Environmentally assisted cracking of aluminium alloys

A short review of the stress corrosion cracking (SCC) behaviour of aluminium alloys is given. Mechanisms of environmentally assisted cracking are outlined. For aluminium alloys, in which stress corrosion cracks propagate predominantly along grain boundaries, anodic dissolution and hydrogen embrittlement have been proposed. Transgranular stress corrosion cracking occurring at severe loading conditions has found particular interest concerning localised corrosion‐deformation interactions. Accelerated test methods for assessing the SCC behaviour are described, including the slow strain rate testing technique and the breaking load method. Results of recent studies on environmentally assisted cracking of aluminium alloys are summarised. Most of the work published in the last two decades has been on aluminium‐lithium based alloys and improved high strength Al‐Zn‐Mg‐Cu alloys in corrosion resistant retrogressed and re‐aged tempers.     

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.     

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.     

Corrosion Protection by electro-deposited aluminium

Aluminium can be deposited on various substrates by using a nonaqueous organic electrolytic system. The metallic deposit has a very high purity and good corrosionprotection properties. It can be used as a substitute for the highly toxic cadium, which is dangerous to the environment. Possible fields of application are protection against corrosion of highstrength steels without any danger of hydrogen embrittlement, the coating of lightweight materials like aluminium, magnesium and titanium alloys and the fabrication of fibre-reinforced metal matrix composites. MBB have installed a facility for the electro-deposition of aluminium and have worked in this field for more than three years. This paper discusses the experience and experimental results of the new technology and the possible advantages compared with other techniques.     

Electrochemical corrosion and mechanical behaviors of the charged magnesium

The electrochemical corrosion and mechanical behaviors of the charged magnesium were investigated using Mott–Schottky (M–S) test and slow strain rate test (SSRT), respectively. The results showed that the hole carrier and vacancy concentrations in the corrosion film increased after cathodic charging due to the formation of magnesium hydroxide. The increasing vacancy concentration caused the increase of the permeation rate of hydrogen to the interior of matrix. When the inner stress caused by synergistic effect of hydrogen pressure and expansion stress of the formation of magnesium hydride was above the fracture strength, crack initiated and propagated. It indicated that hydrogen embrittlement (HE) mechanism for the stress cracking corrosion (SCC) of magnesium and its alloys. After cathodic charging, the corrosion resistance and mechanical properties of matrix deteriorated.     

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 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.     

对非连续增强铝基复合材料的腐蚀、应力腐蚀断裂与氢脆研究的评述

本文就国内外对非连续增强铝基复合材料,主要是SiC、Al_2O_3颗粒和晶须增强铝基复合材料的腐蚀、应力腐蚀断裂和氢脆的研究现状和主要结论进行了介绍和评价,并为今后的进一步研究提供了一些思路。复合材料的腐蚀形态与铝合金相似,但复合材料的增强体对应力腐蚀断裂和氢脆的、作用不同研究者有不同的结论。     

Studies on hydrogen embrittlement of nitrogen containing austenitic alloys

Abstract

The influence of hydrogen on the mechanical properties of four austenitic CrNiMo and CrMn alloys with nitrogen contents up to 0.57 wt-% were investigated concerning their resistance against hydrogen embrittlement in correlation to nitrogen content, chemical composition, and hydrogen diffusivity and solubility. Tensile tests were carried out at room temperature and at strain rates between 10^-4 and 10^-6 s^-1 with simultaneous hydrogen charging and after precharging to different hydrogen contents. The mechanical properties have been found to be dependent on both strain rate and hydrogen content in the material. Simultaneous hydrogen charging and mechanical testing have accelerated the failure process owing to a faster hydrogen uptake especially at low strain rates. Hydrogen has shown a softening effect in the elastic range of the stress strain curve, which becomes more pronounced with decreasing strain rate. The favoured mechanism of hydrogen damage of alloys with high nitrogen contents seems to be the hydrogen enhanced localised plasticity mechanism. A correlation between hydrogen embrittlement and stress corrosion cracking has been discussed. It has been established that both the damaging processes in nitrogen containing steels are influenced by the same mechanism.     

