluded that magnetron sputter coating with AlN_x leads to a higher pitting potential of the three aluminium alloys investigated. The salt spray tested samples also confirm the protective effect of the coatings. In addition we found that AlN_x layers with high nitrogen content lead to a stronger shifting in pitting potential than those with low nitrogen content. Anyway, the results of the salt spray testing show that particularly nitrogen-rich AlN_x layers are less stable towards NaCl electrolyte.     

Corrosion Protection of AA2024-T3 by Cerium Malate and Cerium Malate-Doped Sol-Gel Coatings

Cerium malate (CeMal) was tested as a corrosion inhibitor for AA2024-T3 in this work. Corrosion inhibition on bare AA2024-T3 indicated that the inhibiting effect was a result of the synergistic effect of cerium cations and maleic anions. The corrosion of AA2024-T3 was stagnated by greatly reducing the corrosion current when CeMal was present in NaCl solutions. CeMal was adsorbed on the surface of AA2024-T3 forming a protective film in the initial stage. Then, cerium cations transformed to cerium oxide/hydroxides, precipitating on the cathode sites to inhibit the further corrosion. The electrochemical impedance spectra results of the sol-gel coatings proved that CeMal was an effective corrosion inhibitor in the sol-gel coatings to provide corrosion protection for AA2024-T3.     

Na2 SnO3对铝合金腐蚀的缓蚀作用

目的提高2024-T3铝合金在中性Na Cl溶液中的耐小孔腐蚀性能。方法采用动电位极化曲线测试、扫描电镜(SEM)观察并结合X射线光电子能谱(XPS)等方法,研究2024-T3铝合金在含不同浓度Na2Sn O3的0.1 mol/L Na Cl溶液中的电化学腐蚀行为,分析Na2Sn O3及其浓度对2024-T3铝合金小孔腐蚀和均匀腐蚀的作用。结果电化学测试结果显示,添加一定量(0.05~0.4 g/L)的Na2Sn O3可以使溶液的p H值升高(可从6.6上升至10.1),促进铝合金表面发生钝化,使铝合金孔蚀电位Eb和自腐蚀电位Ecorr的差值增大(最大可达到600 m V),因此降低了铝合金的孔蚀敏感性,提高了其耐小孔腐蚀的能力。但是Na2Sn O3质量浓度较大(0.2、0.4 g/L)时,会促进2024-T3铝合金的均匀腐蚀。SEM和XPS结果显示,小孔及其附近区域Cu含量较多,并有大量的Sn O2颗粒沉积。结论少量(0.05、0.1 g/L)的Na2Sn O3对2024-T3铝合金的小孔腐蚀和均匀腐蚀均具有较好的抑制效果。Na2Sn O3对2024-T3铝合金的缓蚀作用可能源于其水解产生的Sn O2优先在铝合金表面的金属间颗粒(S相)周围发生沉淀,从而屏蔽了铝合金表面的活性点。     

Investigation of pretreatment effects on the formation of lanthanum based conversion coating on AA2024-T3

The effects of a pretreatment process on the formation and properties of lanthanum based conversion coatings on AA2024-T3 was investigated using optical and scanning electron microscopy (SEM), X-ray diffraction (XRD) and open circuit potential (OCP) measurements. The results indicate that the conventional alkaline activation pretreatment with acidic desmutting could not create a suitable condition to produce lanthanum based conversion coating on AA2024-T3. Therefore, in this study, a new approach to produce lanthanum based conversion coating on AA2024-T3 by acidic pretreatment is proposed. The pretreatment of the alloy in acidic and chloride environments (90 s at 30 vol % HCl) creates more cathodic sites on AA2024-T3 compared to alkaline activation with acidic desmutting. Additionally, it was found that the formation stages of lanthanum based conversion coating is similar to those of cerium based conversion coating on AA2024-T3 except lanthanum ions require more localized pH increase to deposit on the alloy.     

Rapidly solidified Al-Ti alloys via advanced melt spinning

Rapidly solidified Al-Ti based binary and ternary alloys containing 3 to 12 wt.% titanium and additions of cerium or vanadium have been produced by melt spinning continuous ribbons, pulverization into powders and consolidation by hot-extrusion into round bars. The mechanical property data show that significantly improved elevated-temperature strengths can be obtained by suitable alloy design and processing. The rapidly solidified Al-Ti base alloys have improved general corrosion and pitting corrosion resistance in comparison to ingot metallurgy Al 7075-T73 alloys and higher resistance to pitting corrosion than rapidly solidified Al-8Fe-2Mo.     

