Polypyrrole films on Al alloys: The role of structural changes on protection performance

Polypyrrole (PPy) films were electrochemically synthesized on AA6082, AA5083 and AA2024 Al alloys by a two-step galvanostatic procedure: (1) activation in nitric acid and (2) deposition in oxalic acid. PPy films were characterized by elemental analysis, cyclic voltammetry, SEM/EDX, RAIR, solid-state ¹³C NMR, and EPR. The corrosion behaviour of PPy-coated alloys was evaluated in near neutral NaCl solution by single-cycle anodic polarization and open circuit potential monitoring. Film adhesion and polymer structure are principally determined by the nature of the Al substrate. Good quality films are obtained on the most reactive AA5083 and AA2024, while highly degraded, partially saturated polymer is electrogenerated on AA6082. The performance of PPy films was observed to depend strongly on the electrochemical test used for corrosion protection evaluation. Accordingly, the rate at which structural changes are driven within the polymeric matrix is of paramount importance to understand the corrosion inhibition properties of conducting polymers.     

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.     

Influence of pre-treatments in cerium conversion treatment of AA2024-T3 and 7075-T6 alloys

The role of pre-treatment in the formation of a cerium conversion coating is investigated for the protection of AA2024-T3 and 7075-T6 alloys. The alloys were alkaline-etched and de-smutted in nitric acid, prior to cerium treatment in Ce(NO₃)₃ at 85 °C with H₂O₂ accelerator. Potentiodynamic polarization studies in 3.5% NaCl solution revealed a large shift of  300 mV of the corrosion potential below the pitting potential for the 7075-T6 alloy, which correlated with the development of a finely-textured, uniform coating. However, the formation of a uniform coating and protection was dependent upon the time of de-smutting, with non-uniform coatings resulting from extended times of de-smutting. In contrast, non-uniform coatings developed on the 2024-T3 alloy, with pitting potential at the corrosion potential, irrespective of the time of de-smutting. Findings for the 2024-T3 alloy indicate that extended de-smutting affects the enrichment of alloying elements.     

基于晶格储存能的航空铝合金腐蚀行为研究

采用多种电子显微技术对AA2024-T3航空铝合金的局部腐蚀行为进行研究。结果表明:AA2024-T3铝合金的局部腐蚀主要以晶间腐蚀的形式出现,相对于晶界沉淀相,晶格储存能更为显著地影响晶间腐蚀的发展,促使其优先发生在晶格储存能更高的晶粒周围。研究结果有助于提高对航空铝合金晶间腐蚀机理的认识,并为航空铝合金的设计提供理论指导,对保障航空安全具有重要意义。     

Variations in stir zone and thermomechanically affected zone of dissimilar friction stir weld of AA5083 and AA6082 alloys

Variations in composition, microhardness (in the thermomechanically affected zone) and texture in the tool domain of the dissimilar friction stir weld of AA5083-O and AA6082-T6 alloys were investigated. The contents of the major alloying elements in the weld zones were determined using inductively coupled plasma?atomic emission spectroscopy. It was observed that a slight drop in the content of the alloying elements results from the friction stir welding process with the Mg content being the most affected amongst the major alloying elements in the two alloys. By relating the mass fractions of the major alloying elements in the parent metals of both alloys to those of the stir zone, the relative proportions of the two alloys in the stir zone were estimated with the results showing that at least 60% of the materials in the stir zone are from the retreating side of the weld. It was also revealed that the changes in the hardness profile in the thermomechanically affected zone of the retreating side are predominantly influenced by changes in grain size in that domain. Finally, the investigation further revealed that the texture component in the tool shoulder domain is different from the texture component in the tool pin domain.     

