采用浸泡实验与电化学循环极化曲线测试研究了7020铝合金在3.5% (质量分数) NaCl溶液中的点蚀行为,并结合金相显微镜 (OM)、扫描电镜 (SEM) 及扫描透射电镜 (STEM) 的微观组织观察结果对相关机理进行了分析和探讨。结果表明：7020铝合金的最大点蚀深度随时间变化的曲线为S型,呈缓慢增长-快速增长-保持稳定的过程。合金中
The pitting corrosion behavior of 7020 aluminum alloy in 3.5%(mass fraction) NaCl solution was investigated by immersion test and cyclic polarization curve, while the corrosion morphology of the alloy was characterized by means of optical microscopy (OM), scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). The results show that the curve of maximum depth of corrosion pits versus time exhibits ''S''-like shape.
Grilli等认为Al-Cu-Fe-Mn第二相是2219铝合金的腐蚀起始位置,且点蚀过程中常作为阴极,Al基体作为阳极而被腐蚀,腐蚀产物Al(OH)3会逐渐积聚在第二相颗粒周围,而Fe则会从Al-Cu-Fe-Mn相中慢慢溶解,长时间的点蚀过程会使第二相被腐蚀产物完全覆盖。Wang等发现影响7A60铝合金发生严重点蚀的主要第二相是MgZn2粒子、Al2MgCu和Mg2Si相,而Al7Cu2Fe颗粒对点蚀性能的影响不大。Sameljuk等研究了铸态Al-Zn-Mg合金的点蚀性能,发现其在3% (质量分数) NaCl溶液中会发生点蚀,主要是富铜相引发了周围含Zn、Mg析出相的溶解。以上研究均说明 (亚) 微米级第二相是影响合金点蚀性能的关键因素。针对7020铝合金,也已进行了相关研究。Ismael认为含Zn的金属间化合物在7020-T6铝合金中作为阳极并控制腐蚀过程,且人工时效会增加Zn含量并减缓腐蚀速率。Al Sammarraie等发现7020铝合金中不同温度下的点蚀主要与GP区、以及GP区长大形成的Mg
本文通过浸泡实验和循环阳极极化曲线研究了7020铝合金在3.5% (质量分数) NaCl溶液中不同时间的点蚀行为,通过扫描电子显微镜 (SEM) 观察了点蚀浸泡前后的表面形貌,用扫描透射电镜 (STEM) 观察了试样内部微观组织,探讨了其点蚀机理。
实验材料为10 mm厚的7020-T5铝合金挤压型材,其化学成分 (质量分数,%) 为：Si 0.084,Fe 0.31,Cu 0.17,Mn 0.31,Mg 1.27,Zn 4.45,Cr 0.22,Zr 0.13,Al 余量。
浸泡实验样品尺寸为15 mm×15 mm×10 mm,经除油、清洗干燥后,在3.5%NaCl溶液中连续浸泡,溶液温度通过恒温水浴控制在 (20±1) ℃,分别浸泡不同时间后取出。采用MX 3000金相显微镜观察了点蚀后试样横截面形貌,同时用FEI-Quanta 200型SEM对试样的初始形貌与浸泡后的表面形貌进行观察,并对一些典型第二相进行能谱 (EDS,Genesis60s) 分析。
STEM样品先经机械研磨减薄至厚度约为80 μm,再冲成直径为3 mm的小圆片,采用MTP-1A型双喷电解减薄仪对圆片试样进行减薄、穿孔。双喷电解液为30%HNO3+70%CH3OH (体积分数),使用液氮将温度控制在-20 ℃以下,电流为50~70 mA,电压为10~20 V。采用FEI Tecnai G2 F20 型STEM (加速电压为200 kV)在高角环形暗场像 (HAADF) 模式下观察试样中的微观组织,并用EDS分析第二相化学成分。
在本实验中,将浸泡不同时间的样品进行循环阳极极化曲线测试。先将样品取出后与Cu线相连,留一面作为工作面,其工作面积为10 mm×10 mm,并用无水乙醇超声波清洗后干燥,非工作面用松香石蜡密封后进行电化学测试。用IM6EX型电化学综合测试仪,根据ASTM G59-97 (2003) 标准采用动电位扫描法测定极化曲线闭合环。在3.5%NaCl溶液中,采用三电极体系,7020铝合金样品为工作电极,Pt片为辅助电极,饱和甘汞电极为参比电极,测定开路电位后,先进行阴极极化,再进行阳极极化。扫描范围为
The authors have declared that no competing interests exist.