Corrosion protection of aluminium alloy 2024-T3 in 0.05 M NaCl by cerium cinnamate

The corrosion protection of AA 2024-T3 in 0.05 M NaCl by cerium cinnamate has been studied. Polarization measurements demonstrate that cerium cinnamate is markedly effective for suppressing anodic process of the alloy corrosion during the initial 72 h of immersion. The protection mechanism during the immersion period appears to involve two stages: the deposition of cerium cinnamate, and then hydrolysis of cerium ions forming a cerium oxide/hydroxide, prevailing over the foregoing deposition. This study also elucidates the inhibition effect of cerium cinnamate on the early corrosion attack occurred from the second phase particles by X-ray photoelectron spectroscopy and electron-probe microanalysis.     

Effect of gelatin additions on the corrosion resistance of cerium based conversion coatings spray deposited on Al 2024-T3

The corrosion resistance of cerium based conversion coatings on Al 2024-T3 was improved by the addition of a water soluble gelatin to the coating solution. Auger electron spectroscopy depth profiling showed that coatings deposited from solutions containing 800-3200 ppm gelatin were ~ 400 nm thick, while coatings deposited from solutions with 0-200 ppm gelatin were ~ 850 nm thick. The thinner coatings exhibited reduced surface cracking and spalling. Open circuit potential measurements during deposition showed that adding gelatin to the coating solution resulted in a more negative and stable potential with increasing gelatin concentrations. Visually, increasing gelatin concentrations promoted the formation of stable bubbles that covered panel surfaces, which limited transport of cerium species to the surface and decreased the deposition rate. X-ray diffraction analysis revealed that only coatings deposited from solutions containing 400-3200 ppm gelatin could be converted to CePO₄H₂O during post-treatment, potentially improving the corrosion resistance compared to coatings deposited from solutions without gelatin.     

The effect of nitrate on the corrosion behavior of 7075-T651 aluminum alloy in the acidic NaCl solution

The effect of nitrate on the corrosion behavior of 7075-T651 aluminum alloy in an acidic NaCl solution is investigated by electrochemical investigation and morphology characterization. Localized corrosion initiated from intermetallic particles could be observed in the solution with and without NaNO₃. The nitrate plays a controversial role in the corrosion of 7075-T651 aluminum alloy. It could enhance the performance of passive film and reduce the probability of pitting corrosion initiation. However, the pitting corrosion would be promoted by nitrate, once stable pitting corrosion is initiated.     

Influence of cerium molybdate containers on the corrosion performance of epoxy coated aluminium alloys 2024-T3

Cerium molybdate containers loaded with 2-mercaptobenzothiazole were incorporated into epoxy coatings onto aluminium alloys 2024-T3 and investigated with respect to the corrosion protection of the metallic surfaces. The coatings were deposited via the dip-coating process. The morphology of the coatings was examined by Scanning Electron Microscopy. Their composition and structure were investigated by Fourier Transform Infrared Spectroscopy and Energy Dispersive X-ray Analysis. The corrosion resistance of these coatings was investigated by using electrochemical impedance spectroscopy and open circuit potential. After exposure to 0.05 M NaCl solution for 28 days, the coatings with the loaded containers exhibit improved corrosion performance.     

Effect of porosity sealing treatments on the corrosion resistance of high-velocity oxy-fuel (HVOF)-sprayed Fe-based amorphous metallic coatings

High-velocity oxy-fuel-sprayed FeCrMoMnWBCSi amorphous metallic coatings were sealed with sodium orthosilicate (Na₃SiO₄), aluminium phosphate (AlPO₄), and cerium salt sealants. The microstructure of the sealed coatings was characterised by scanning electron microscopy, energy dispersive spectrometer, and X-ray diffraction. Corrosion behaviour was examined using electrochemical methods of potentiodynamic polarisation, cyclic polarisation, electrochemical impedance spectroscopy, and Mott-Schottky tests. The results indicated that the uniform corrosion resistance of the three sealed coatings was enhanced greatly, and the passive current densities were decreased by one order of magnitude after the sealing treatments. The AlPO₄ sealant can penetrate the coatings by no less than 50 μm and enhance their hardness, which exhibited a more uniform corrosion resistance, fairly good pitting corrosion resistance, and can be applied in long-term corrosive and/or abrasive environments. The cerium salt-sealed coating showed better pitting corrosion resistance but inferior corrosion resistance in the local regions of micro-cracks, which was practically used for temporary corrosion protection. The Na₃SiO₄-sealed coating showed better uniform corrosion resistance and inferior pitting corrosion resistance, which can be applied in short-term corrosion environments. The stability of the passive film affected the corrosion behaviour of the sealed coatings. The AlPO₄-sealed coating performed better as a protective passive film during the long-term immersion test for a lower defect concentration and a more protective passive film.     

