Volume 42, Issue 5 - Special Issue: Environmentally-Assisted Cracking; Issue Editors: N.J. Henry Holroyd, Fritz Friedersdorf, Mehdi Amiri, Asuri K. Vasudevan and Ronald M. Latanision

Environment-induced cracking (EIC) research spanning the last 80 years for ferrous and non-ferrous metals in aqueous environments at ambient and elevated temperatures has concentrated on crack propagation. Studies clearly reveal EIC involves two differentiable processes, one controlling initiation and the other propagation. Utilization of advanced high-resolution electron microscopy over the last 20 years has enabled more focused studies of crack initiation for stainless steel and nickel-based alloys at elevated temperatures exposed to environments associated with the nuclear industry. More recently, when coupled with advanced in-situ experimental techniques such as time-lapse X-ray computed 3D-tomography, progress has also been made for aluminum alloys suffering EIC at ambient temperatures. Conventional wisdom states that chemical processes are typically rate-controlling during EIC initiation. Additionally, experimental evidence based on primary creep exhaustion ahead of the introduction of an aggressive environment indicates that time-dependent mechanically-driven local microstructural strain accommodation processes (resembling creep-like behavior) often play an important role for many metals, even for temperatures as low as 40 % of their melting points (0.4 T m ). EIC studies reveal initial surface conditions and their associated immediate sub-surface alloy microstructures generated during creation (i.e. disturbed layers) can dictate whether or not EIC initiation occurs under mechanical loading conditions otherwise sufficient to enable initiation and growth. The plethora of quantitative experimental techniques now available to researchers should enable significant advances towards understanding EIC initiation.

In this article, a new partially immersed slow strain rate (SSR) and corrosion-fatigue (CF) test methods are described and discussed. Partial immersion of a specimen in 3.5 % NaCl solution creates an interface between the solution and air. We call this a “chemical notch” stress concentration factor. The outcome of this demonstrates that the SSR test condition on a smooth specimen will fail at the interface if the machined notch stress concentration factor is less than the “chemical notch” stress concentration factor. This “chemical notch” concept can be applied to correlate corrosion-fatigue S–N data for a specific material/environment system.

Environment effects in plasticity and fracture of metals, well studied for several decades, still pose many unanswered questions. A micro-mechanics explanation of how dislocation activity is influenced by the material surface state, that can answer these questions, has been elusive. We build on a recently discovered effect in metal cutting – organic monolayer embrittlement (OME) – wherein metal surfaces are rendered brittle by long-chain organic adsorbates, to explore how material state variables influence surface plasticity and fracture. In particular, cutting experiments with Al containing Self Assembled Monolayers (SAMs), show that the OME is controlled by surface stress ( f ) induced by the adsorbates. This is contrary to many instances of environment-assisted fracture which are usually attributed to surface energy changes, and wherein f is largely ignored. Other contributions include (a) a cantilever technique to measure surface stress, (b) demonstration of strong effect of SAM molecule chain length on f , (c) characterization of how dislocation activity at crack-tips is affected by adsorbate-induced f , and (d) large improvements in machining processes enabled by controlled environment-assisted fracture. We make the case that surface stress, due to adsorbates, likely influences all environmentally assisted cracking (EAC) phenomena, warranting a revisit of extant models of EAC.

In order to achieve a full mechanistic understanding of stress corrosion cracking (SCC), the key operating mechanisms need to be identified but also quantified. In this study, we summarize and rationalize key findings from the last 15 years of high-resolution characterization of SCC in our group. A comprehensive characterization of a set of austenitic alloys with different Ni content and constant Cr level, tested under simulated pressurized water reactor (PWR) primary water conditions at various temperatures, has revealed evidence for at least two operating mechanisms: one diffusion-related and the other deformation-related. For their relevance to the nuclear industry, two additional alloys with increased Cr content were also studied (A800 and A690). Key precursors for SCC initiation and propagation are identified and their effect on alloy degradation discussed. A list of key materials’ properties that ensure low SCC susceptibility is proposed.

The comparative chloride stress corrosion cracking (Cl-SCC) susceptibility for 304 L, 316 L, 317 L, and AL-6XN was evaluated using double cantilever beam (DCB) specimens in boiling MgCl 2 by measuring the threshold stress intensity (K ISCC ), crack initiation time, and stage-II crack growth rate (CGR) for each material using in situ monitoring. Materials 304 L, 316 L, and 317 L were measured to have K ISCC upper limit values of 2.5–5 MPa√m, substantially lower than previously measured for 304 L in DCB specimens in boiling MgCl 2 . The K ISCC for AL-6XN was measured to be substantially higher, as would be expected in this more highly alloyed material. This work describes the challenge in measuring low K ISCC values and how this was accomplished in the present work. More prototypical conditions were used to evaluate Cl-SCC behavior of 304 L using a DCB-like geometry with a smooth gauge area in a low humidity environment in the presence of MgCl 2 salt. Finite element modeling (FEM) was used to establish the relationship between applied loads and stress and strain. Loading to 15 % strain led to low temperature creep that masked in situ Cl-SCC measurements. Below the yield stress (YS), no low temperature creep was observed, and Cl-SCC cracking could be distinguished using in situ methods.

The influence of applied loading rate (dK/dt) on stress corrosion cracking (SCC) behavior in annealed 304L stainless steel immersed in 4.7 M MgCl 2 solution was assessed at varying temperatures from 23 °C to 70 °C. Measured crack growth rates obtained under rising K loading (dK/dt > 0) are compared to those obtained during static K testing. A rising K-based loading protocol was found to yield more conservative crack growth rates across all temperatures, with the variation in crack growth rates (and therefore the dependence in loading rate) decreasing with increasing environmental severity.