Evidence on the corrosion-induced hydrogen embrittlement of the 2024 aluminium alloy

The present work aims to provide evidence of corrosion‐induced hydrogen embrittlement of the aircraft aluminium alloy 2024. An extensive experimental investigation involving metallographic and fractographic analyses as well as mechanical testing was performed. The corrosion exposure led to a moderate reduction in yield and ultimate tensile stress and a dramatic reduction in tensile ductility. Metallographic investigation of the specimens revealed a hydrogen‐rich embrittled zone just below the corrosion layer. Furthermore, fractographic analyses showed an intergranular fracture at the specimen surface followed by a zone of quasi‐cleavage fracture and further below an entirely ductile fracture. Mechanical removal of the corroded layers restored the yield and ultimate stress almost to their initial values but not the tensile ductility. The tensile ductility was restored to the level of the uncorroded material only after heat treatment at 495°C. Measurement of hydrogen evolution with temperature showed that by heating the corroded alloy at 495°C, the trapped hydrogen is released.     

Microstructural study of a high-strength stress-corrosion resistant 7075 aluminium alloy

A heat-treatment procedure providing for enhanced stress-corrosion cracking resistance without any sacrifice of yield strength in 7075 aluminium alloy is investigated using transmission electron microscopy. It is suggested that the heat treatment (known as retrogression and re-ageing) provides for large grain-boundary precipitates and coherent matrix precipitates. The latter provides for the high strength levels while the grainboundary precipitates provide for enhanced stress-corrosion cracking resistance. A hydrogen embrittlement mechanism of stress-corrosion cracking is assigned to this alloy system.     

Engineering property comparisons of 7050-T73651, 7010-T7651 and 7010-T73651 aluminium alloy plate

A comparison of some engineering properties of 7050-T73651, 7010-T7651 and 7010-T73651 plate has been made. The properties investigated were strength, stress corrosion resistance, fracture toughness and fatigue crack propagation resistance under flight simulation loading.

It was found that both 7050 and 7010 are high strength deep hardenable alloys with only minor differences in crack tolerance properties. The fracture toughness of both alloys is equivalent, while 7050 possesses slightly better resistances to stress corrosion cracking and fatigue crack propagation under flight simulation loading.     

Corrosion behaviour of aluminium alloys in ethanol fuels

In this study, the corrosion behaviour of several aluminium alloys in ethanol fuels was investigated by immersion and polarization tests. The corrosion properties of cast aluminium alloys (Al–17wt%Si–4wt%Cu–Mg, Al–8wt%Si–3wt%Cu, Al–7wt%Si–Mg and Al–17wt%Si–4wt%Cu–Mg with a chemically deposited nickel layer) in ethanol blended gasoline fuels were examined at various ethanol and water contents and various temperatures. Electrochemical and gravimetric measurements revealed a pronounced acceleration of the corrosion process above the boiling point. Additions of water restrain the corrosion. Increasing the ethanol content and the temperature leads to a higher corrosion sensitivity of the aluminium alloys. Furthermore, the nickel layer is very protective in all tested fuels. For aluminium alloys, a theory of the corrosion process in ethanol blended gasoline fuels is proposed.     

Mechanical properties in the semi-solid state and hot tearing of aluminium alloys

This review represents a comprehensive coverage of results reported in the literature over last 50 years on the methods of studying hot tearing and mechanical properties of semi-solid aluminium alloys; the mechanical properties of these alloys in the semi-solid state; and hot tearing criteria. While compiling this review, the authors attempted to include in it all available sources including quite a few works never published in English before. The review consists of three parts. The first part introduces the reader to the phenomenon of hot tearing. The second part describes different techniques for testing metallic alloys in the semi-solid state and summarizes reported results on strength and ductility of semi-solid model and commercial aluminium alloys. The third part describes the methods for assessing hot tearing susceptibility of aluminium alloys, gives the results on hot cracking of various aluminium alloys and discusses different hot tearing criteria.     

Stress Corrosion Crack Growth in Pipe Grade Steels in Near Neutral pH Environment

This paper is concerned with an accelerated testing and modeling of stress corrosion cracking (SCC) phenomena in pipe grade steels in near neutral pH environment. In modeling of SCC, the authors adopt the crack layer theory that provides formalism to account for contributions to crack growth rate such processes as electro-chemical corrosion, hydrogen embrittlement and mechanical loading. Special attention is paid to the hydrogen diffusion, a precursor to hydrogen embrittlement. The energy-momentum tensor (Eshelby's tensor) is employed to evaluate the thermodynamic forces responsible for SC crack growth. Griffith' crack equilibrium condition is used to derive a quasi-equilibrial SC crack growth equation. A parametric study and comparison with the experimental results of corrosion fatigue tests for various maximal stress, stress ratio and electric potential are performed to examine the validity of the proposed model.     

