Stress Corrosion Cracking Model Research Articles

Data analysis of the influence of microstructure, composition, and loading conditions on stress corrosion cracking behavior of Mg alloys

Stress corrosion cracking (SCC) can be a crucial problem in applying rare earth (RE) Magnesium alloys in environments where mechanical loads and electrochemical driven degradation processes interact. It has been proven already that the SCC behavior is associated with microstructural features, compositions, loading conditions, and corrosive media, especially in-vivo. However, it is still unclear when and how mechanisms acting on multiple scales and respective system descriptors predictable contribute to SCC for the wide set of existing Mg alloys. In the present work, suitable literature data along SCC of Mg alloys has been analyzed to enable the development of a reliable SCC model for MgGd binary alloys. Pearson correlation coefficient and linear fitting are utilized to describe the contribution of selected parameters to corrosion and mechanical properties. Based on our data analysis, a parameter ranking is obtained, providing information on the SCC impact with regard to ultimate tensile strength (UTS) and fracture elongation of respective materials. According to the analyzed data, SCC susceptibility can be grouped and mapped onto Ashby type diagrams for UTS and elongation of respective base materials tested in air and in corrosive media. The analysis reveals the effect of secondary phase content as a crucial materials descriptor for our analyzed materials and enables better understanding towards SCC model development for Mg-5Gd alloy based implant.

Investigating the susceptibility of Fe39Mn20Co20Cr15Si5Al1 high entropy alloy to stress corrosion cracking in a 3.5 wt.% NaCl environment

This study examines the stress corrosion cracking (SCC) behavior of the Fe39Mn20Co20Cr15Si5Al1 (at.%) high entropy alloy in a salt solution at room temperature. The research employs slow strain-rate tensile tests and advanced analytical techniques, such as electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM), to understand the SCC behavior. Results show that the alloy has a high susceptibility to SCC. The TEM analysis supports a slip-step dissolution model for SCC. The EBSD results suggest that cracks form at boundaries between martensite and austenite phases and propagate through interphase and inter-variant boundaries. Reducing such boundaries may mitigate the alloy's SCC susceptibility.

Prediction of crack propagation kinetics through multipoint stochastic simulations of microscopic fields

Prediction of crack propagation kinetics in the components of nuclear plant primary circuits undergoing Stress Corrosion Cracking (SCC) can be improved by a refinement of the SCC models. One of the steps in the estimation of the time to rupture is the crack propagation criterion. Current models make use of macroscopic measures (e.g. stress, strain) obtained for instance using the Finite Element Method. To go down to the microscopic scale and use local measures, a two-step approach is proposed. First, synthetic microstructures representing the material under specific loadings are simulated, and their quality is validated using statistical measures. Second, the shortest path to rupture in terms of propagation time is computed, and the distribution of those synthetic times to rupture is compared with the time to rupture estimated only from macroscopic values. The first step is realized with the cross-correlation-based simulation (CCSIM), a multipoint simulation algorithm that produces synthetic stochastic fields from a training field. The Earth Mover’s Distance is the metric which allows to assess the quality of the realizations. The computation of shortest paths is realized using Dijkstra’s algorithm. This approach allows to obtain a refinement in the prediction of the kinetics of crack propagation compared to the macroscopic approach. An influence of the loading conditions on the distribution of the computed synthetic times to rupture was observed, which could be reduced through a more robust use of the CCSIM.

SCC susceptibility of solution-annealed 316L SS in hydrogenated hot water below 288 °C

