Effect of applied tensile stress on intergranular corrosion of AA2024-T3

The effect of uniaxial tensile stress on intergranular corrosion (IGC) of AA2024-T3 was studied using the foil penetration technique. Standard ASTM G49 fixed-displacement jigs were modified to allow the use of sheet samples, which were then attached to an electrochemical cell as in the foil penetration setup. The time for IGC to penetrate samples of varying thickness was monitored. This method provides a new approach to bridge the gap between IGC and intergranular stress corrosion cracking (IGSCC). Samples with various orientations relative to the rolling direction were studied in 1.0 M NaCl at controlled anodic potentials. Potentiodynamic polarization measurements indicated that the two breakdown potentials typically observed for AA2024-T3 were lower for stressed samples than for unstressed samples, and the current at a given potential was higher. The penetration rate depended on potential and was higher for stressed samples than for unstressed samples. The primary form of attack above the higher breakdown potential was IGSCC, whereas pitting dominated between the two breakdown potentials. Stress had a larger effect on penetration rate at higher applied potentials, indicating that pitting is less susceptible to the effects of stress than a properly oriented IGC crevice. The effects of stress on the penetration rates in various orientations were strongly linked to the anisotropic microstructure. X-ray microfocal radiography and optical microscopy of cross-sections were used to characterize IGSCC defects in thin penetrated foils. In certain orientations, crack faces were parallel rather than perpendicular to the stress direction as a result of the constraints of the microstructure on the orientation of the IGC. Implications for the mechanisms of IGC and IGSCC are discussed.     

Corrosion susceptibility of peened friction stir welded 7075 aluminum alloy joints

Effects of surface treatment techniques like laser and shot peening on stress corrosion cracking (SCC) susceptibility of friction stir welded (FSW) 7075 aluminum alloy joints were investigated. This study had two parts; the first part investigated the peening effects on stress corrosion cracking susceptibility in FSW samples by slow strain rate testing in a 3.5% NaCl solution. The second part of the study investigated the effects of peening on corrosion while submerged in a 3.5% NaCl solution with no external loads applied. No signs of corrosion pitting or SCC were evident on any of the tensile samples during the slow strain rate testing. The FSW plates exposed in 3.5% NaCl solution for 60 days were inspected periodically for signs of corrosion and stress corrosion cracking in the areas expected to have residual stresses due to welding. Pitting corrosion was seen on the samples, but even after 60 day exposure no stress corrosion cracking was detected on any of the peened or unpeened samples.     

Stress-corrosion cracking of aged AlCuMg alloys in NaCl solution

Pitting potentials and stress corrosion life-times of AlCuMg alloys (mainly 2024 alloy) with various ageing structures have been measured in a de-aerated 1M NaCl solution under conditions of controlled potential. The aged alloy, which has the higher susceptibility to stress-corrosion cracking, showed two pitting potentials corresponding to pitting at the grain boundaries and within the grains. The susceptibility of the alloys to intergranular stress-corrosion cracking occurred at potentials above the pitting potential of the grain boundaries. The intergranular stress-corrosion cracking is caused not by the dissolution of the grain boundary precipitates (S phase) but by the pitting dissolution of the solute-denuded zones along the grain boundaries. Aspects of SCC in the alloys are similar to those in the Al-4%Cu alloy without Mg.     

On the Stress corrosion cracking behaviour of aluminium alloy sheet in an aqueous solution of 3% NaCl + 0.3% H2O2

The stress corrosion cracking (SCC) behaviour of aluminium alloy sheet was investigated in the long transverse direction using the slow strain rate testing technique. The synthetic environment used was an aqueous solution of 3% NaCl + 0.3% H₂O₂. No indications of SCC sensitivity are observed for the alloys 2024-T351, 8090-T81, and 2091 CPHK-T8X. The alloys 2091 T8X and 6061-T4 are found to be susceptible to intergranular stress corrosion cracking. At strain rates below 4 · 10^?7 s^?1, the slow strain rate testing technique indicates a slight SCC sensitivity with alloy 6013-T6. Fractography reveals transgranular stress corrosion cracking. Transgranular stress corrosion cracking is also observed with 6061-T4 specimens which are dynamically strained at strain rates below 5 · 10^?7 s^?1. Aqueous 3% NaCl solution with hydrogen peroxide addition promotes pitting and intergranular corrosion. The loss of ductility caused by these corrosion processes interferes with the evaluation of the results of the slow strain rate testing technique.     

