96 Corrosion And Hydrogen Embrittlement 961 Introduction

Corrosion presents a major threat to the structural integrity of aging aircraft structures. As the time of an aircraft structure 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 in the form of widespread cracking at regions of high-stress gradients; it can result in loss of structural integrity and may lead to fatal consequences. Thus, the effect of corrosion on the damage tolerance ability of advanced aluminum alloys calls for a very diligent consideration of the problems associated with the combined effect of corrosion and embrittling mechanisms. There has been, recently, an increasing attention of basic research and development concerning structural integrity taking into account the related corrosion aspects[29-31, 36]. It has been realized that the establishment of damage functions for quantifying the simultaneous accumulation of corrosion and fatigue-induced damage is very complex and difficult. Therefore, despite the advancements in modeling fatigue crack growth [32-35] and multiple-site damage phenomena [29-31], the assessment of structural degradation in aging aircraft is still relying heavily on test data. To face the corrosion-induced structural degradation issue, available data usually refer to accelerated laboratory corrosion tests and, more rarely, to in-nature atmospheric or marine exposure corrosion tests.

With the exception of the atmospheric corrosion test where, according to the relevant specification the tensile properties of corroded specimens are measured as well, these tests are used for evaluating the corrosion susceptibility of the materials by measuring weight loss and characterizing depth and type of corrosion attack. The above methodology toward understanding corrosion susceptibility of a material does not relate corrosion to their effect on the materials mechanical behavior and residual properties. Yet, it is exactly these missing data that are needed to face structural integrity problems of corroded aircraft components. Corrosion-induced mechanical degradation studies have been based mainly on the results of stress corrosion cracking tests [37, 38] or, more rarely on the results of fatigue tests performed in the presence of a corrosive environment [29, 30]. Both types of tests provide useful results; they refer, however, to the case where a material is loaded in a corrosive environment but not to situations where a corroded material is subjected to mechanical loads. Present-day considerations of the corrosion-induced structural degradation relate the presence of corrosion with a decrease of the load-bearing capacity of the corroded structural member [36, 39].

This decrease is associated with the presence of corrosion notches that lead to local increase of stress promoting fatigue crack initiation as well; in addition, corrosion-induced reduction of the members' load-bearing thickness which, in the case of the thin alloy skin sheets, may be essential, can lead to appreciable increase of stress gradients [39]. Corrosion-induced material embrittlement is not accounted for. The above consideration of the corrosion-induced structural integrity issue is consistent with the classical understanding of the corrosion attack of aluminum alloys as the result of complex oxidation processes at the materials surface [40].

Regarding corrosion-induced material embrittlement, Pantelakis et al. [41, 42] claimed that hydrogen embrittlement could be responsible for the dramatic degradation of toughness and ductility of 2091 and 8090 Al-Li alloys as well as conventional 2024-T3 alloy in several types of accelerated corrosion tests. In other alloy systems there is mounting evidence connecting embrittlement and stress corrosion cracking to hydrogen penetration. Speidel [37] reviews recent results, mainly for Al-Mg-Zn alloys. Studies by Scamans et al. [43] of Al embrittlement in humid air, point to the major role of hydrogen. In particular, the intergranular crack path and the reversibility of the phenomenon (recovery of ductility after degassing) support a hydrogen, rather than an anodic dissolution, mechanism. Also, Scamans and Tuck [44] measured H2 permeability and stress corrosion resistance of the Al-Mg-Zn alloy, as functions of quench rate and aging treatment, and found similar trends. However, the stress-corrosion-resistant Al-Mg-Si alloy does not allow hydrogen permeation through its matrix, though the volume of hydrogen produced by surface reaction with the water in humid air is even higher than that of the Al-Mg-Zn alloy [44]. It has been suggested [37] that hydrogen plays a major role in stress corrosion cracking of aluminum alloys exposed to aqueous solutions as well. An indication in favor of this argument is provided by measurement, in Al-Mg-Zn alloys, of hydrogen permeation [45] and stress corrosion crack growth rates [46]. These parameters are found to vary similarly as functions of the electrode potential. Despite the lack of a universally accepted hydrogen embrittlement mechanism, a generally recognized common feature is that some critical concentration of hydrogen must buildup at potential crack sites, for failure to initiate. Thus, the distribution of hydrogen inside the metal and its pattern of migration are of paramount importance in understanding the phenomena and designing alloys with improved behavior.