Stress corrosion cracking (SCC) susceptibility of solution-annealed 316L stainless steel was evaluated by slow strain rate test in oxygenated, deaerated and hydrogenated water below 288 °C. At 288 °C, no SCC was found in the specimens tested in oxygenated water, while in hydrogenated water, SCC occurred irrespective of dissolved-hydrogen (DH) content in the late stage of deformation where cold-working was followed by necking. The SCC susceptibility diminishes as temperature decreased from 288 °C to RT, which was accompanied by a reduction of oxide film thickness. At 220 °C, SCC occurred at the highest DH content only, which suggests a combined effect of DH content and test temperature on the SCC susceptibility. The combined effect can be explained by anodic reaction rate, hydrogen diffusion rate and hydrogen trapping ability. The SCC susceptibility appeared to increase with DH content, which was reasonably explained by suppression of the protective oxide film formation on specimen surface and increasing hydrogen content in the specimen. Based on an SCC model that hydrogen reduces cohesive force in austenitic stainless steel lattice, the SCC behavior can be interpreted in terms of hydrogen-assisted SCC (HASCC) where corrosion-induced hydrogen production, suppression of the formation of protective oxide film caused by hydrogen, work-hardening, and hydrogen-induced reduction of cohesive force of the lattice atoms are necessary.

Statistical model of stress corrosion cracking based on extended form of Dirichlet energy: Part 2

The mechanism of stress corrosion cracking (SCC) has been discussed for decades. Here I propose a model of SCC reflecting the feature of fracture in brittle manner based on the variational principle under approximately supposed thermal equilibrium. In that model the functionals are expressed with extended forms of Dirichlet energy, and Dirichlet principle is applied to them to solve the variational problem that represents SCC and normal extension on pipe surface. Based on the model and the maximum entropy principle, the statistical nature of SCC colony is discussed and it is indicated that the crack has discrete energy and length under ideal isotropy of materials and thermal equilibrium.

A three-dimensional model of non-slipping stress corrosion cracking under low loads

Stress corrosion cracking (SCC) of 316L single-crystal austenitic stainless steel subjected to low loads (σnom = 20-40 MPa) in a 45 % boiling MgCl2 solution was studied using synchrotron X-ray computed tomography, finite element analysis and so on. Results show that there was no surface slip band around the nucleation sites and the tips of short cracks. Three-dimensional reconstruction of discontinuous zig-zag surface SCC crack indicates that the crack was continuous inside the specimen. The obtaining through two-surface trace analysis manifests, rather than {1 1 1} slip planes, the cracks extended along {1 0 0} planes with the lowest surface energy. It is considered that microcleavage and local dissolution synergistically led to the SCC advance, and microshear was also one of the primarily microscopic SCC mechanisms under the high load. The three-dimensional SCC model was created at the low stress level, where a main SCC crack grew along the MPD due to anodic dissolution, and a microdefect was formed on the crack front. When the microdefect enlarged to a critical size, secondary microcracks nucleated at the stress-concentrated microdefect shoulders. Then, the microcracks propagated to two sides of the MPD through anodic dissolution, microcleavage or mciroshear, resulting in the formation of the river-like fractography and the discontinuous surface SCC cracks with or without surface slipping.

Oxidation of nickel-base welds 182 and 82 in simulated primary water of pressurised water reactors

To date, nickel-base welds 182 and 82 used in pressurised water reactors have shown a higher susceptibility to stress corrosion cracking (SCC) during laboratory tests than in power plants. However, the increasing number of cracks reported in American, Swedish and Japanese nuclear power plants on Alloys 182/82 enlightens the need for a predictive initiation model of SCC. Initiation of SCC involves several variables interacting together such as material microstructure and composition, primary water chemistry and loading history. Building such a model requires studying each contribution of individual phenomenon, in order to have a better understanding of the mechanisms involved at a local scale in crack initiation. Before SCC initiation, some grain boundaries exposed to primary water present intergranular oxidation reaching several hundreds of nanometres. The oxide penetration kinetics could be modified by several parameters such as local metallurgy, dissolved hydrogen content of the primary water and mechanical history before oxidation. The grain boundaries affected by oxidation during a tensile test demonstrate a lower resistance to failure compared to ‘healthy’ grain boundaries. The main objective of this study is to determine key local parameters controlling grain boundary oxidation in order to calibrate an oxidation model which will be integrated to SCC initiation modelling.

Stress corrosion cracking under low stress: Continuous or discontinuous cracks?

