Corrosion-induced hydrogen embrittlement in aluminum alloy 2024

doi:10.1016/j.corsci.2005.05.015

Corrosion Science

Volume 48, Issue 5, May 2006, Pages 1209-1224

https://doi.org/10.1016/j.corsci.2005.05.015 Get rights and content

Abstract

The present paper focuses on the observed corrosion-induced embrittlement of alloy 2024 and tries to answer the key question on whether the observed embrittlement is attributed to hydrogen uptake and trapping in the material. Hydrogen is produced during the corrosion process and is being trapped in distinct energy states, which correspond to different microstructural sites. The formation of a hydrogen-affected zone beneath the corrosion layer is supported by fractographic analysis. Removal of the corrosion layer leads to complete restoration of yield strength but only partial restoration of ductility. Additional heat treatment to release the trapped hydrogen leads only to complete restoration of ductility.

Introduction

The structural integrity of aging aircraft structures can be affected by corrosion. As the time of an aircraft in service increases, there is a growing probability that corrosion will interact with other forms of damage, such as single fatigue cracks or multiple-site damage. The aging aircraft may have accumulated corrosion damage over the service life and its residual strength depends on possible degradation stemming from corrosion-induced embrittling mechanisms. One characteristic example where failure was attributed to multi-site damage (MSD) has been the Aloha Airlines accident in 1988. Damage was attributed to growth and linkage of multiple fatigue cracks, emanating from rivet holes [1]. Recent investigations on fire fighting planes of the Hellenic Aerospace Industry has also shown considerable corrosion damage around rivet holes [2].

There are two key questions regarding this issue: (1) Is there a corrosion-induced degradation of ductility, which in turn degrades damage tolerance and the residual strength of aerostructures? and (2) What is the underlying corrosion-induced embrittling mechanism? The answer to the first question has been given by a series of experiments [3], [4], [5], involving mechanical testing of pre-corroded alloy 2024. It was shown that (i) degradation of ductility and of fatigue life increases with corrosion exposure time and (ii) removal of the corrosion layer restores strength but not ductility. These results were attributed to the operation of a bulk corrosion-induced embrittlement mechanism, and it was suggested that hydrogen might be a possible underlying cause.

Other researchers have also considered hydrogen as an embrittlement mechanism in Al-alloys. Studies by Scamans et al. [6] of Al embrittlement in humid air, argued for a major role of hydrogen. In particular, the intergranular crack path and the reversibility of the phenomenon (recovery of ductility after degassing) supported a hydrogen rather than an anodic dissolution mechanism. Also, Scamans and Tuck [7] measured hydrogen permeability and stress corrosion resistance of the Al–Mg–Zn alloy, as functions of quench rate and aging treatment, and found similar trends. Speidel [8] reviewed results up to 1992, mainly for Al–Mg–Zn alloys. More recently, Young and Scully [9] considered the kinetics of crack growth of aluminum alloy 7050 in a humid air environment and confirmed that hydrogen embrittlement was the controlling mechanism. Jones [10] summarized evidence for hydrogen uptake and its contribution to crack growth for the low-strength alloy 5083.

Hydrogen is produced by surface corrosion reactions and part of it is absorbed in atomic form into the material [11]. In particular, the production of atomic hydrogen by a single-electron transfer process, according to the reactionH₂O + e⁻ → OH⁻ + Hmakes water an aggressive environment for aluminium alloys [9]. Absorbed hydrogen diffuses towards the interior of the material and may be retained at various preferential locations. More specifically, it has been shown [12], [13] that lattice defects (vacancies, dislocations, grain boundaries) and precipitates provide a variety of trapping sites. Hydrogen traps are mechanistically classified as reversible and irreversible [14], depending on the steepness of the energy barrier needed to be overcome by hydrogen to escape from them.

Thermal desorption has been successfully used to study hydrogen diffusion and trapping in pure aluminium [15], [16], Al–Cu, Al–Mg₂Si [13] and Al–Li alloys [17]. It has also been combined with accelerated corrosion tests in order to characterize corrosion and hydrogen absorption in alloy 2024 [18], [19]. In the last two works, the existence of multiple trapping states was verified and the quantity and evolution pattern of hydrogen was discussed. The goal of the present study is to link hydrogen uptake and trapping to material embrittlement.

