Volume 35, Issue 4-5 - Special issue: Environment-induced cracking / Issue editors: A.K. Vasudevan, N.J. Henry Holroyd and Ronald M. Latanision

Environmentally assisted failures involving poor materials selection (or heat treatment), along with some examples where the specified material was (inadvertently) not used, are described. The materials discussed are martensitic steels, stainless steels, aluminium alloys, and copper alloys. The examples discussed include some cases where the material-selection issue was with welds, coatings, or insulation rather than the component material per se. The failure modes discussed are hydrogen embrittlement, stress-corrosion cracking, corrosion fatigue, metal-induced embrittlement, galvanic corrosion, selective corrosion (dealloying), and intergranular corrosion. The characteristics of fracture/corrosion, which contribute toward correctly diagnosing the modes and causes of failure, are also outlined along with comments on the mechanisms involved.

Traditionally, airframe structures are designed for immediate mechanical performance and loads-only structural response; the lifetime of aircraft structures is predicted on these analyses and environmental degradation of properties over the life cycle and during operations is often an afterthought. Although the maintenance of aircraft structures is primarily determined by material degradation, galvanic management of airframe designs and corrosion-resistant material selection have never been done systematically. From end-of-life tear-down inspections, we know that, predominantly, structural failures are initiated from corrosion features, especially those accelerated by dissimilar material coupling. In its most simplistic form, this environmental exposure, “loading”, creates corrosion features, such as pitting, that produce crack initiation morphologies; cracks nucleate from these features and then grow under the combined influence of mechanical stress and corrosion, eventually leading to structural failure. There is clearly a strong correlation between corrosion and structural damage, which we think of as corrosion fatigue and stress corrosion cracking. Office of Naval Research’s Sea-Based Aviation program is developing computational approaches to corrosion activity prediction, crack initiation and crack growth, with the ultimate aim of predicting service life in terms of the combination of mechanical and chemical stress. This approach is intended to be the basis for design of durable aircraft structures, using design principles that will take into account both stress and corrosion in the design phase, rather than designing for stress and then maintaining for corrosion.

This work reports an electrochemical investigation of corrosion and repassivation in NaCl solutions of structural Al 7075-T6 and Al 2024-T3 as a function of permanent load in bending by means of single-cycle anodic polarization. Experimental variables other than the load level were the extent of corrosion, chloride ion concentration and initial pH of the test solution, in addition to the pre-exposure in the aggressive environment and of the viscosity of the test solution for Al 7075-T6. The susceptibility to local stress during corrosion and repassivation depends on the alloy microstructure and corrosion mechanisms. In the case of Al 2024-T3, the mechanical activation is driven by cathodically controlled dissolution of this alloy at open circuit in concentrated NaCl, being indicative of a chemomechanical effect. Conversely, for Al 7075-T6, the effect of applied load is better discerned from the repassivation response. Film rupture/formation sequence and related interfacial (electro)chemical processes determine the stress-enhanced metastable repassivation of this alloy.

Environment-assisted cracking (EAC) of aluminum alloys in corrosive atmospheres is a significant maintenance and safety issue. EAC is influenced by the interaction of stress, environment, and microstructure. Atmospheric conditions and corrosion kinetics are dynamic due to diurnal cycles and changing operating conditions. Temperature, relative humidity, and surface contaminants interact to control thin-film electrolyte properties. Within a crack, the separation of the anode and cathode may occur due to concentration gradients between the crack tip, mouth, and external surface. Conventional immersion testing is not well suited to study factors and interactions leading to atmospheric EAC because the bulk electrolyte conditions for immersion testing are different from the thin-film properties. Additionally, standard three-electrode immersion measurements are not well suited to directly investigate the variation and distribution of cathodic and anodic currents on a sample surface and within an EAC crack. In this work, atmospheric electrochemical tests have been conducted using a segmented, multielectrode sensor with an artificial crevice to quantify local, dynamic anodic and cathodic current distributions. These tests are compared to EAC growth rate measurements. Maximum EAC growth rates are observed when high cathodic current is measured at the tip of artificial crevices, suggesting a hydrogen embrittlement mechanism.

Basic concepts, related to the crack tip driving forces in relation to the material resistance, are analyzed for the elastic and elastic-plastic crack growth condition. This defines the crack initiation and growth conditions, as well as for crack arrest. Environment provides an additional driving force, thereby reducing the mechanical driving force required for the crack to grow. It is shown that (a) crack initiation and its growth are inseparable and (b) the magnitude of the applied and/or internal stresses; their gradients are also important for initiation and continuous growth of a crack. Elastic-plastic crack growth is also analyzed using the discrete dislocation models. The results show that its behavior is similar to that of an elastic crack. These concepts are valid for all subcritical crack growth. Mechanical and mechanical equivalent of chemical forces are defined for estimating the life prediction of a component in service. Failure diagrams are defined based on the extension of classical Kitagawa-Takahashi diagram that bridges the behavior of smooth and fracture mechanics specimens. Connections between crack initiation, growth, arrest, and overload fracture are established via these failure diagrams. Application of these diagrams for engineering components in service is outlined for diagnostic and prognostic purposes.

An overview is presented of recent experimental measurements of environmentally assisted crack growth rates for steam turbine blades in simulated condensate solutions. The most notable observation is the complex dependence of stress corrosion and corrosion fatigue crack growth rates on crack size and solution conductivity in the low conductivity solutions associated with turbine condensates. Recent modelling of crack electrochemistry for this system has enabled rationalisation of most of these observations. An intriguing implication of the merging of experimental data and modelling is that the mechanism of cracking could also be dependent on crack size and solution conductivity.