Microstructure and corrosion resistance of molybdenum and aluminum coatings thermally sprayed on 7075-T6 aluminum alloy

Pitting corrosion upon 7075-T6 high strength aluminum alloy, often associated with cathodic intermetallic particles decreases its fatigue life by a factor of about 6 to 8. In order to improve the corrosion resistance of this alloy, arc spray coatings of molybdenum and aluminum are applied. The open circuit potential and potentiodynamic polarization measurements made in 3.5% NaCl naturally aerated solution reveal that the molybdenum coating, which has an excellent hardness, shifts the 7075-T6 corrosion potential (E _corr) to noble values and increases slightly the corrosion current density (i _corr). On the contrary, when the aluminum coating alloy is applied, both E _corr and i _corr are shifted to better values. The increase of i _corr of the alloy when molybdenum coating is applied can be attributed to the high porosity present into the coating. On the other hand, microstructure observations of the aluminum coating reveal a small porosity, which helps the formation of passive oxide film that protects the coating against a further corrosion.     

Formation of subsurface crevices in aluminum alloy 2024-T3 during deposition of cerium-based conversion coatings

The processing variables that contributed to the formation of subsurface crevices under cerium-based conversion coatings on AA 2024-T3 were investigated. Focused ion beam milling revealed the presence of subsurface crevices underneath a small fraction (∼ 10%) of coated areas, typically in areas with large cracks through the coatings. A solution of sodium chloride and H₂O₂ etched AA 2024-T3 and produced features similar to subsurface crevices, which confirmed that crevices formed during deposition due to the composition of the coating solution. Using sodium nitrate in place of sodium chloride resulted in no etching of the substrate. Thus, coatings free of subsurface crevices could be produced by using cerium nitrate instead of cerium chloride in the coating solution. Electrodeposited coatings, even those deposited from solutions containing chloride ions and H₂O₂, were also free of subsurface crevices. As a result, subsurface crevices are not inherent to cerium-based conversion coatings, but rather were formed due to certain process parameters, specifically the presence of chloride ions and hydrogen peroxide in the coating solution.     

Al-Cu-Li合金在盐溶液中的腐蚀行为以及腐蚀机理研究进展

Al-Cu-Li合金是航天航空工业中重要的轻质结构材料，已成为国产大飞机结构件的关键材料之一。飞行器在海洋等潮湿环境服役时，易受到具有腐蚀性的卤化物阴离子的侵蚀，尤其是在Cl^-离子侵蚀作用下，Al-Cu-Li合金构件表面易出现点蚀、晶间腐蚀和剥落腐蚀现象。Al-Cu-Li合金的局部腐蚀主要归因于合金相与合金基体间存在电势差，进而导致在腐蚀介质中形成微型腐蚀原电池。综述了Al-Cu-Li合金在NaCl溶液中的腐蚀行为以及热处理工艺对合金耐腐蚀性能的影响，重点分析了粗大第二相颗粒和时效析出相对Al-Cu-Li合金腐蚀性能的影响，研究了典型第3代Al-Cu-Li（2A97-T3、2A97-T6、2060-T8和2099-T83）合金以及航空用常规高强铝合金2024-T4在3种不同浓度NaCl溶液中的侵蚀行为以及在质量分数3.5%NaCl溶液中的电化学行为。综合分析各试样的微观腐蚀形貌、腐蚀电化学参数以及腐蚀程度，最终得出各试样的耐腐蚀性能由强至弱为：2A97-T3>2A97-T6>2024-T4>2060-T8>2099-T83。最后揭示了Al-Cu-Li合金在腐蚀介质中的腐蚀机理，总结了在海洋环境下铝合金的防腐措施。本文为后续Al-Cu-Li合金防腐性能的发展和飞机耐腐蚀性能的提升提供了参考。     