We report the crack growth on peak aged Al 7075 under tensile loading in a saltwater environment to investigate whether hydrogen plays a role in the initiation and the subsequent growth of the crack. As the hydrostatic stress is at a maximum ahead of the notch, it has been speculated that hydrogen would diffuse to region of maximum hydrostatic stress and initiate the crack. An elastic-plastic finite element (FE) model is developed to determine the stress distribution around the circular notch at the edge of the plate. The crack growth was monitored using a camera attached with an optical microscope, and various regimes corresponding to hydrogen evolution, pit formation and growth, crack nucleation and initial growth and rapid crack growth leading to failure have been identified. We observe the crack was nucleated in a region associated with the highest hoop stress instead of the maximum hydrostatic stress from the base of a pit at the circular notch. We demonstrated the crack growth rate in a salt water environment is governed mostly by the dissolution of grain boundary MgZn 2 precipitate, rather than the hydrogen induced decohesion mechanism.

It is common to observe that monotonic K 1c (lab air) and K 1scc (NaCl solution) decrease with increasing YS of an alloy. K 1c is measured in lab air and K 1scc in an aqueous solution such as NaCl. It is noted that K 1c is not considered as a subcritical parameter while K 1scc is. Interestingly, for a given alloy, both these parameters seem to be inter-related. That is, K 1scc is linearly related to K 1c , such that K 1scc increases with increasing K 1c . This may indicate that plasticity is affecting the K 1scc behavior. This article looks into the conditions that affect K 1scc for steels, Al-alloys, and Ti-alloys. This linear variation of K 1scc with K 1c seems to be independent of alloy YS, E, microstructure, and work hardening rate. This observation seems similar in all three systems of alloys. How these parameters are interrelated is analyzed and discussed in detail.

This article describes the relationship that relates the static threshold parameter of K 1SCC (at ∼10 −10   m/s) to Δ K th and K max,th (at ∼10 −10  m/cycle) under fatigue load at high R-ratio like R  = 0.85. At these high R -ratios (>0.75), applied K max  >  K 1SCC and the fracture path has a larger component of static fracture. 5083 Al alloy is adapted as an example to demonstrate this relationship at room temperature their relationship with and without GB-β anodic precipitates in inert and a 3.5 % NaCl solution environments. We are interested in the mechanical cyclic Δ K th and K max,th and quasi-static K ISCC driving forces affecting the damage in the presence of a chemical environment due to anodic dissolution of the GB-β precipitates. Detailed discussions involve how these parameters are interrelated.

Environmentally assisted cracking can significantly affect the performance of high strength alloys and limit material selection to minimize the risk of subcritical crack growth in service. UNS N07718 is widely used in marine service applications and under a variety of conditions, such as: alternate immersion, different levels of cathodic protection, and freely corroding galvanic couples, because of its demonstrated corrosion and fracture resistance in these environments. In this work we developed a representative model of the material microstructure including the metal grains, the material texture, and the precipitates along the grain boundaries and within the grains. The microstructural model was subjected to the boundary conditions identified at the notch root of a fracture mechanics sample and the results are used as input for a simulation of hydrogen diffusion from the surface of the notch, assuming the material has been introduced to a hydrogen producing environment. The diffusion of hydrogen was modeled by Fick’s law and included both hydrostatic stress and mobile dislocation velocity as driving forces. The influence of immobile dislocations was also modeled to account for the irreversible trapping. The results show that hydrostatic stress and immobile dislocation trapping can significantly alter the highest concentration of hydrogen and its location within the microstructure towards the fracture process zone. Mobile dislocation velocity has a small influence in determining the hydrogen distribution near the fracture process zone.

To address the need for reduced vehicle weight and improved environmental sustainability, the automotive industry has increasingly turned to mixing lightweight materials and alloys with metal alloys. However, this integration of dissimilar materials has heightened the risk of galvanic corrosion. This study addresses the gap in modeling of galvanic corrosion under dynamic thin film electrolyte by incorporating data derived from real-world weather conditions and finite element simulations. The presented model successfully captures the trend of galvanic corrosion rate for a given atmospheric environmental condition. The model predictions are compared with experimental data in the literature. Good agreements are observed. The model is further used for prediction of galvanic corrosion of two identical vehicles located in two different geographic locations (i.e., Miami Beach in Florida and Wendover in Nevada) in the year 2021 leveraging weather station data. Additionally, a Bayesian estimation method is used to account for uncertainties in the model parameters and estimation of the probability of failure.

This short review paper describes the use of a reverse life plot, σ a versus 1/ N f , for estimating a fatigue endurance/limit, σ FL . The method is applicable for different alloy-environment systems and load R -ratios. Due to inherent scatter in the fatigue data approaching fatigue limit, a ‘staircase’ method is often utilized that requires relatively large number of specimens (around 15–30) to be tested just for fatigue endurance/limit determination alone. The proposed method uses only high-cycle-fatigue (HCF) S-N data. The estimated fatigue endurance/limit is verified against the data from staircase method for 7000 Al alloy in air and corrosive 0.5 % NaCl solution environment. The comparison with the staircase method shows fairly good agreement. An additional example shows how this method estimates endurance limits for 4140 steels tested in three environments: dry air, air with 93 % RH, and aerated 3 % NaCl solution.