Effect of W on stress corrosion cracking susceptibility of newly developed Ni-saving duplex stainless steels

Effect of W on stress corrosion cracking behavior (SCC) of Ni-saving duplex stainless steels (Fe18Cr6Mn3Mo0.4N (2.13, 5.27)W, in wt%) was investigated in 4 M NaCl solution using a slow strain rate test method. The change in the W content from 2.13 to 5.27 wt% marginally affected the tensile properties of the investigated DSSs. Alloying W clearly improved the pitting initiation resistance and repassivation tendency of the investigated alloys, but the SCC susceptibility was not remarkably decreased by addition of W. The slight enhancement in the SCC resistance of the alloy containing 5.27 wt% W was revealed to be correlated with the accelerated galvanic corrosion between the ferrite and austenite phases as a result of the W partitioning preferentially into the ferrite phase which could encourage the propagation of pitting.     

Effect of Gd on microstructure and stress corrosion cracking of the AZ91-extruded magnesium alloy

AZ91 and AZ91–xGd (x = 0.5, 1.0, 1.5 wt%) magnesium alloys are extruded into plates. The addition of Gd promotes the formation of Al₂Gd, effectively reducing the volume fraction of the β-Mg₁₇Al₁₂ phase and making the banded structures of the extruded magnesium alloys thinner. The corrosion weight loss tests and electrochemistry analyses demonstrate that Gd significantly improves the pitting resistance of the AZ91 in 3.5-wt% NaCl solution saturated with Mg(OH)₂. Slow strain rate tensile tests show that in a corrosive environment, compared with AZ91, the elongation to failure of the AZ91–1.0Gd alloy is increased by 47%, and the alloy exhibits excellent stress corrosion resistance in this study. The fracture mode of AZ91 is changed from typical intergranular fracture to a mixture of transgranular and intergranular fracture in the corrosion solution by adding Gd. The mechanism of Gd to improve the stress corrosion resistance of the AZ91 magnesium alloy is that Gd increases the corrosion resistance, especially the pitting of AZ91.     

Stress Corrosion Cracking Behavior of Peened Friction Stir Welded 2195 Aluminum Alloy Joints

The surface treatment techniques of laser and shot peening were used to investigate their effect on stress corrosion cracking (SCC) in friction stir welded (FSW) 2195 aluminum alloy joints. The investigation consisted of two parts: the first part explored the peening effects on slow strain rate testing (SSRT) in a 3.5% NaCl solution, while the second part investigated the effects of peening on corrosion while submerged in a 3.5% NaCl solution with no external loads applied. For the SSRT, the laser-peened samples demonstrated superior properties to the other samples, but no signs of corrosion pitting or SCC were evident on any of the samples. For the second part of the study, the FSW plates were inspected periodically for signs of corrosion. After 60 days there were signs of corrosion pitting, but no stress corrosion cracking was noticed in any of the peened and unpeened samples.     

Corrosion behavior of two heat treatment Al-Zn-Mg-Cu alloys in different intergranular corrosion solution

The electrochemical behavior of two kinds of artificial aged Al-Zn-Mg-Cu alloys in two intergranular cor-rosion (IGC) solutions were studied using electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) at steady-state. EDAX result indicates that different artificial ageing methods change the composition and content of Cu and Zn in different zones. Zn/Cu depleted precipitation-free zone that plays a very important role in IGC is formed by heating the solubilized Al alloy for 135℃ at 16 h. All impedance spectra of the two alloys in two IGC solutions can be divided into three types. The two different states A1 alloys takes on one time constant and two capacitive arcs at high-mediate frequency and low frequency in the NaCl (NH4 )2SO4 solution respectively; but in the NaCl HCl solution, impedance displays one capacitive arc at the high-mediate frequency and an inductive loop at low frequency. OCP results show that more micro-galvanic cells in the NaCl (NH4)2SO4 solution than that in the NaCl HCl solution results in more potential fluctuation amplitude, and long-term drift of OCP is due to the long-term variation of the cathodic and anodic corrosion processes.     