Section snippets

Experimental procedures

The material used for the present study was alloy 2024-T351, supplied in thicknesses 1.6–3.0 mm. The chemical composition of the alloy (wt.%) is: Al–4.35Cu–1.5Mg–0.64Mn–0.5Si–0.5Fe. Exfoliation corrosion testing (EXCO) was performed according to ASTM specification G34-90 [20]. It included exposure at 25 ± 0.5 °C, in a solution containing 234 g NaCl, 50 g KNO₃ and 6.3 ml concentrated HNO₃ (70 wt.%) diluted to 1 L of distilled water. Exposure times in the EXCO solution ranged from 15 min to 96 h. Specimen

Microstructural description of corrosion

Corrosion in this alloy starts in the form of pitting. Pits begin to appear as early as after 15 min of exposure and are mainly located at intersections of cracks in the protective surface oxide layer (Fig. 1a). With exposure time pits become deeper and start to be connected by a network of intergranular corrosion paths (Fig. 1b). This process of pit-to-pit interactions leads to pit clustering and coalescence (Fig. 1c). From that point on, corrosion does not penetrate much deeper but instead

Conclusions

The experiments performed in this work lead to the following conclusions regarding corrosion-induced hydrogen embrittlement in aircraft Al-alloys:

1.
Corrosion damage starts with pitting and proceeds to pit-to-pit interactions, intergranular attack and exfoliation.
2.
Hydrogen is produced during the corrosion process and is being trapped in distinct states in the interior of the material.
3.
The temperature of hydrogen evolution and the variation of hardness with heating provide indirect evidence of the

Acknowledgment

Part of this work has been financially supported by the Greek Secretariat of Research and Technology (GSRT) and by AirBus.

References (26)

S.G. Pantelakis et al.
Tensile and energy density properties of 2024, 6013, 8090 and 2091 aircraft aluminum alloy after corrosion exposure
J. Theor. Appl. Mech.
(2000)
G.M. Scamans et al.
Corros. Sci.
(1976)
D.E Azofeifa et al.
Determination of hydrogen absorption in Pd coated Al thin films
Thin Solid Films
(1997)
G. Itoh et al.
Evidence for the transport of impurity hydrogen with gliding dislocation in aluminium
Scr. Mater.
(1996)
G.A. Young et al.
The diffusion and trapping of hydrogen in high purity aluminium
Acta Mater.
(1998)
E. Charitidou et al.
Characterization of trapped hydrogen in exfoliation corroded aluminium alloy 2024
Scr. Mater.
(1999)
T. Ishikawa et al.
The diffusivity of hydrogen in aluminium
Acta Metall.
(1986)
P. Sofronis et al.
Hydrogen induced shear localization of the plastic flow in metals and alloys
Eur. J. Mech.—A/Solids
(2001)
Y. Liang et al.
Interaction of hydrogen with crack-tip plasticity: effects of constraint on void growth
Mater. Sci. Eng. A
(2004)
Aloha Airlines, Flight 243, NTSB Report Number AAR-89-03, Washington, DC, adopted on June 14,...

Z. Riga, Hellenic Aerospace Industry, private communication,...

S.G. Pantelakis et al.

Effect of corrosive environment on the mechanical behavior of the advanced Al–Li alloys 2091 and 8090 and the conventional aerospace alloy 2024

METAL

(1993)

A.T. Kermanidis, Ph.D. Thesis, Department of Mechanical Engineering and Aeronautics, University of Patras, Greece,...