Two distinct modes of environment-induce cracking (EIC) can initiate in AA5083-H131 during slow strain rate testing (SSRT) in laboratory air (50% RH) at nominal strain rates around 10 −6 /s, for material either sensitized in distilled water (DW) at 80°C or when material sensitized in dry air is subsequently pre-exposed to DW at room temperature. Type-1 EIC is the “classic” form of intergranular stress corrosion cracking (IGSCC), which during SSRT in laboratory air, initiates at intergranular corrosion (IGC) sites promoted during exposure to DW and provides the prerequisite local stress intensity factor of around 5 MNm −3/2 required for crack initiation. When Type-1 EIC cracks become sufficiently deep for local plane-strain stress intensity factors to exceed 12–15 MNm −3/2 , Type-2 EIC initiation triggers a series of sudden large load-drops and SSRT failures with extremely low fracture stresses (30–65 MPa). Pre-exposure to DW at room temperature can enhance propensity to both Type-1 and Type-2 EIC. SSRT in dry air, as opposed to humid laboratory air, exhibits failure consistently as a 45° slant failure via a predominantly ductile microvoid coalescence fracture mode, with Type-1 EIC very rarely initiating and any Type-2 EIC restricted to isolated internal patches that only form when relative elongations exceed ~9%.

Stress corrosion characteristics of three alloys, AL-3Li, AL-2Li-3Cu, and 5083, were analyzed. All three alloys have anodic grain boundary (GB) precipitates (ppts) (δ, T 1 , and β, respectively) that show different corrosion rates with respect to the lattice H diffusion. These GB phases grow with heat treatment time. These phases affect room-temperature fracture toughness in AL-3Li and AL-2Li-3Cu alloys but not in 5083 alloy. When exposed to NaCl solution, all three GB phases dissolve. Under an applied stress, the resulting stress corrosion threshold stress intensity K 1scc varies inversely with size and spacing, when the local crack tip environmental condition is established. The mechanism involved is mostly anodic dissolution with a secondary effect from hydrogen.

The paper summarizes the results obtained by authors concerning near-neutral stress corrosion cracking (SCC) phenomena on buried pipelines. Stress corrosion tests according to different methodologies and fatigue propagation tests were executed on traditional steel grades. Tests were performed also on specimens pre-corroded according to a procedure developed to produce localized attacks similar to those observed during failure analysis on pipelines, acting as preferential sites for crack nucleation. Electrochemical tests were carried out in order to evaluate the influence of environmental parameter on pit formation. The main role of continuous plastic deformations on near-neutral SCC is confirmed. The effect of pH, CO 2 , and bicarbonate concentrations on pitting formation is discussed.

A theory is proposed to predict the initiation of fatigue cracks using cyclic dislocation dynamics (DD) simulations. The evolution of dislocation networks in a grain is simulated over several cycles. It is shown that the dislocation density and the energy stored in the dislocation networks increase with the number of cycles. The results of the DD simulations are used to construct an energy balance expression for crack initiation. A hypothetical crack is inserted into the grain, and the Gibbs energy consisting of the energy of the dislocation structure, the surface energy of the hypothetical crack, and the reduction in continuum energy is evaluated. Once the Gibbs energy attains a maximum, the dislocation structure becomes unstable, and it becomes energetically more favorable to form a real crack. The proposed method is applied to oxygen-free high conductivity copper, and the results are compared against experiments. Finally, it is shown how the method can be amended to account for environmental effects.

To better understand the mechanics of environmentally assisted cracking, and particularly hydrogen embrittlement, a correct description of the hydrostatic stress field is indispensable. The concentration of hydrogen in the proximity of the crack tip is indeed dependent of the hydrostatic stress effect on the microstructural lattice of the material. The overall parameters of the hydrostatic stress, including peak value, its location, gradient, and distribution size are fundamental to assess the effect on hydrogen distribution near the crack tip, specifically considering hydrogen-enhanced decohesion mechanism, or the HEDE mechanism. Hydrostatic stress is hence widely analysed in studies related to hydrogen embrittlement contribution in stress corrosion cracking or corrosion fatigue of metallic alloys. However, recent studies highlighted that the hydrogen-enhanced local plasticity (HELP) mechanism can be more relevant than HEDE in hydrogen-assisted fatigue failure of metallic alloys. In order to investigate the contribution of the HELP mechanism, detailed finite element modelling is reported for notched Ti-6Al-4V specimens, based on experimental fatigue data. The material is modelled with elastic-perfectly plastic behaviour, reproducing actual geometry of the notches and the fatigue crack, from measurements and replicas conducted during testing. Strain data are obtained in initial and final crack configuration, to discuss the HELP contribution on environmentally assisted cracking, and compare it with HEDE contribution linked to hydrostatic stress.

The United States Department of Defense (DoD) estimated that the annual cost of corrosion to weapon systems and infrastructure in 2014 exceeded $18 billion and that the number was likely to continue to rise. Corrosion affects military readiness by taking critical weapon systems out of action, due to the degradation of equipment. Unfortunately, as the warfighters demand more from their systems, corrosion prevention and control is frequently traded during the acquisition cycle for weapon system performance. As a result, the DoD remains entrenched in a find-and-fix corrosion management philosophy which is expensive and unsustainable. Better standardized laboratory procedures are needed to help the DoD develop (1) a fundamental understanding of corrosion damage, (2) material performance data relevant to corrosion damage, (3) prediction methodologies to help mitigate the effects of corrosion nucleated damage and (4) to develop an understanding of how corrosion preventative coatings can slow mechanical damage. This paper addresses the effect of the corrosion inhibitors strontium chromate and calcium molybdate in concentrations relevant to service on corrosion fatigue damage as well as presents development of a test methodology for the examination of the corrosion pit-to-fatigue crack transition to help the DoD improve corrosion protection system selection.