Pitting potential of high purity binary aluminium alloys—I. AlCu alloys. Pitting and intergranular corrosion

The effect of solute content on the pitting potential of Al-Cu alloys in 1M NaCl solutions was studied. In solution treated Al-Cu alloys the pitting potential of the alloy was found to increase with increasing Cu content. The maximum pitting potential value was limited by the solubility of Cu in the Al. It was found that, while the pitting potential of pure aluminium was ca. ?0.52 V, the pitting potential of Al-5.22% Cu was ?0.33 V.The ageing of the alloy affected the pitting potential. The pitting potential of the aged alloy was found to be determined by the copper content of the solute depleted zones formed during ageing. The formation of G.P. zones did not modify the pitting potential of the alloy. The formation of θ′′ phase, in an Al-3.5 Cu alloy, produced a decrease of 30 mV in the pitting potential, while the formation of θ′ phase produced a decrease of > 100 mV.As ageing was more rapid in the grain boundaries, there was a localized decrease in the pitting potential, which was followed by susceptibility of the alloy to intergranular corrosion.     

Intergranular Corrosion of Cryorolled and Aged D16 Aluminum Alloy

Structural and phase factors influencing the intergranular corrosion (IGC) of D16 aluminum alloy after quenching, isothermal rolling at liquid-nitrogen temperature, and subsequent natural and artificial aging were studied by optical metallography, X-ray diffraction, and transmission and scanning electron microscopy. It is established that the alloy aged at temperatures below 150°C demonstrate low depth and intensity of corrosion attack, which sharply increase with aging temperature up to 190°C. The alloy behavior is caused by the accelerated decomposition of the preliminary supersaturated aluminum solid solution, accompanied by formation of the mixed structure of the matrix with a bimodal distribution of main strengthening phases. It is concluded that IGC of the cryorolled and aged alloy has an equal nature and mainly caused by phase factors.     

Stress corrosion cracking of zirconium and Zircaloy-4 in halide aqueous solutions

Zirconium and Zircaloy-4 in 1 M NaCl, 1 M KBr and 1 M KI aqueous solutions were found to be susceptible to stress corrosion cracking (SCC) only at potentials above the pitting potential. In all the tested systems the following steps were found: first electrochemical breakdown of the passive film, followed by intergranular attack due to anodic dissolution assisted by stresses; and finally a fast transgranular propagation. This last step was identified as the “true” SCC process. The analysis of the possible mechanisms involved during this process led to the conclusion that the surface-mobility SCC mechanism can be used to explain the experimental results found in the present work.     

拉应力条件下微弧氧化膜对铝合金腐蚀及电化学行为的影响

为研究在拉应力条件下微弧氧化膜对铝合金腐蚀及电化学行为的影响,采用恒载荷应力环在3.5% NaCl溶液中研究了经微弧氧化（MAO）处理后的7050铝合金（AA7050）应力腐蚀行为。用原位电化学阻抗谱（EIS）的方法评价在拉应力条件下,膜层的腐蚀破坏随浸泡时间的变化,并建立了相应的等效电路模型。结果表明,在3.5% NaCl溶液中,微弧氧化膜在有无拉应力的条件下都可以提高AA7050的耐蚀性和减少AA7050的塑性损失。在400 MPa拉应力条件下,微弧氧化膜的阻抗在应力腐蚀的过程中呈现出先减小后增大,再减小最后趋于稳定的规律;另外,AA7050在有拉应力的条件下,拉应力会促进基体的点蚀形核,提高腐蚀速率,微弧氧化膜的疏松层在拉应力的作用下会失去对基体的保护作用。     

不同晶粒尺寸5083铝合金晶间腐蚀和应力腐蚀开裂的差异性

通过冷轧和退火获得具有不同晶粒尺寸(8.7～79.2μm)的5083铝合金板.研究其微观结构、晶间腐蚀(IGC)、应力腐蚀开裂(SCC)和裂纹扩展行为.结果表明,粗晶粒样品表现出更好的抗IGC性能,其腐蚀深度为15μm.慢应变速率测试结果表明,细晶粒样品表现出更好的抗SCC性能,敏感性指数ISSRT为11.2％.此外,...     