Two-dimensional and three-dimensional crack morphologies of stress corrosion cracking (SCC) were studied by serial-sectioning and synchrotron-based X-ray computed tomography. Discontinuous surface cracks were actually continuous inside the specimen, which matched typical river-like fractographs and finite element simulations. A low stress SCC model was created, where a main crack continuously grew along the main propagation direction (MPD) due to anodic dissolution; then, discontinuous secondary microcracks emanated from MPD, angularly extending to the two sides of MPD. Finally, some of the secondary microcracks reached the sample surface, resulting in the formation of discontinuous surface cracks.

Environmental Crack Driving Force

The effect of environment on the crack driving force is considered, first by assuming quasistatic extension of a stationary crack and second, by use of stress corrosion cracking (SCC) crack growth rate models developed previously by this author and developed further here. A quasistatic thermodynamic energy balance approach, of the Griffith-Irwin type, is used to develop stationary crack threshold expressions, \( \tilde{J}_{\rm c} \), which represent the conjoint mechanical and electrochemical conditions, below which stationary cracks are stable. Expressions for the electrochemical crack driving force (CDF) were derived using an analysis that is analogous to that used by Irwin to derive his “strain energy release rate,” G, which Rice showed as being equivalent to his mechanical CDF, J. The derivations show that electrochemical CDFs both for active path dissolution (APD) and hydrogen embrittlement (HE) mechanisms of SCC are simply proportional to Tafel’s electrochemical anodic and cathodic overpotentials, ηa and ηc, respectively. Phenomenological SCC models based on the kinetics of APD and HE crack growth are used to derive expressions for the kinetic threshold, Jscc, below which crack growth cannot be sustained. These models show how independent mechanical and environmental CDFs may act together to drive SCC crack advance. Development of a user-friendly computational tool for calculating Tafel’s overpotentials is advocated.

TEM investigations of the oxide layers formed on a 316L alloy in simulated PWR environment

Atomic scale observations of the oxide formed on stainless steels, under simulated nuclear reactor conditions, are performed to estimate the oxide layer contribution on stress corrosion cracking (SCC) mechanisms. A duplex oxide composed of a chromium enriched inner layer (Fe1.5Cr1.5O4) and an outer layer composed of magnetite crystallites (Fe3O4) is found. The oxide layer structure evolves from amorphous, for oxidation times of 1 min, to nano-crystalline at 2 min and mono-crystalline after 5 h. IFFT images, calculated from Cs-corrected HRTEM images recorded on grains oriented in the 〈111〉 direction, highlight a double network of dislocations with ½ 〈10-1〉 and ½ 〈−110〉 Burgers vectors. This network leads to the decrease in non-relaxed deformation and favors an epitaxial growth between steel and oxide. Both crystal structure transformations and epitaxial relations between metal and oxide have provided relevant information which contributed to progress on SCC modeling.

Equations of state for crystalline zirconium iodide: The role of dispersion

We present the first-principle equations of state of several zirconium iodides, ZrI2, ZrI3, and ZrI4, computed using density functional theory methods that apply various methods for introducing the dispersion correction. Iodides formed due to reaction of molecular or atomic iodine with zirconium and zircaloys are of particular interest due to their application to the cladding material used in the fabrication of nuclear fuel rods. Stress corrosion cracking (SCC), associated with fission product chemistry with the clad material, is a major concern in the life cycle of nuclear fuels, as many of the observed rod failures have occurred due to pellet–cladding chemical interactions (PCCI) [A. Atrens, G. Dannhäuser, G. Bäro, Stress-corrosion-cracking of zircaloy-4 cladding tubes, Journal of Nuclear Materials 126 (1984) 91–102; P. Rudling, R. Adamson, B. Cox, F. Garzarolli, A. Strasser, High burn-up fuel issues, Nuclear Engineering and Technology 40 (2008) 1–8]. A proper understanding of the physical properties of the corrosion products is, therefore, required for the development of a comprehensive SCC model. In this particular work, we emphasize that, while existing modeling techniques include methods to compute crystal structures and associated properties, it is important to capture intermolecular forces not traditionally included, such as van der Waals (dispersion) correction. Furthermore, crystal structures with stoichiometries favoring a high I:Zr ratio are found to be particularly sensitive, such that traditional density functional theory approaches that do not incorporate dispersion incorrectly predict significantly larger volumes of the lattice. This latter point is related to the diffuse nature of the iodide electron cloud.