Cited by (128)

The role of lithium in hydrogen trapping and embrittlement of Al-Cu-Li alloys: Experimental study and DFT calculations
2024, Journal of Alloys and Compounds
The role of lithium in hydrogen embrittlement (HE) and trapping of Al-Cu-Li alloys were investigated in experimental and density functional theory (DFT) calculations. The tensile curves and fracture morphology show that the Li-contained samples are more sensitive to HE than the Li-free samples and the HE sensitivity increases with the aging time expansion. The Transmission Electron Microscope (TEM) analyses reveal that the number density of T₁ (Al₂CuLi) precipitates increases about 66.8% with aging process, suggesting that HE sensitivity is exacerbated by T₁ precipitates. In addition, the lithium in T₁ precipitates, solid solution matrix and solute segregation grain boundaries (GBs) always has a high affinity to hydrogen according to DFT calculations, where T₁ precipitates have the highest hydrogen trapping energy with 0.736 eV/H atom. Moreover, the influence of hydrogen on Al-Cu-Li alloys was investigated using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and electrochemical corrosion analysis. These analyses indicate that hydrogen exists in the form of hydrogen atoms, which results in a reduction in corrosion potential (E_corr) from −676.3 mV to −699.3 mV by approximately 20 mV and an increase in corrosion current density (i_corr) from 2.1×10⁻⁶ A/cm² to 10.8×10⁻⁶ A/cm² by an order of magnitude. The accumulated hydrogen atoms eventually lead to corrosion pits and blisters on the surface of alloys.
Stress corrosion cracking induced by the combination of external and internal hydrogen in Al-Zn-Mg-Cu alloy
2024, Scripta Materialia
Stress corrosion cracking (SCC) behaviors induced by the combination of external and internal hydrogen (H) in an Al-Zn-Mg-Cu alloy were systematically investigated via in-situ 3D characterization techniques. SCC of the Al-Zn-Mg-Cu alloy could initiate and propagate in the potential crack region where the H concentration exceeded a critical value, in which the nanoscopic H-induced decohesion of η-MgZn₂ precipitates resulted in macroscopic cracking. External H that penetrated the alloy from the environment played a crucial role during the SCC of the Al-Zn-Mg-Cu alloy by generating gradient-distributed H-affected zones near the crack tips, which made Al alloys in water environment more sensitive to SCC. Additionally, the pre-existing internal H was driven toward the crack tips during plastic deformation. It was involved in the SCC and made contributions to both the cracks initiation and propagation.
The role of hydrogen in the corrosion-induced reduction of plane-stress fracture toughness and strain-induced intergranular cracking of AA2024
2024, Procedia Structural Integrity
The role of diffusible hydrogen in the embrittlement of AA2024 after short-term exposure in the standard exfoliation corrosion (EXCO) test solution (ASTM G34) was investigated. Slow strain rate KR-curves were established – utilizing the unloading compliance method of the ASTM E561 Standard – on 3.2 mm thick compact tension (C(T)) specimens for the following four (4) sets of samples: (i) as-received (unexposed), (ii) 2 h EXCO exposed, (iii) unexposed and heat-treated, and (iv) 2 h EXCO exposed and heat-treated samples of AA2024. A significant degradation (≈ 12.0 ± 1.8 %) was observed in the effective slow strain rate KC toughness after short-term exposure of the AA2024-T3 specimens to the EXCO-solution. Post-exposure heat-treatments appear to have restored the plane-stress fracture toughness to its original values. The formation of secondary and primary intergranular cracks in the plastic zone of the C(T) samples were studied using SEM. The presence of intergranular secondary surface cracks in the plastic zones of the C(T) samples was, however, not altered by the heat treatment, and did not appear to influence the fracture toughness results. Thermal desorption mass spectroscopy was exploited to evaluate the extent of hydrogen absorption due to the corrosive exposure, and the effect of the subsequent heat treatment in removing it.
The fatigue crack propagation in nanocrystalline materials in the hydrogen environment
2023, International Journal of Fatigue
