Effect of hydrogen in aluminium and aluminium alloys: A review

Susceptibility of aluminium and its alloys towards hydrogen embrittlement has been well established. Still a lot of confusion exists on the question of transport of hydrogen and its possible role in stress corrosion cracking. This paper reviews some of the fundamental properties of hydrogen in aluminium and its alloys and its effect on mechanical properties. The importance of hydrogen embrittlement over anodic dissolution to explain the stress corrosion cracking mechanism of these alloys is also examined in considerable detail. The various experimental findings concerning the link between hydrogen embrittlement and stress corrosion cracking are also discussed.     

ENVIRONMENTAL EFFECTS ON CRACK PROPAGATION IN ALUMINIUM ALLOYS

Abstract Crack propagation rates have been measured in two aluminium alloys under cyclic and static loading, in air, and in salt solution. On the basis of these results, a model is proposed, whereby corrosion fatigue crack propagation may be interpreted in terms of fatigue and static stress corrosion characteristics. Two interacting processes are operative; one is "stress assisted dissolution", which tends to inhibit mechanical failure by crack blunting and microbranching. The other is "environment assisted fracture" which occurs too rapidly for dissolution to occur. One or other of these processes is always observed to be dominant. This proposal is discussed in relation to other recent models for corrosion fatigue cracking. The effects of frequency, waveform and mean stress variations are also considered.     

The mechanism of hydrogen embrittlement: the stress interaction between a crack, a hydrogen cluster, and moving dislocations

Mechanisms of dissolvent anodic chemical reaction and hydrogen embrittlement have been proposed as stress corrosion cracking (SCC) mechanisms. The former is feasible for the case of plastic deformation dominant metals (low-yield stress), and the latter is for high-strength metals such as high-strength steels. However, in spite of low-yield stress, a discontinuous cleavage-like fracture is sometimes observed during SCC for ductile fcc alloys, which concerns the interaction between dislocations and the hydrogen cluster. The problem of when these mechanisms will be dominant remains. In this paper, the stress corrosion cracking model on the basis of hydrogen diffusion and concentration toward the elastic-plastic stress field around a crack and the interaction of dislocations and hydrogen around a crack tip are proposed to clarify the mechanism of stress corrosion cracking for ductile and brittle materials. We conducted numerical analyses using these proposed models.     

50th Anniversary Article: The Origin and Management of Residual Stress in Heat‐treatable Aluminium Alloys

The presence of macroscopic residual stresses in heat‐treatable aluminium alloys can give rise to machining distortion, dimensional instability and increased susceptibility to in‐service fatigue and stress corrosion cracking. This paper presents and reviews details about the residual stress magnitudes and distributions introduced into wrought aluminium alloys by the thermal operations associated with heat treatment. Experimental measurement data and the results of finite element analysis are presented and discussed. The available technologies by which residual stresses in aluminium alloys can be relieved are reviewed. The limitations of these techniques are described, and recommendations are made as to selecting the most appropriate technique to manage residual stresses. Opportunities for the future optimisation of these techniques are also presented.     

电子显微分析技术在奥氏体合金应力腐蚀开裂研究领域中的应用

奥氏体合金广泛应用于核电领域。应力腐蚀开裂是核电材料主要的失效形式之一,奥氏体合金的应力腐蚀开裂关系到核电站的安全运行。综述了评估应力腐蚀开裂的试样方法以及运用现代电子显微分析技术表征应力腐蚀开裂的方法。对这些电子显微分析技术的优点进行了总结,并指出未来电子显微分析技术在应力腐蚀开裂研究中的发展方向。     

The mechanism of hydrogen embrittlement: the stress interaction between a crack, a hydrogen cluster, and moving dislocations

Mechanisms of dissolvent anodic chemical reaction and hydrogen embrittlement have been proposed as stress corrosion cracking (SCC) mechanisms. The former is feasible for the case of plastic deformation dominant metals (low-yield stress), and the latter is for high-strength metals such as high-strength steels. However, in spite of low-yield stress, a discontinuous cleavage-like fracture is sometimes observed during SCC for ductile fcc alloys, which concerns the interaction between dislocations and the hydrogen cluster. The problem of when these mechanisms will be dominant remains. In this paper, the stress corrosion cracking model on the basis of hydrogen diffusion and concentration toward the elastic-plastic stress field around a crack and the interaction of dislocations and hydrogen around a crack tip are proposed to clarify the mechanism of stress corrosion cracking for ductile and brittle materials. We conducted numerical analyses using these proposed models.     