Static crack growth and fatigue modeling for silicon MEMS

Fatigue in silicon microstructures has been widely observed but is currently not well understood. In this paper, it is shown that typical silicon MEMS fatigue can be described by a ‘classic’ stress corrosion cracking (SCC) model for glass fracture. The model can be used to describe the slow crack propagation and ultimate failure of MEMS structures due to mechanical stress under different conditions. With this SCC model it is possible to do full lifetime predictions, and these correspond very well with measured fatigue data from literature, as a function of applied stress and temperature. This suggests that at least part of the literature data available can be explained by static fatigue, which is better described by a time to failure than cycles to failure. However, not all failures can be explained by SCC alone, a notable exception being those of MEMS devices with very thin surface oxides.

Understanding and Modeling of Stress Corrosion Cracking (SCC)

Stress corrosion cracking (SCC) is a brittle fracture of a ductile material under severe environment. Due to the complexity of mechano-chemical degradation during SCC formation, the scientific community is still far from the complete understanding of this phenomenon. Moreover, it is commonly misunderstood that polymeric materials is ‘SCC-free’, but it should be noticed that SCC is universal phenomenon for all engineering materials including polymers. In this paper, the similarity and differences of SCC in different materials, such as carbon steels and engineering polymers, are observed and reported. The SCC modeling in carbon steels and engineering polymers is also compared and discussed.

Stress Corrosion Cracking—Crevice Interaction in Austenitic Stainless Steels Characterized By Acoustic Emission

Stress corrosion cracking (SCC) susceptibility of austenitic EN1.4301 (AISI 304) and EN1.4404 (AISI 316L) stainless steels was studied using the constant load method and polymer (PTFE) crevice former in order to study the effects of crevice on SCC susceptibility. The uniaxial active loading tests were performed in 50 pct CaCl2 at 373 K (100 °C) and in 0.1 M NaCl at 353 K (80 °C) under open-circuit corrosion potential (OCP) and electrochemical polarization. Pitting, crevice, and SCC corrosion were characterized and identified by acoustic emission (AE) analysis using ∆t filtering and the linear locationing technique. The correlation of AE parameters including amplitude, duration, rise time, counts, and energy were used to identify the different types of corrosion. The stages of crevice corrosion and SCC induced by constant active load/crevice former were monitored by AE. In the early phase of the tests, some low amplitude AE activity was detected. In the steady-state phase, the AE activity was low, and toward the end of the test, it increased with the increasing amplitude of the impulses. AE allowed a good correlation between AE signals and corrosion damage. Although crevice corrosion and SCC induced AE signals overlapped slightly, a good correlation between them and microscopical characterization and stress-strain data was found. Especially, the activity of AE signals increased in the early and final stages of the SCC experiment under constant active load conditions corresponding to the changes in the measured steady-state creep strain rate of the specimen. The results of the constant active load/crevice former test indicate that a crevice can initiate SCC even in the mild chloride solution at low temperatures. Based on the mechanistic model of SCC, the rate determining step in SCC is thought to be the generation of vacancies by selective dissolution, which is supported by the low activity phase of AE during the steady-state creep strain rate region.

Waste package performance evaluations for the proposed high-level nuclear waste repository at Yucca Mountain.

The evaluation studies of the proposed repository for long-term storage of spent nuclear fuel and high-level nuclear waste at Yucca Mountain, Nevada, are underway. Fulfillment of the requirements for limiting dose to the public, which includes containment of the radioactive waste emplaced in the proposed repository and subsequent slow release of radionuclides from the Engineered Barrier System (EBS) into the geosphere, will rely on a robust waste container design, among other EBS components. Part of the evaluation process involves sensitivity studies aimed at elucidating which model parameters contribute most to the waste package and overlying drip shield degradation characteristics. The model parameters identified for this study include (1) general corrosion rate parameters and (2) stress corrosion cracking (SCC) parameters. Temperature dependence and parameter uncertainty are evaluated for the general corrosion rate model parameters while for the SCC model parameters, uncertainty treatment of stress intensity factor, crack initiation threshold, and manufacturing flaw orientations are evaluated. Based on these evaluations new uncertainty distributions are generated and recommended for future analyses. Also, early waste package failures due to improper heat treatment were added to the waste package degradation model. The results of these investigations indicate that the waste package failure profiles are governed by the manufacturing flaw orientation model parameters.

