Volume 37, Issue 5 - Special issue: Stress-assisted corrosion damage / Issue editors: A.K. Vasudevan, N.J. Henry Holroyd and Ronald M. Latanision

The operating environment of Navy aircraft varies to a good degree depending upon the squadron location, flight requirements, and other field and ground activities. All these conditions promote corrosion of one type or the other. The aircraft operations will also have influence on the type of corrosion. Thus, building an environment history that can monitor and track the damage development in many areas of the aircraft structure based on aircraft activities, operating environment, and service history data is crucial. The development of such environmental history builder has two main advantages: first, it provides a tool to treat corrosion as a structural issue, and second, it accounts for time variation of environmental factors such as relative humidity (RH) and temperature rather than average environmental data. This paper will demonstrate how the environmental history builder could be used, in conjunction with predictive models, to predict corrosion damage.

There have been several reviews and numerous papers on hydrogen-embrittlement phenomena and mechanisms in the past few years, but long-standing controversies regarding mechanisms of embrittlement (when hydride phases are not involved) are no closer to being resolved despite all the recent research. These controversies have arisen partly because there have been significant misunderstandings of some of the proposed mechanisms, and some misinterpretation of experimental observations. There has also been a lack of consideration of all the evidence, with some researchers ignoring observations that do not fit their favoured hypothesis. Some recent reviews and papers illustrating the above points are discussed in the present paper, although a comprehensive coverage is not attempted. It is concluded that the adsorption-induced dislocation-emission/void-coalescence mechanism predominates for cleavage-like and intergranular fracture modes that exhibit microscale or nanoscale dimples. Hydrogen-enhanced decohesion, hydrogen-enhanced localised plasticity, and vacancy-based mechanisms may play secondary roles for such fracture modes and could be more important in other cases.

The formation of small and long cracks and their propagation or arrest are treated drawing special attention to the (a) impact of environment and (b) several loading parameters ( R -ratio, stress/strain-amplitude, constant-amplitude and variable-amplitude loading, superimposed loading, ultrasonic fatigue loading, and frequency effects) for three groups of metallic materials (two high-strength steels, 7075 and 2024 Al alloys, polycrystalline copper). The influence of these parameters on lifetimes and fatigue crack propagation behavior being determined by microstructural features on all levels of magnification (ranging from several millimeters to nanometers) is presented. A review of the state of knowledge according to literature is given in the introduction. The following results were obtained, and models for their interpretation were presented: The development (growth or arrest) of small cracks into a long crack is driven by several competing processes (due to material, way of loading, environment, etc.). The environment plays a predominant role. Especially in the high-cycle and very-high cycle regime, the complexity of interacting processes needs further – mainly experimental – investigations. Some further studies relate to different loading conditions and possibilities of testing considering newly developed material. Experiments in high vacuum have to form the basis for studying environmentally assisted fatigue response.

Pitting corrosion damage is a major problem affecting material strength and may result in difficult to predict catastrophic failure of metallic material systems and structures. Computational models have been developed to study and predict the evolution of pitting corrosion with the goal of, in conjunction with experiments, providing insight into pitting processes and their consequences in terms of material reliability. This paper presents a critical review of the computational models for pitting corrosion. Based on the anodic reaction (dissolution) kinetics at the corrosion front, transport kinetics of ions in the electrolyte inside the pits, and time evolution of the damage (pit growth), these models can be classified into two categories: (1) non-autonomous models that solve a classical transport equation and, separately, solve for the evolution of the pit boundary; and (2) autonomous models like cellular automata, peridynamics, and phase-field models which address the transport, dissolution, and autonomous pit growth in a unified framework. We compare these models with one another and comment on the advantages and disadvantages of each of them. We especially focus on peridynamic and phase-filed models of pitting corrosion. We conclude the paper with a discussion of open areas for future developments.

The KBS-3 method for long-term disposal of spent nuclear fuel is designed with an external self-standing copper shell, which provides the most important barrier against corrosion and escape of radionuclides, and an internal nodular cast iron insert, which provides the load-bearing structure against external loads. The material intended for the load-bearing insert is ferritic nodular cast iron EN 1563 grade EN-GJS-400-15U. In this paper, hydrogen uptake and sensitivity to hydrogen-induced cracking of the cast iron were studied using tensile testing under continuous electrochemical charging in 1 N H 2 SO 4 solution. Hydrogen uptake was measured by using the thermal desorption method. It was found that the hydrogen desorption profile manifests three distinct peaks at initial locations of 400, 500, and 700 K with a heating rate of 6 K/min. Plastic deformation results in a remarkable increase of the 400 K peak, which indicates hydrogen uptake during deformation. In the constant extension rate tests (CERT) and the constant load tests (CLT), electrochemical hydrogen charging reduced markedly the elongation to fracture and time to fracture, respectively. In CLT, hydrogen charging increased dramatically the creep rate at the applied load of about 0.7 yield stress. Ligaments between the graphite nodules exhibit brittle cleavage facets in the presence of hydrogen, while the ligaments show a characteristic ductile appearance of shear and small dimples when testing in air or distilled water. The obtained results are discussed in terms of the known mechanisms of hydrogen-induced cracking and the role of the graphite nodules in the embrittlement of ductile cast iron.

Temper embrittlement induced by segregation of metalloid solutes to grain boundary (GB) was evaluated by a shift of the ductile-brittle transition temperature (DBTT). DBTT was found to be linearly correlated with the amount of metalloid on the GB ( X gb ) for both dynamic and static displacement rates (d δ /d t ) in high and medium hardness steels. Recent first-principles calculations have determined the GB embrittling potency (Δ e p ) of segregated Sb, Sn and P. In both high and medium hardness steels, the slope ( α ) of DBTT vs. X gb was found to be linearly dependent on Δ e p regardless of the segregated solutes. In high hardness steels, the slope is independent of d δ /d t , while in medium hardness steels the α is dependent on d δ /d t . An Arrhenius plot of d δ /d t vs. the reciprocal DBTT was used to drive the thermal activation energy ( E act ), which represents a barrier to plasticity. It was found that E act correlates to a reduction in the GB fracture surface energy. The E act depends strongly on GB decohesion in high hardness steels but only weakly depends on it in medium hardness steels.