Investigation of the SCC behaviour of alloy 2024 using the slow strain rate technique

The stress corrosion cracking (SCC) behaviour of 2024 plate in T351 and T851 tempers was investigated in short transverse direction performing accelerated tests under constant deformation, constant load and slow strain rate conditions. Corrosive media used were: aqueous 3.5% NaCl solution, an aqueous solution of 2% NaCl + 0.5% Na₂CrO₄ at pH 3 (according to LN 65666), an aqueous solution of 3% NaCl + 0.3% H₂O₂, and substitute ocean water according to ASTM D1141. Alternate immersion tests in 3.5% NaCl solution indicated the low SCC resistance of the alloy 2024-T351 as well as the improved SCC behaviour due to aging to T851 condition. Similar results were obtained from constant load tests under permanent immersion conditions in the acidified chloride-chromate solution, in 3% NaCl solution with peroxide, and in substitute ocean water, whereas no SCC failure was observed with specimens which were permanently immersed in 3.5% NaCl solution. Using the slow strain rate method, 3% NaCl + 0.3% H₂O₂ and substitute ocean water were found to be effective synthetic environments. The other two electrolytes did not promote severe stress corrosion cracking with alloy 2024-T351. The SCC behaviour of 2024-T851 was difficult to determine employing the slow strain rate technique. Large scatter in data, observed even in inert environment, and the low elongation of the aged material, exacerbated by a further degradation of ductility due to pitting and intergranular corrosion, precluded an evaluation.     

Corrosion behavior of 2195 and 1420 Al-Li alloys in neutral 3.5% NaCl solution under tensile stress

1 Introduction Al-Li alloys, compared with traditional Al alloys, possess many excellent properties, such as lower density, greater elastic modulus and higher specific strength. 2195 Al-Li alloy with high strength and good ability to weld, was applied to …     

2050铝锂合金薄板拉伸性能及晶间腐蚀与时效的相关性

采用拉伸性能测试、浸泡腐蚀及透射电镜(TEM)观察,研究了2050铝锂合金薄板T6及T8态时效时后的拉伸性能、晶间腐蚀(IGC)及微观组织。结果表明,T8态时效2050铝锂合金强度及伸长率均明显高于T6态时效。T6及T8态时效时,合金IGC敏感性随时效延长至峰时效阶段而逐渐降低;进一步延长至过时效阶段,IGC敏感性有所增加。另外,T8态时效时IGC敏感性明显低于T6态时效。基于提高强度、伸长率及降低IGC敏感性的综合考虑,2050铝锂合金薄板适宜的时效制度为T8态峰值时效,相应合金具有最高的强度(抗拉强度529 MPa)、良好的伸长率(10%)及最低的IGC敏感性。     

微弧氧化处理对AA7075铝合金在3.5%NaCl溶液中局部腐蚀行为的影响(英文)

采用微弧氧化法在AA7075铝合金上生成氧化铝膜层,并对氧化铝膜层在3.5%NaCl溶液中的腐蚀和应力腐蚀开裂行为进行测试。采用电化学阻抗谱来研究膜层随浸泡时间的变化,构建合适的等效电路图。对常载荷应力腐蚀开裂测试后的试样进行金相观察。结果表明,微弧氧化膜可以有效避免富Cu、Fe金属间化合物相引发的严重局部腐蚀而导致的合金过早失效。     

Untersuchungen über die interkristalline Korrosion und Spannungsrißkorrosion einer reinen AlZnMg 2-Legierung in NaCl-Lösung

Investigations Concerning the Intercrystalline Corrosion and Stress Corrosion Cracking of a Pure AIZnMg 2-Alloy in NaCl-Solution In a pure AIZnMg 2-alloy diversified fully step aged or overaged in one step intergranular corrosion (IG) and stress corrosion cracking (SCC) in airsaturated 1 N NaCl-solution or in deaerated 0,1 N NaCl-solution at 303 K were investigated under potentiostatic control. The pure AlZnMg 2-alloy had the following composition (weight percent): Zn: 4.60; Mg: 2.00; Fe: 0.005; Si: 0.006; balance Al. In the two step aged AlZnMg 2-alloys the IG-sensitivity is lowered by an increase of the first step aging time, while SCC-susceptibility and the yield point are raised. As possible mechanism of SCC hydrogen embrittlement is suspectet. At potentials noble to IG-breakdownpotential an interaction of intergranular corrosion and SCC brings about crack velocities, which are to be regarded as a sum of the velocity of intergranular corrosion, which is virtually independent of tensile stress, and the velocity of crack propagation due to SCC. In the overaged AlZnMg 2-alloy IG-attack was observed although in this state of aging no precipitate free zone anodic to the matrix can be assumed. A model to explain this variation of intergranular corrosion, that proofs to be dependent of tensile stress, is represented.     