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.     

A filiform corrosion and potentiodynamic polarisation study of some aluminium alloys

The aim of the present investigation is a combined study of filiform corrosion of aluminium alloys by accelerated exposure tests and potentiodynamic polarisation measurements. The accelerated exposure tests are performed on binary Al-Cu, Al-Mg, Al-Si and Al-Zn model alloys, a ternary Al-MgSi alloy and on the two commercial alloys, AA2024-T351 and AA7075-T651, with variations of composition and surface treatments. The surface treatments cover simple degreasing, chromate and cerium based treatments. A trend of a higher filiform corrosion susceptibility with increasing alloying elements concentrations was observed for all model systems. Furthermore, the filiform corrosion susceptibility varies with the solute atom, in particular Cu was found to have a detrimental effect on the filiform corrosion properties. Both chromating and cerating improve the filiform corrosion resistance of the alloys significantly. To explain the trends observed in the exposure tests, polarisation measurements were performed on the untreated Al-Cu and Al-Zn alloys in bulk anolyte and catholyte solutions which are characteristic for the local anodic and cathodic sites in the filaments on the aluminium substrates. From these measurements a filiform corrosion current, defined as the intercept of the anodic and cathodic curves, can be determined. The present set of experiments shows a correlation between the filiform corrosion properties during accelerated exposure tests and the potentiodynamic polarisation measurements for the Al-Cu alloys. When comparing the results for the Al-Cu and Al-Zn binary alloys it can be concluded that the correlation factor differs significantly with the solute atom and the filiform corrosion current proves to be a non-uniquely discriminating parameter for the filiform corrosion susceptibility of the model alloys. The difference in correlation factor for the Al-Cu and Al-Zn alloys is attributed to differences in the electrochemical behaviour of these alloys with local variations in substrate composition. For the Al-Cu and Al-Zn model alloys the filiform corrosion initiation characteristics are related to the passive range and thus implicitly to the ease of pitting of the alloy. A smaller passive range corresponds to a higher filiform site density for both the Al-Cu and Al-Zn alloys.     

Material influence on the stress corrosion cracking of rock bolts

Rock bolt stress corrosion cracking (SCC) has been investigated using the linearly increasing stress test (LIST). One series of experiments determined the threshold stress of various bolt metallurgies (900 MPa for 1355AXRC, and 800 MPa for MAC and MA840B steels). The high values of threshold stress suggest that SCC begins in rock bolts when they are sheared by moving rock strata. SCC only occurred for environmental conditions which produce hydrogen on the sample surface, leading to hydrogen embrittlement and SCC. Different threshold potentials were determined for a range of metallurgies.Cold work was shown to increase the resistance of the steel to SCC. Rock bolt rib geometry does not have a direct impact on the SCC resistance properties of the bolt, although the process by which the ribs are produced can introduce tensile stresses into the bolt which lower its resistance to SCC.     

Environmental Embrittlement in A_3B-type Intermetallic Alloys

Environmental embrittlement in A3B-type intermetallics based on Ni3Al and Fe3Al has been studied in this paper. For the Ni3Al doped with 120 wt ppm B and Ni,(Al,Cr.Zr) doped with 80 wt ppm B,their elongation and ultimate tensile strength decreased in the sequence:of vacuum > air >hydrogen. while for Ni,(Al,Mn) doped with 400 wt ppm B no envifonmental degradation was ob served, although a -Ni3(Al,Mn) alloy without B showed a decrease in ductility when tested in air in stead of oxygen. It is supposed that boron and hydrogen compete for the occupation of interstitial sites near grain boundaries. If boron content is sufficiently low, hydrogen embrittlement occurs ;however, if its content is sufficiently high. boron addition is capable of eliminating envjronmental ef fect in Ni3Al-based alloysi As to the micromechanism of hydrogen embrittlement in Ni3Al+B. S EM in situ observations showed that both grain boundary decohesion and a high stress concentration con tributed to hydrogen-assisted jntergranu lar cracking in this alloy. For the Fe3Al and Fe3 (Al.Cr) alloys.their mechanical properties depended strongly on grain size / grain shape and testing environment. A strain rate effect on ductiIity and fracture strength was also observed in the Fe3Al and Fe,(Al,Cr)+B aIloys. Preoxidation increased the ductility of the Fe,(Al,Cr)+B alloy. All these results can be rationalized from a hypothesis that surface reaction is the controlling process in embrittling Fe3Al-based alloys.