Statistical model for intergranular corrosion growth kinetics

A statistical model has been developed to predict the anisotropy of intergranular corrosion (IGC) kinetics and effect of microstructure on IGC kinetics in high strength AA2024-T3 alloys. The methodology is to estimate the statistical distribution of minimum IGC path length that will be found through the thickness of a given thickness ST sample using the principle of order statistics. A brick-wall model is adopted for wrought Al alloy microstructure and different distributions of grain dimension are assumed for the purpose of the estimation of total IGC path length. Numerical stimulation using a gamma distribution model stimulated the anisotropy of localized corrosion in AA2024-T3 alloys, which quite agrees with the growth rate anisotropy experimentally determined by the foil penetration technique. Some limitations regarding the statistical model are discussed. The statistical model provides a new approach to predicting and quantifying localized corrosion kinetics on basis of the alloy microstructure.     

The Effect of Homogenization on the Corrosion Behavior of Al–Mg Alloy

The effect of homogenization on the corrosion behavior of 5083-O aluminum alloy is presented in this paper. The intergranular corrosion and exfoliation corrosion were used to characterize the discussed corrosion behavior of 5083-O aluminum alloy. The variations in the morphology, the kind and distribution of the precipitates, and the dislocation configurations in the samples after the homogenization were evaluated using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The effects of the highly active grain boundary character distribution and the types of constituent particles on the corrosion are discussed on the basis of experimental observations. The results indicated that the corrosion behavior of 5083-O alloy was closely related to the microstructure obtained by the heat treatment. Homogenization carried out after casting had the optimal effect on the overall corrosion resistance of the material. Nevertheless, all samples could satisfy the requirements of corrosion resistance in marine applications.     

Corrosion protection of AA 2024-T3 by bis-[3-(triethoxysilyl)propyl]tetrasulfide in neutral sodium chloride solution. Part 1: corrosion of AA 2024-T3

This study consists of two parts. In the first part, the corrosion of 2024-T3 aluminum alloy (AA 2024-T3) was studied using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results showed that the anodic S phase (Al₂CuMg) particles dealloyed Al and Mg during the 3.5 h of immersion in a neutral 0.6 M sodium chloride (NaCl) solution; with the dealloying of Mg being the most severe. Simultaneously, a heavy dissolution was also observed for the surrounding Al matrix of the S phase particles. This Al dissolution is likely to be caused by a local alkalization resulting from the coupled cathodic reaction (water and/or oxygen reduction). Such corrosion in AA 2024-T3, however, can be inhibited efficiently after the treatment of bis-[3-(triethoxysilyl)propyl]tetrasulfide (bis-sulfur silane). The associated studies on bis-sulfur silane treated AA 2024-T3 will be presented in the second part.     

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。     

Study of localized corrosion in AA2024 aluminium alloy using electron tomography

SEM based tomography of localized corrosion has been achieved using selective detection of backscattered electrons. The high resolution tomography provides direct evidence that links the surface appearance of corroded alloy, the alloy microstructure and the corrosion propagation path. Stable localized corrosion of AA2024-T351 aluminium alloy was initiated at locations where large clusters of S phase particles were buried beneath the surface. Propagating away from the initiation sites, corrosion developed preferentially along the grain boundary network. The grain boundary attack started beneath the alloy surface, proceeded along preferred grain boundaries and may emerge at the alloy surface.     

Semi-solid near-net shape rheocasting of heat treatable wrought aluminum alloys

Flexibility of the CSIR-RCS, induction stirring with simultaneous air cooling process, in combination with high pressure die casting is successfully demonstrated by semi-solid rheocasting of plates performed on commercial 2024, 6082 and 7075 wrought aluminum alloys. Tensile properties were measured for the above mentioned rheocast wrought aluminum alloys in the T6 condition. The results showed that tensile properties were close to or even in some cases exceeded the minimum specifications. The yield strength and elongation of rheocast 2024-T6 exceeded the minimum requirements of the wrought alloy in the T6 condition but the ultimate tensile strength achieved only 90% of the specification because the Mg content of the starting alloy was below the commercial alloy specification. The strengths of rheocast 6082-T6 exceeded all of the wrought alloy T6 strength targets but the elongation only managed 36% of the required minimum due to porosity, caused by incipient melting during solution heat treatment, and the presence of fine intermetallic needles in the eutectic. The yield strength of rheocast 7075 exceeded the required one and the ultimate tensile strength also managed 97% of the specification; while the elongation only reached 46% of the minimum requirement also due to incipient melting porosity caused during the solution heat treatment process.     