Beitrag zur Wachstumskinetik der interkristallinen Korrosion von ausgehärteten Al-Cu-Legierungen – Teil I: Ergebnisse der Folien-Durchbruchsmethode

Contribution to the growth kinetics of the intergranular corrosion of age-hardened Al-Cu alloys – Part I: Results of the foil penetration technique The growth kinetics of the intergranular corrosion (IC) of age-hardened Al-Cu-alloys in aqueous chloride solutions under potentiostatic conditions have been investigated using the foil penetration technique originally developed for pit growth measurements. The two tested systems were a pure binary Al-4% Cu-alloy (sheet thicknesses 0.2 and 0.5 mm) in 0.01 m NaCl, pH 11, and a commercial AA 2024 type alloy (various sheet thicknesses ranging from 0.2 to 1.0 mm) in 0.1 m NaCl, pH 7, respectively. Both alloys were tempered to maximum IC susceptibility and have been tested at potentials where selective anodic dissolution of the grain boundary regions occurs. As a reference system, the pit growth kinetics in commercial pure aluminum (sheet thicknesses ranging from 0.2 to 1.0 mm) in 0.01 m NaCl + 0.01 m Na₂SO₄ (as pit initiation inhibitor), pH 11, have been investigated at an applied potential considerably higher than the pitting potential. The experimental results have shown that, for the IC in the finegrained commercial alloy, a uniform growth kinetic valid for the whole range of sheet thicknesses, as for the case of pitting, cannot be formulated. The correlation of the current-time-curves and the attack morphologies (after penetration) for different sheet thicknesses with the penetration times leads to the conclusion that the growth kinetics of intergranular attacks are related to the number of sites of active attack (cracks) per metal volume. This specific number of actively growing cracks depends on the grain size and on the electrochemical conditions and, for given parameters, on the exposure time and therefore on the sheet thickness. For the commercial alloy examined, the following three stages of attack with decreasing penetration velocity of the IC could be distinguished:

• activation stage
• transition stage
• stable, macroscopic grain boundary dissolution.

The coarse-grained pure binary alloy showed a markedly higher penetration velocity of the IC under milder electrochemical conditions. This alloy is suitable for a model investigation of the first stage of attack but no quantitative kinetic information could be obtained from the only two disposable sheet thicknesses. The pit growth measurements in pure aluminum showed that the square root-of-time growth law previously found for thin foils and sheets is valid for deep pits too. The discussion explains that the electrochemical mechanisms of pitting of aluminum and of the IC in Al-Cu-based alloys are identical and that the basic difference lies in the geometry of the sites of attack. The ohmic control of the aluminum dissolution and therefore of the growth kinetics of pits and intergranular cracks is governed by the total anodic area and its different time dependence during the growth of the sites of local attack.     

喷砂处理对A7N01铝合金应力腐蚀开裂和点蚀性能的影响

采用慢应变速率试验和循环极化试验,研究了喷砂对A7N01铝合金耐应力腐蚀开裂和点蚀性能的影响。结果表明:在3.5%NaCl(质量分数)溶液中,与未喷砂合金相比,喷砂处理后A7N01铝合金的应力腐蚀敏感指数增大,抗应力腐蚀性能下降。喷砂后合金的点蚀电位、保护电位均降低,滞后环增大,且随浸泡时间增长,点蚀电位和保护电位负移,滞后环增大,喷砂处理使合金的抗点蚀能力和钝化膜修复能力降低。喷砂处理会造成A7N01铝合金表层晶粒细化、硬度增加,但同时喷砂也会造成表面粗糙度增加,并有微裂纹等表面损伤发生,从而降低合金的耐蚀性。