Corrosion electrochemical behaviors of silane coating coated magnesium alloy in NaCl solution containing cerium nitrate

Sol–gel coatings cannot provide adequate corrosion protection for metal/alloys in the corrosive environments due to their high crack‐forming potential. This paper demonstrates the possibility to employ cerium nitrate as inhibitor to decrease the corrosion development of sol–gel‐based silane coating on the magnesium alloy in NaCl solution. Cerium nitrate was added into the NaCl solution where the silane coating coated magnesium alloy was immersed. Scanning electron microcopy (SEM) was used to examine surface morphology of the silane coating coated magnesium alloy immersed in NaCl solutions doped and undoped with cerium nitrate. The corrosion electrochemical behaviors were investigated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests. The results showed that the introduction of cerium nitrate into NaCl solution could effectively inhibit the corrosion of the silane coating coated magnesium alloy. Moreover, the influence of concentration of cerium nitrate on the corrosion inhibition and the possible inhibiting mechanism were also discussed in detail.     

Comparison of the corrosion resistance of an Al–Cu alloy and an Al–Cu–Li alloy

ABSTRACT

In this study, the corrosion mechanisms of the AA2024-T3 and the AA2098-T351 were investigated and compared using various electrochemical techniques in 0.005?mol?L^?1 NaCl solution. The severe type of corrosion in the AA2098-T351 was intragranular attack (IGA) although trenching and pitting related to the constituent particles were seen. On the other hand, the AA2024-T3 exhibited severe localised corrosion associated with micrometric constituent particles, and its propagation was via grain boundaries leading to intergranular corrosion (IGC). Electrochemical techniques showed that the corrosion reaction in both alloys was controlled by diffusion. The non-uniform current distribution in both alloys showed that EIS was not a proper technique for comparing the corrosion resistance of the alloys. However, local electrochemical techniques were useful for the evaluation of the corrosion resistance of the alloys.     

Prior corrosion and fatigue of 2024-T3 aluminum alloy

Pit-to-crack transition experiments were conducted on two thicknesses of 2024-T3 aluminum alloy. Specimens were corroded using a 15:1 ratio of 3.5% sodium chloride solution and hydrogen peroxide prior to fatigue loading. Cracks originating from corrosion pits were visually investigated using various microscopy techniques in order to gain insight into the pit-to-crack transition process.All pre-corroded specimens in this study fractured from cracks associated with pitting. Pit-to-crack transition was successfully observed using digital video techniques. The more aggressively corroded 2024-T3-4.064 mm specimens experienced more of an overall fatigue life reduction than 2024-T3-1.600 mm specimens. Results indicated that quantities such as pit surface area and surrounding pit proximity are as important as pit depth in determining when and where a crack will form.     

Detecting in-situ active corrosion by a SQUID magnetometer

The magnetic field distributions above the surface of in-situ active corroding 2024-T3 and 7075-T6 aircraft aluminum alloy plates have been measured using a high-resolution superconducting quantum interference device (SQUID) magnetometer. The magnetic field distributions and their variation with time are clearly different for the two aluminum alloys in an identical solution and for 2024-T3 in two different solutions. It is believed that these results demonstrate theability of SQUID to noninvasively detect in-situ active corrosion in aircraft aluminum alloys in a way that present corrosion-detection methods do not allow.     

Electrochemical study of modified non-functional bis-silane layers on Al alloy 2024-T3

In the last few years great efforts have been made in order to find and to develop environmentally friendly substitutes for Cr⁶⁺ pre-treatments applied on aluminium alloys used in the aircraft industry. Among the potential substitutes, silane layers have attracted considerable interest from researchers and from the industry. The present work investigates the anti-corrosion behaviour of (bis-1, 2-(triethoxysilyl) ethane (BTSE)) silane layers modified with Ce ions and/or silica nanoparticles applied on Al alloy 2024-T3 substrates. The corrosion behaviour was investigated in 0.1 M NaCl solution via d.c. polarization and electrochemical impedance spectroscopy (EIS). Contact angle measurements and XPS were used to assess information on the chemistry of the silane pre-treated surfaces. The results have shown that the introduction of additives improves the corrosion protection properties of the silane layer.