The effect of inhibitor structure on the corrosion of AA2024 and AA7075

A range of structurally-related compounds were tested for their capacity to inhibit corrosion on aluminium alloys AA2024-T3 and AA7075-T6 in 0.1 M NaCl solution. It was found that the thiol group, positions para- and ortho- to a carboxylate, and substitution of N for C in certain positions strongly inhibited corrosion. The hydroxyl group was slightly inhibitive, while the carboxylate group provided little or no corrosion inhibition on its own. In several cases, different activities were found on the different alloys, with some compounds (particularly thiol-containing compounds) being more effective on AA2024 than on AA7075.     

Effect of tool geometry on friction stir processing and fatigue strength of MIG T welds on Al alloys

Several tool geometries were developed and their effect on weld morphology, material flow, microstructure and hardness of processed regions was analyzed. Their effect on fatigue strength of welds was also examined for the most promising tools. The feasibility of FSP MIG T welds was proved. Quality of processed regions is very influenced by tool geometry. FSP removes defects in the MIG weld toe and increases its radius of curvature. Also promotes significant grain refinement in processed regions, reduces hardness in welds on AA 6082-T651 and hardens AA 5083-H111 welds. Only tools with concave and rounded edge shoulder and cylindrical threaded pin allow the improvement of fatigue strength of MIG welds on both alloys.     

Development and analysis of frits for enamelling AA2024, AA6082 and AA7075 aluminium alloys

This study reports the development of frits with different composition, time, and firing temperature for mitigating the corrosion of AA2024, AA6082, and AA7075 aluminium alloys in chloride-containing solutions. The melting points of the frits were determined using a heating microscope, while the thermal expansion was measured using dilatometry. Several frits were prepared, of which seven were coated on AA2024, AA6082 and AA7075 aluminium alloys. The corrosion resistance of the coated samples was tested in 5 wt% NaCl solution at 25°C using potentiodynamic curve measurements. The corrosion resistance of the coated samples increased compared with the respective uncoated aluminium alloys. The composition and in-depth homogeneity of the best-performing coated samples were confirmed using a gas cluster ion beam sputtering source associated with X-ray photoelectron spectroscopy analysis. Furthermore, O₂⁺ sputtering was performed in association with time-of-flight secondary ion mass spectrometry to describe the three-dimensional composition of the frits.     

Electrochemical study of the surface metal matrix composite developed on AA 2024-T351 by the friction stir process

ABSTRACT

This work is focused on the evaluation of the corrosion behaviour of an AA2024-T351/SiC surface composite produced by friction stir processing (FSP). The surfaces composite show a quite homogenous distribution of the SiC particles in the stir zone, together with a significant grain size reduction as Electron Backscatter Diffraction analysis confirms. The corrosion behaviour was evaluated in 3.5 wt-% NaCl solution using electrochemical techniques. The complete study shows that all processed samples have similar corrosion resistance at the stirred region and differ from the base material behaviour. The results indicate that the galvanic coupling metal matrix–SiC particles are less active than the galvanic coupling metal matrix–Cu-rich second phases. Although the initial corrosion resistance of the base aluminium alloy is the worst of all systems studied, for long immersion times the development of corrosion through grain boundaries was found to progressdeeper in the 2024Al/SiC surface produced by FSP.     