文章选取了我国某类型高速列车常用的A7N01S-T5铝合金材料,进行了恒载荷应力腐蚀试验,对A7N01S-T5材料的应力腐蚀机理、应力腐蚀强度因子K1SCC和裂纹生长机制进行了研究.结果表明,A7N01S-T5铝合金的应力腐蚀破坏敏感性很高,其临界应力场强度因子K<sub>1</sub>SCC为5.2 MPa·m1/2,仅为断裂韧度K<sub>1</sub>C的0.15,对应的临界应力门槛值б为123MPa,仅为拉伸强度б<sub>b</sub> 0.27.裂纹断口面有明显的舌状凸起和凹坑的存在.A7N01S-T5铝合金的应力腐蚀裂纹以沿晶开裂为主,形貌呈树枝状,同时相邻二次裂纹之间有竞争生长的关系.
The pitting corrosion behaviors of 7A60 aluminum alloy in the retrogression and re-aging (RRA) temper were investigated by electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) techniques, and the microstructure and the second phase content of the alloy were observed and determined by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The results show that there exist two different corrosion stages for 7A60 alloy in 3.5% NaCl solution, and the corrosion process can be detected by the appearance of EIS spectrum with two capacitive time constants and the wavelet fractal dimension D extracted from EN. SEM and EDS results also demonstrate that severe pitting corrosion in 7A60 alloy is mainly caused by electrochemical active MgZn2 particles, secondly by Al2MgCu and Mg2Si. Al7Cu2Fe particles make little contribution to the pitting corrosion of 7A60 alloy.
The corrosion behaviour of Al–5Zn–3Mg–0.6Cu–0.8Zr–0.25Cr–0.15Ni–0.15Ti alloys, produced by traditional and powder technologies, with similar thermo-mechanical treatments, in 3% sodium chloride solution, has been examined by electrochemical methods, scanning electron microscopy, transmission electron microscopy and X-ray microanalysis. The alloys reveal similar precipitation but of different shape, size and distribution; further, both alloys experience localized corrosion. Copper-rich precipitates initiate the dissolution of surrounding particles, enriched in Zn and Mg. As a result, the surface is enriched with other alloying elements after a full polarisation run. Cast material has lower corrosion properties because of the higher heterogeneity of the structure. The structure heterogeneity of the cast material involves a more non-uniform distribution of the precipitates, larger Zn- and Mg-rich particles, and depletion of the matrix and areas around the precipitates by alloying elements compared with the powder material.
Effects of pH solution and chloride (Cl) ion concentration on the corrosion behaviour of alloy AA6061 immersed in aqueous solutions of NaCl have been investigated using measurements of weight loss, potentiodynamic polarisation, linear polarisation, cyclic polarisation experiment combined with open circuit potential transient technique and optical or scanning electron microscopy.The corrosion behaviour of the AA6061 aluminum alloy was found to be dependant on the pH and chloride concentration [NaCl] of solution. In acidic or slightly neutral solutions, general and pitting corrosion occurred simultaneously. In contrast, exposure to alkaline solutions results in general corrosion. Experience revealed that the alloy AA6061 was susceptible to pitting corrosion in all chloride solution of concentration ranging between 0.00302wt% and 5.502wt% NaCl and an increase in the chloride concentration slightly shifted both the pitting and corrosion potentials to more active values. In function of the conditions of treatment, the sheets of the alloy AA6061 undergo two types of localised corrosion process, leading to the formation of hemispherical and crystallographic pits.Polarisation resistance measurements in acidic (pH02=022) and alkaline chloride solutions (pH02=0212) which are in good agreement with those of weight loss, show that the corrosion kinetic is minimised in slightly neutral solutions (pH02=026).