In this article, the effect of the external corrosion environment on dislocation emission is introduced, and the microscopic mechanism of fatigue crack propagation (FCP) in a hydrogen environment is studied. Meanwhile, based on the discrete distributed dislocation method, the corrosion FCP model of nanocrystal materials is established. In the framework of our model, the FCP is mainly determined by the irreversibility of the crack tip dislocation. The research results indicate that the aggregation of hydrogen atoms at the crack tip (CT) promotes the dislocations emission and increases the FCP rate. Meanwhile, considering the application of materials in different hydrogen circumstances, the effects of changes in the distance between hydrogen atoms and the crack tip and the radius of hydrogen atoms on crack propagation rate are studied. The results show that the accumulation of hydrogen atoms at the crack tip promotes the fatigue crack growth rate in nanocrystalline materials. Compared with the air environment, the maximum increase in crack propagation rate is about 45% under the action of hydrogen environment. Meanwhile, increasing the grain size and dislocation emission angle helps to increase the crack propagation rate. The research in this paper provides a theoretical basis for evaluating the corrosion fatigue life of metal materials.
Effects of Sn addition on the improved hydrogen intrusion and emission behaviors in 1.5-GPa-grade Al-Si-coated hot press forming steels
2023, Journal of Alloys and Compounds
High-strength hot press forming (HPF) steel sheets are coated mostly by Al-Si to prevent the decarburization or oxidation of sheet surfaces, but become vulnerable to hydrogen embrittlement (HE) as H atoms supplied from moisture in a furnace penetrate through the molten coating into the steel substrate during the HPF process. In this study, 0.1-wt% of Sn was added to a 1.5-GPa-grade reference HPF steel (referred to as 0.1Sn and Ref steels), and their coating microstructures including Sn-enriched zone and Kirkendall and surface voids were examined after laboratory-scale HPF simulation tests. The content of diffusible H atoms immediately after the HPF simulation at 900 °C was 0.111 wt.ppm in the Ref sheet, decreased slowly as the elapsed time increased, and remained to be 0.030 wt.ppm even after 17 days. In the 0.1Sn sheet, it was about half of that of the Ref sheet (0.061 wt.ppm), and decreased rapidly to nil after 3 days. These results indicated that the 0.1Sn steel showed excellent H-intrusion and emission behavior, which would favorably work for better resistance to HE, by optimally controlling the multi-mechanisms of enrichment of Sn solutes and population of coating voids. The presence of Sn-enriched zone and voids played an important role in blocking the H intrusion to the substrate and in accelerating the H emission to the coating surface, respectively.
Addressing the hydrogen entrapment in Al2024 alloys using scanning transmission electron microscopy and density functional theory
2023, Journal of Alloys and Metallurgical Systems
The effect of precipitate coherency in an aged aluminum (Al) 2024 alloy is investigated using scanning transmission electron microscopy (STEM) and density functional theory (DFT) methods. The focus of the study is on understanding the effects of hydrogen (H₂) entrapment in the alloy, which leads to the degradation of its corrosion resistance. We employed STEM to analyze the structure and strain properties of the alloy at high spatial resolution. This has been achieved by applying the geometrical phase analysis (GPA) to the acquired HRSTEM images. The determination of strain field around the precipitates was utilized to gauge the degree of coherency at the interface of the precipitates with Al matrix. We also applied density functional theory (DFT) simulations to investigate the interfacial energy at the interfaces of the precipitates with the Al matrix. The obtained results of the DFT simulations showed that the interfacial energy at the semi-coherent precipitates was determined to be 0.06 eV per atom, which was higher compared to the coherent precipitates. Our findings suggest that the semi-coherent precipitates have a less stable interface with the Al matrix. Furthermore, the study revealed that the H₂ entrapped at the semi-coherent precipitate regions exhibits a binding energy which is 3.2 eV lower than that of the coherent precipitates. This implies that hydrogen binds more strongly at the semi-coherent precipitate cites, which enhances the process of the alloy embrittlement.

View all citing articles on Scopus

View full text

Corrosion-induced hydrogen embrittlement in aluminum alloy 2024

Abstract

Introduction

Section snippets

Experimental procedures

Microstructural description of corrosion

Conclusions

Acknowledgment

J. Theor. Appl. Mech.

Corros. Sci.

Thin Solid Films

Scr. Mater.

Acta Mater.

Scr. Mater.

Acta Metall.

Eur. J. Mech.—A/Solids

Mater. Sci. Eng. A

Effect of corrosive environment on the mechanical behavior of the advanced Al–Li alloys 2091 and 8090 and the conventional aerospace alloy 2024

METAL