The mechanics and mechanisms of fracture in stress corrosion cracking of aluminium alloys

A fracture mechanics approach to stress corrosion cracking is highlighted. The mechanisms of stress corrosion cracking is presented. Experiments on 2024 and 7075 aluminium alloys are carried out to determine their mechanical properties, microstructure and plane strain fracture toughness (K_IC). Stress corrosion cracking tests, namely, cantilever beam tests as well as wedge opening loading tests using sea water as a corrosive medium, are conducted to establish the critical stress intensity factor for stress corrosion cracking (K_ISCC) for each aluminium alloy. It is found that the K_ISCC is in the range of (1/5) to (1/6) of the plane strain fracture toughness, K_IC, depending on the alloy. The scanning electron microscopy of fracture surfaces reveals a great dependence of the cracking and/or pit severity on the applied stress intensity factor. A brief discussion on the dislocation's role in stress corrosion cracking is given.     

Stress corrosion cracking model in 7075 aluminium alloy

The stress corrosion cracking is a typical fracture process in metals and alloys. Among aluminium alloys, the 7075 alloy, presents a high performance in the mechanical properties but it is susceptible to stress corrosion cracking. This paper presents a semiempiric model of crack growth by stress corrosion cracking for the above alloy. This model only uses macroscopic parameters from fracture mechanic theory and experimental tests which are easy to obtain. The model quantifies the fissure rate related to environmental condition, microstructure and loading level, permitting the evaluation of the crack growth process at different environmental conditions and heat treatments. The model results are compared with the experimental data obtained. The theoretical model reproduces adequately the stress corrosion cracking process for the 7075 alloy.     

Passivation and stress corrosion cracking tendency of manganese stainless steels

Stress corrosion cracking (SCC) behaviour of three grades of nickel-substituted manganese stainless steels have been studied at room temperature in 1M HCl under constant load at different applied potentials. Repassivation studies have also been carried out using a scratching electrode technique. Selective dissolution and film formation similar to that of films formed on alpha brass in ammoniacal solutions, were found to induce SCC in manganese stainless steels. With decrease in nickel content, these alloys were found to be highly susceptible to SCC.     

Effect of sulfide ions on the stress corrosion behaviour of Al-brass and Cu10Ni alloys in salt water

The stress corrosion cracking (SCC) behavior of Al-brass and Cu10Ni alloys was investigated in 3.5% NaCl solution in absence and in presence of different concentrations of Na₂S under open-circuit potentials using the constant slow strain rate technique. The results indicated that the Cu10Ni alloy is more susceptible to stress corrosion cracking than as-received Al-brass at strain rate of 3.5 × 10^–6 s^–1 in 3.5% NaCl in presence of high concentration of sulfide ions (1000 ppm). The sulfide ions (up to 500 ppm) has no effect on the stress corrosion cracking of the annealed Al-brass in 3.5% NaCl at two strain rates of 7.4 × 10^–6 and 3.5 × 10^–6 s^–1. The results support film rupture for Al-brass and sulfide stress corrosion cracking assisted with pitting corrosion for Cu10Ni at slip steps as the operating mechanisms.     

Measurement of crack bridging stresses in environment-assisted cracking of duplex stainless by synchrotron diffraction

Crack propagation studies have been conducted in age‐hardened Zeron 100 duplex stainless steel, which in the 475 °C embrittled condition exhibits environment‐assisted cracking under cathodic conditions in aqueous 3.5% NaCl solution. Values of the threshold stress intensity for environment‐assisted cracking, K_1SCC, measured by crack arrest, show R‐curve behaviour. Diffraction experiments on the high‐energy beamline ID15A at the European Synchrotron Radiation Facility (ESRF) were performed to measure the crystal lattice strains in ferrite and austenite in wedge open loaded (WOL) stress corrosion specimens. The observations demonstrate that significant crack bridging stresses develop in the wake of the crack. This is due to branching of the environment‐assisted crack. Simple bridging models for the effect of the measured stresses are in agreement with the observed R‐curve behaviour.     