The aim of this work is to analyze hydrogen transport in a low alloy steel by applying the electrochemical permeation technique to matrices subject to cyclic loading conditions, up to the yield strength and beyond this limit. The results indicate that, with an increase in the applied maximum stress, a decrease in the apparent diffusivity takes place, along with a marked and instantaneous reduction of diffusing hydrogen in the lattice and an increase in hydrogen solubility. An effect on the permeation current was observed, together with a variation of hydrogen diffusion kinetics ascribable to the activation on new trapping sites, with an appreciable effect under cyclic loading already at 55% of the yield limit, which becomes more relevant in the plastic domain.

Environmentally induced cracking (EIC) in a sensitized high-strength AA5083 H131 alloy has been investigated using time-lapse synchrotron X-ray computed tomography combined with post-mortem correlative characterization. Small corrosion features deliberately introduced in a pre-exposure step were found to be the site of initiation for over 95% of the 44 EIC cracks that developed under slow strain rate testing. Detailed analysis using three-dimensional electron backscatter diffraction and energy-dispersive spectroscopy analysis of a single crack confirmed the intergranular nature of the cracks from the start and that the pre-exposure corrosion was associated with an α-AlFeMnSi particle cluster. It also appears that several cracks may have initiated at this site, which later coalesced to form the 300-μm-long crack that ultimately developed. Of further note is the fact that initiation of the EIC cracks across the sample started below the yield strength and continued beyond the ultimate tensile strength. The most rapid crack propagation occurred during sample extension following a period of fixed displacement.

The role of tensile overload superimposed on a constant amplitude cycling results in compressive residual stresses at the crack tip that cause crack growth retardation. The degree to which this effect manifests depends on whether the tests are done at a constant driving force (K max ) or at a constant crack growth rate (da/dN). It is observed that depending on the magnitude of the overload at a given applied base stress intensity, these residual stresses can have significant effect on the crack growth in both the inert (vacuum) and the chemical (NaCl) environments. In general, cracks will grow only if the total crack tip driving force K total exceeds the long crack intrinsic threshold K*max,th.${\rm{K*}}_{{\rm{max,th}}}^{}.$ The crack growth retardation results can be attributed to the combined effects of the crack tip chemical reaction rates and the overload compressive residual stresses.

In this contribution, we introduce a simple approach to quickly estimate the environment-induced crack velocity (CV) as a function of the calculated applied stress intensity factor (K) developed during the slow strain rate testing of aluminum alloys exposed to aqueous or humid air-type environments. The CV-K behavior for a commercial aluminum-magnesium alloy, AA5083-H131, sensitized and pre-exposed to a 0.6 m NaCl solution has been estimated from slow strain rate test data. The predicted threshold K and crack velocities match recently published data for the same alloy in similarly sensitized conditions where the CV-K data were obtained using state-of-the-art fracture mechanics-based testing.

Although a great deal of success in structural integrity management has been achieved, it may be realized by excessively conservative assumptions, high inspection burdens, and aggressive maintenance and repair strategies. Corrosion has been found to reduce fatigue life, but methods to account for it in fatigue modeling are still limited. There is a recognized need to improve structural integrity calculations through the inclusion of the effect of existing corrosion damage on fatigue. The feasibility of a corrosion modified fatigue analysis process that uses finite element analysis and linear elastic fracture mechanics techniques to predict fatigue life of a corroded aircraft component based on the corrosion damage location is demonstrated in this work. A corrosion modified equivalent flaw size was successfully used to predict fatigue crack growth from AA7075-T651 specimens with two different notch geometries and corrosion damage at the notch center.

This work investigates the effects that the confined volume of atmospheric electrolytes has on the galvanic corrosion kinetics of a martensitic stainless steel alloy, UNS S13800, coupled with UNS A97075 in simulated atmospheric environments at relative humidity values that span the range of operational exposures. Restricted volumes found in thin films and droplets have been shown to control reduction reaction kinetics and are an ongoing challenge to characterize and standardize. This, along with the dynamic and high concentration of aggressive ions found in confined electrolytes, creates a unique corrosion system that requires a multifaceted approach to evaluate varied conditions, compare them with traditional measurements, and more accurately predict galvanic atmospheric corrosion. In this work, corrosion currents in galvanic couples were obtained under two environmental conditions: (1) bulk electrolytes, in a standardized test configuration, with chemistries relevant to atmospheric electrolytes; and (2) deliquesced droplets formed and equilibrated at a given temperature and relative humidity value. The corrosion currents for the same galvanic couple specimens were evaluated, using an atmospheric corrosion model, under a thin film electrolyte while statically loaded and unloaded, at two nominally different locations, e.g. Alexandria, VA, and Miami, FL, on the same date, using recorded weather conditions. The modeled corrosion currents were then compared with the currents obtained from the experimental conditions.

High strength-to-weight ratio Ti-6Al-4V alloy is used in many engineering applications. Its surface oxide film can protect the substrate from interaction with a lot of corrosive environments. Unfortunately, this surface layer can be damaged under mechanical and chemical actions with a consequent reduction in corrosion resistance. A characterization of the untreated alloy under quasi-static loading is here provided. Inert and aggressive environments have been investigated and the influence of the notch geometry of the alloy has also been analyzed.