It has been shown [47, 48] that lattice defects (vacancies, dislocations, grain boundaries) and precipitates provide a variety of trapping sites for diffusing hydrogen. Hydrogen traps have mechanistically been classified by Pressouyre [49] as reversible and irreversible, depending on the steepness of the energy barrier needed to be overcome by hydrogen to escape from the trap. For example, during a degassing experiment reversible traps will release hydrogen continuously, while irreversible ones will do so only after a critical temperature has been reached. This is the temperature at which the probability of a single jump out of the steep trap becomes nonnegligible. Reversible and irreversible traps may play different roles during an actual experiment [50]. In particular, irreversible traps will always act as sinks for hydrogen, whereas reversible traps may act as sinks or sources depending on initial hydrogen charging of the lattice. A uniform distribution of irreversible traps is believed to provide a beneficial effect in alloy behavior under embrittling conditions, by arresting diffusing hydrogen and thus delaying its buildup at the crack sites [51]. When crack nucleation and growth is along the grain boundaries, boundary chemistry may be playing an important role. Various studies on Al-Mg-Zn alloys [52-54] have indicated that alloying elements (and in particular Mg) are segregated on the grain boundary. Tuck [55] proposed that Mg hydride forms at grain boundaries and is responsible for material embrittlement. In an effort to explain the connection between Mg-H interaction and material embrittlement, Song et al. [56] recently showed that stress corrosion and fatigue crack growth rates increase with the concentration of solid solution Mg on grain boundaries. The same authors theoretically calculated a decrease in the intergranular fracture work with both Mg and H segregation.

Useful insight in the nature and intensity of hydrogen traps can be offered by studying the temperature needed to break these bonds. Thus, thermal analysis techniques have been used for a variety of alloys [55, 57]. In particular, thermal desorption has been successfully used to study hydrogen partitioning in pure cast aluminum [58] and in Al-Cu and Al-Mg2Si alloys [47] and hydrogen diffusion in Al-Li alloys [59]. Among other findings, these studies show that, for aluminum alloys, the energy of chemisorption is lower than the energy for lattice diffusion. Thus, the layer of passive oxide—formed on the surface of aluminum alloys—does not mask the bulk trapping states, and the results of thermal analysis are meaningful. Accelerated corrosion tests were recently used by Haidemenopoulos et al. [60] to characterize corrosion and hydrogen absorption in the less studied but widely used Al-Cu alloy 2024. In [61] hydrogen evolution from the corroded specimen of Al alloy 2024 was systematically measured as a function of temperature. The exfoliation test [62] was used as an accelerated corrosion method, and different exposure times were tested. The existence of multiple trapping states was verified and the quantity and evolution pattern of hydrogen is discussed.

The investigations summarized above indicate that characterization of corrosion susceptibility should involve information on the residual mechanical properties of a structural material following exposure to corrosive environment. In the following the effect of corrosion and hydrogen embrittlement on the mechanical behavior of aluminum aircraft alloys is discussed. The work is based on extended, new experimental data of the aircraft aluminum alloys 2024, 6013, 2091, and 8090. A short overview of the currently used corrosion resistance characterization procedures is first made along with a critical appraisal of their suitability for characterization of aluminum alloy corrosion susceptibility. Evaluation of corrosion resistance is performed based on the tensile behavior following several types of corrosion tests; to the evaluation conventional metallographic and stereoscopic characterization of corroded specimens is also employed. The results are referred to current considerations of the effect of corrosion on structural integrity analysis of aged aircrafts. Results on the fatigue behavior of corroded 2024 alloy specimens are presented as well. The results include stress-life (S-N) curves, as well as, fatigue crack growth tests for several R ratios. Finally, the obtained results are discussed under the viewpoint of hydrogen embrittlement. For investigating the possible links between material embrittlement and corrosion-induced hydrogen evolution, experimental determination of hydrogen uptake has been performed followed by controlled heating experiments in order to determine the hydrogen trapping states in the material.

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