On the mixed Mode II/III fatigue threshold behaviour for aluminium alloys 2014‐T6 and 7075‐T6

This paper describes an investigation into the fatigue threshold behaviour of two structural aluminium aerospace alloys, Al 2014‐T6 and Al 7075‐T6, when subjected to Mode II, Mode III and mixed Mode II/III loading. A unique four‐point shear loading test rig was employed to cyclically load sharply edge‐notched square bar specimens using an increasing load technique. The main aim of the work has been to generate Mode II–Mode III interaction diagrams for the fatigue threshold in each case, in order to facilitate improved design procedures for components fabricated from these alloys, which are susceptible to fatigue cracking under predominantly shear type loading. Aircraft are subjected to structural loads consisting of: pressurization, tension/compression, bending, shear and torsion, both on the ground and in flight. Representative fatigue fracture surfaces have been examined using scanning electron microscopy.     

Improving the properties of aluminium alloys by retrogression and reageing

Retrogression and reageing heat treatments offer the potential of improved tensile properties in combination with greatly increased resistance to stress corrosion cracking. The potential of this technique is reviewed with respect to the current application in the European and North American Aerospace Industry. To illustrate the performance increase associated with RRA treatments, the stress corrosion cracking performance of the established aerospace plate and forging alloy 7010 has been evaluated using an alternate immersion constant load tensile type test (ECSS‐Q‐70‐37A) Specimens were cut from a large aerospace rectilinear forging and tested in three different tempers, T652, T7452 and a retrogressed and reaged condition (RRA). In the T652 condition the material has been shown to be highly susceptible to intergranular corrosion and stress corrosion cracking. In the T7452 and RRA conditions 7010 showed much improved resistance to SCC but pitting corrosion resulted in failure of some specimens within the 30day requirement of the test standard.     

The role of magnesium in CF and SCC of 7000 series aluminum alloys

Preliminary fatigue crack growth and surface reaction experiments were carried out on a 7075-T651 aluminum and an AZ31 magnesium alloy to examine the chemical role of magnesium in corrosion fatigue and stress corrosion cracking susceptibility of 7000 series (AlMgZn and AlMgZnCu) alloys. The evidence, although limited, strongly supports the further reactions of magnesium with water as being the cause for environmental cracking (CF and SCC) susceptibility of 7000 series aluminum alloys. This explanation is very different from the current hypotheses that are based on deformation (e.g., slip planarity), and opens up new avenues for improving the environmental cracking resistance of high strength aluminum alloys.     

原子力显微镜评价奥氏体不锈钢晶间腐蚀敏感性的研究

由13Cr可焊马氏体不锈钢(WMSGSS)制造的油气管线由于价格便宜,相对于昂贵的耐腐蚀合金来说是一个很有竞争力的选择。但是实验室试验和现场经历表明,13Cr马氏体不锈钢焊接接头热影响区发生了沿晶应力腐蚀开裂(IGSCC)。对于低等级的马氏体不锈钢来说,沿晶应力腐蚀开裂机理被认为是在晶界的碳化物附近存在贫铬区。对于中等级和高等级马氏体不锈钢来说,只在实验室试验发现有应力腐蚀开裂。它们的应力腐蚀开裂机理还没有明确,虽然最可能的应力腐蚀开裂机理也被认为和低等级马氏体不锈钢的应力腐蚀开裂机理类似。原子力显微镜(AFM)已开始广泛用于材料显微组织和环境敏感断裂的研究中。高等级马氏体不锈钢焊接接头对晶间腐蚀/应力腐蚀开裂敏感性很有希望由原子力显微镜检测到。在原子力显微镜技术应用于这项研究之前,用由304不锈钢准备的不同敏化度的试样来确定它的适用性是十分有益的。本文用原子力显微镜研究了不同敏化程度的304不锈钢的晶间腐蚀敏感性并和SEM和EDS结果进行了对比。     

Stress corrosion cracking and component failure: Causes and prevention

The century-old problem of stress corrosion cracking (SCC) in metals and alloys has been reviewed highlighting the metallurgical factors like grain boundary precipitation and grain boundary segregation contributing to the process, the electrochemical aspects of the phenomenon and the current knowledge of the mechanisms which can be broadly classified as dissolution-based and cleavage-based models. The introduction covers some field examples of SCC. SCC test methods and preventive measures for SCC have also been discussed. Liberal help has been taken from the review articles of Scully (1979), Newman & Procter (1990), Parkins (1992), Jones & Ricker (1992) and the text book by Jones (1992) in the preparation of this article.     