Co-operative corrosion phenomena

The corrosion of aluminium alloy 2024-T3 (AA2024-T3) was studied as a function of immersion time from 2.5 to 120 min in 0.1 M aqueous NaCl solution. At immersion times as short as 5 min, rings of corrosion product of 100 to 200 μm diameter, containing smaller domes of corrosion product, were observed using SEM. The domes of corrosion product had greater chloride concentrations than elsewhere on the surface and represented sites of anodic attack. As the immersion time was increased, significant grain boundary attack was observed within the rings of corrosion product. Analyses of Particle Induced X-ray Emission (PIXE) maps of the corroded surfaces showed a significantly higher number of IM particles around the chloride attack sites than the average particle density for the maps, indicating clustering of IM particles. These results suggest a co-operative corrosion effect as a result of clustering of the IM particles. A mechanism for the generation of the corrosion rings is discussed.     

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。     

Microstructure and mechanical properties of friction stir welded aluminium alloys with special reference to AA 5083 and AA 6082

Abstract

Aluminium alloys AA 5083 and AA 6082 have been friction stir welded and the mechanical properties and microstructures of the welds have been evaluated. Alloy AA 5083 mainly fractured near the centre of the weld, while fracture in AA 6082 mainly occurred in the heat affected zone. The tensile strength of welded joints in AA 6082 was lower than the base material strength, but still met classification societies' requirements. Hardness was approximately constant across the welded zone in AA 5083, while a minimum in hardness was found in the AA 6082 welds. The location of the fracture closely matched the minimum hardness region. Very fine scale precipitation in AA 6082 was significantly affected by the weld thermal cycle. In the zone of lowest hardness, the hardening precipitate (β″-Mg₅Si₆) had transformed to the non-hardening β′-Mg_1.7Si. This is probably the main reason for the minimum in hardness, the fracture location, and the decreased tensile strength. Results are compared to a similar investigation of aluminium alloy AA 7075.     

Base material location dependence of corrosion response in friction-stir-welded dissimilar 2024-to-5083 aluminum alloy joints

The effects of the base material (BM) location on the mechanical properties and the exfoliation corrosion performance of friction-stir-welded (FSWed) dissimilar 2024-to-5083 aluminum alloy joints were investigated. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), tensile tests and electrochemical experiments were conducted. The results revealed that the BM location had little effect on the tensile properties of the joints. The grain orientation spread (GOS) value of 2024 alloy side was lower than that of 5083 alloy side. Intergranular corrosion occurred mainly on the 2024 alloy side, while the grain interior of the 5083 alloy side was corroded due to the higher GOS value and dislocation density. The FSWed dissimilar joints with a superior exfoliation corrosion resistance could be achieved when the 5083 aluminum alloy with better corrosion performance was positioned on the retreating side.     

Corrosion behaviour of AA2024 aluminium alloy in different tempers in NaCl solution and with the CeCl3 corrosion inhibitor

The paper analyses the corrosion behaviour of naturally and artificially aged AA2024 alloy in NaCl solution and in the presence of an environment-friendly corrosion inhibitor, CeCl₃. On the basis of the values of polarisation resistance and corrosion current density, the corrosion resistance of the protective inhibitor film is established as well as the general corrosion resistance of this aluminium alloy. Resistance to pit formation is determined based on the difference in pitting and corrosion potentials while resistance to pit growth is determined based on the amount of charge consumed during pit growth. A scanning electron microscope is used to examine the morphology of the pits formed during the pitting corrosion testing, as well as to determine the cerium content on intermetallic particles and the matrix AA2024 alloy. The corrosion behaviour of AA2024 alloy is investigated after different test periods in NaCl solution and in the same solution with the CeCl₃ inhibitor. The corrosion resistance of both tempers of AA2024 alloy is more than one order of magnitude higher in the presence of CeCl₃. An explanation of the observed differences in the corrosion behaviour of the naturally and artificially aged AA2024 alloy is proposed. Different corrosion behaviour of the alloy after different test periods is also explained.