The corrosion anisotropy of 7050鈥揟7451 Al alloy thick plate in NaCl solution was investigated by immersion tests, slow strain rate testing (SSRT) technique, potentiodynamic and anode polarization measurements, optical microscropy (OM) and scanning electron microscopy (SEM) observations. The results show that the thick plate exhibits severe corrosion anisotropy due to the microstructure anisotropy. The observations of immersion surfaces together with the analysis of polarization curves reveal that the differences of the corrosion morphologies on various sections in this material are mainly related to the area fraction of the remnant second phase, and higher area fraction displays worst corrosion resistance. The stress corrosion cracking (SCC) susceptibility of different directions relative to the rolling direction is assessed by SSRT technique, ranked in the order: S direction >L direction >T direction. The result show that the smaller the grain aspect ratio, the better the corrosion resistance to SCC.
Corrosion behaviour, residual stress, intermetallics and microstructure of friction stir welded aerospace Al–Zn–Mg and Al–Zn–Mg–0.10Sc–0.10Zr (wt.%) alloys were investigated comparatively and in detail. The thermo-mechanically affected zones adjacent to weld nugget are most susceptible to corrosion in two weld joints. In addition to the enrichment of coarse Si and Fe intermetallics, wide precipitate free zones (in Al–Zn–Mg weld joint) and high tensile residual stress are the key factors. Secondary Al3ScxZr161x nanoparticles can effectively improve stress corrosion cracking resistance and decrease exfoliation corrosion rates of Al–Zn–Mg weld joints by restraining the formation of precipitate free zones and refining grains.
The corrosion behaviour of cast and heat-treated Al–6%Zn–1%Mg and Al–6%Zn–1%Mg–1%Ag alloys and metal matrix composites (MMCs) were investigated using dynamic polarisation techniques, followed by scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS). The addition of silver and the incorporation of continuous Altex fibres into the alloys have a positive effect on the materials’ corrosion resistance. It was observed that due to the presence of second phase particles and microsegregation, pitting occurs preferentially at the grain boundary and fibre/matrix interface regions in the cast alloys and composites, respectively.
The corrosion behavior of commercial Al alloys was studied in neutral 0.6 mol L 611 NaCl by using single-cycle polarization. Qualitative interpretation of pitting scans in both deaerated and naturally aerated NaCl solution, with the aid of corrosion morphology characterization, allowed for inference of the features of localized corrosion as a function of Al substrate nature. Electrochemical characteristic parameters such as pitting ( E pit), repassivation or protection ( E prot) potentials and pit transition potential ( E ptp) were determined. The validity of the difference Δ E = | E pit 61 E prot| as criterion for susceptibility to localized corrosion of aluminium alloys is discussed.
Initiation of localized corrosion upon high strength aluminum alloys is often associated with cathodic intermetallic particles within the alloy. Electrochemical measurements and metallurgical characterization have been made to clarify and quantify the physical properties of Al 7Cu 2Fe particles in AA7075-T651. Prior studies regarding either the stereology or electrochemical properties of Al 7Cu 2Fe are scarce. Quantitative microscopy revealed a significant population of Al 7Cu 2Fe in the alloy; comprising up to 65% of the constituent particle population and typically at a size of 1.7 ± 1.0 μm. It was determined that Al 7Cu 2Fe may serve as a local cathode in the evolution of localized corrosion of AA7075-T651 and is capable of sustaining oxygen reduction reactions at rates of several hundreds of μA/cm 2 over a range of potentials typical of the open circuit potential (OCP) of AA7075-T651 in NaCl solution of various concentrations and pH. The presence of Al 7Cu 2Fe leads to the development of pitting at the particle–matrix interface.
Exfoliation experiments were performed on two different 7000 series aluminium alloys in two different tempers and two different surface conditions according to ASTM G34 standard (EXCO test). The results of this experiment were evaluated with regard to surface condition, temper and alloy composition. The results after an EXCO test were evaluated according to the specification in the standard (in comparison to reference pictures), by visually ranking the samples, and by metallographic investigation. According to the ASTM standard, almost no changes could be seen with regard to the temper. Ranking and metallography revealed that AA7010 generally is less susceptible to exfoliation compared to AA7349. The differences between the temper conditions are not significant. It could be elaborated that the higher zinc content of AA7349 is detrimental for the exfoliation behaviour.