Das Verhalten von Aluminium-Werkstoffen in der Hausinstallation

The corrosion behaviour of aluminium alloys . The corrosion behaviour of the aluminium alloys AlMgMn, AlMgSi 0,8 and AlZn 1 was investigated at two temperatures in two natural waters with different contents with and without oxygen. These measurements were performed as well separated as in contact with copper, brass, iron and zinc; the electrode potentials and the galvanic currents were measured and registrated. The results of the metallographic investigations concerning pitting corrosion of the aluminium samples and of the measurements under potentiostatic conditions were summarized in especial corrosion diagrams. Out of these the corrosion risks of the aluminium materials can be seen. The investigated aluminium alloys – except AlZn 1 – showed unpolarized a good corrosion resistance. The best of them was AlMgMn. In contact with iron and copper containing alloys aluminium were polarized in anodic direction depending of the different metals and of oxygen content of the water. The limit of the pitting corrosion has been exceeded at times. In the cold waters zinc polarized the aluminium alloys cathodic. References were given for applications of aluminium building parts in the home installation and it was informed about experiences which corresponds with these results.     

Cracking of aluminium alloy valve body used in attitude control of satellite launch vehicle – A metallurgical investigation

Aluminium alloys emerged as preferred material for aerospace application, due to their high specific strength, high specific stiffness and ease for fabrication, Al–4.5Zn–1.5Mg aluminium alloy was used for the fabrication of a valve body for an attitude control system of satellite launch vehicles. During one of the functional qualification test, while charging the gaseous medium, a pressure drop was noticed. This was attributed to a leak through a crack opening of the valve body. Detailed metallurgical analysis indicated that the failure was due to stress corrosion cracking (SCC).This paper brings out the metallurgical analysis carried out and remedial measures suggested.     

Stress corrosion cracking of AISI 321 stainless steel in acidic chloride solution

The stress corrosion cracking (SCC) of AISI 321 stainless steel in acidic chloride solution was studied by slow strain rate (SSR) technique and fracture mechanics method. The fractured surface was characterized by cleavage fracture. In order to clarify the SCC mechanism, the effects of inhibitor KI on SCC behaviour were also included in this paper. A study showed that the inhibition effects of KI on SCC were mainly attributed to the anodic reaction of the corrosion process. The results of strain distribution in front of the crack tip of the fatigue pre-cracked plate specimens in air, in the blank solution (acidic chloride solution without inhibitor KI) and in the solution added with KI measured by speckle interferometry (SPI) support the unified mechanism of SCC and corrosion fatigue cracking (CFC).     

火电机组锅炉受热面合金烟气腐蚀与应力协同作用失效行为研究进展

为响应国家提出的2030年碳达峰及2060年碳中和的目标, 火电发电机组蒸汽参数(如蒸汽温度与压力)尚需进一步提高, 这无疑为火电机组的安全运行带来严峻挑战。作为影响火电机组锅炉安全服役的两个重要因素, "高温烟气腐蚀"与"应力"协同作用导致锅炉受热面材料失效常被简化或独立研究。本文首先从烟气腐蚀和应力失效着手, 简述锅炉受热面合金烟气腐蚀机理, 分析影响烟气腐蚀性能的材料因素(金属种类、合金元素、金属表面状态)及环境因素(温度、腐蚀气氛及煤灰成分), 并从腐蚀与应力相互影响的角度, 讨论火电机组受热面合金在高温烟气腐蚀与应力协同作用下的失效行为。此类合金虽然在高温烟气腐蚀环境中不存在应力腐蚀开裂倾向, 但腐蚀产物会显著影响材料的高温蠕变及持久寿命。同时, 应力造成的缺陷会改变材料的腐蚀过程。因而, 本文重点介绍了火电机组锅炉环境下, 高温烟气腐蚀与应力协同作用对不同材料性能的影响, 并以18-8型奥氏体钢Super304H为例, 完整分析从烟气腐蚀行为到腐蚀与应力耦合作用行为。最后, 对未来烟气腐蚀与应力之间的相互作用及协同作用下材料的失效机理进行展望。