Quantitative Assessment of Submodes of Stress Corrosion Cracking on the Secondary Side of Steam Generator Tubing in Pressurized Water Reactors: Part 3

Abstract The work in this article is part of a project to develop a quantitative description of stress corrosion cracking (SCC) on the secondary side of pressurized water reactor (PWR) steam generator tubing based on existing information from operating plants and from laboratory experiments. This work is the second step in developing a predictive model for SCC on the secondary side. The first step involved developing a statistical framework into which dependencies of the various submodes of SCC can be inserted. The results of the present work will lead to quantitative descriptions of corrosion processes that, in turn, will be incorporated into the statistical framework. The chemistry of heat-transfer crevices will then be assessed to determine the proper inputs to the dependencies of the various submodes of SCC, and these will be connected to bulk environments. The modeling here is directed toward predicting the early occurrence of SCC that is too shallow to be detected by nondestructive examination (NDE)...

Quantitative Assessment of Submodes of Stress Corrosion Cracking on the Secondary Side of Steam Generator Tubing in Pressurized Water Reactors: Part 1

Abstract The work in this article is part of a project to develop a quantitative description of stress corrosion cracking (SCC) on the secondary side of pressurized water reactor (PWR) steam generator tubing based on existing information from operating plants and from laboratory experiments. This work is the second step in developing a predictive model for SCC on the secondary side. The first step involved developing a statistical framework into which dependencies of the various submodes of SCC can be inserted. The results of the present work will lead to quantitative descriptions of corrosion processes that, in turn, will be incorporated into the statistical framework. The chemistry of heat-transfer crevices will then be assessed to determine the proper inputs to the dependencies of the various submodes of SCC, and these will be connected to bulk environments. The modeling here is directed toward predicting the early occurrence of SCC that is too shallow to be detected by nondestructive examination (NDE)...

Stress Corrosion Crack Growth in Pipe Grade Steels in Near Neutral pH Environment

This paper is concerned with an accelerated testing and modeling of stress corrosion cracking (SCC) phenomena in pipe grade steels in near neutral pH environment. In modeling of SCC, the authors adopt the crack layer theory that provides formalism to account for contributions to crack growth rate such processes as electro-chemical corrosion, hydrogen embrittlement and mechanical loading. Special attention is paid to the hydrogen diffusion, a precursor to hydrogen embrittlement. The energy-momentum tensor (Eshelby's tensor) is employed to evaluate the thermodynamic forces responsible for SC crack growth. Griffith' crack equilibrium condition is used to derive a quasi-equilibrial SC crack growth equation. A parametric study and comparison with the experimental results of corrosion fatigue tests for various maximal stress, stress ratio and electric potential are performed to examine the validity of the proposed model.

Formation kinetics and rupture strain of Ni-Cr-Fe alloy corrosion films formed in high-temperature water

Nickel alloys such as Alloy 600 undergo stress corrosion cracking (SCC) in pure water at temperatures between about 260 °C and the critical point. Increasing the level of Cr in Ni-Fe-Cr alloys increases SCC resistance in aerated and deaerated water. The mechanism for Cr influence is not understood. The effect of Cr composition on the in-situ oxide rupture strain and corrosion kinetics of Ni-9Fe-Cr alloys was determined experimentally, to evaluate whether the rupture-dissolution model for SCC can account for the effect of Cr on SCC. The alloy corrosion rate and corrosion product oxide microstructure and mechanical properties are strongly influenced by Cr composition. As Cr concentration increases from 5 to 30 pct, oxide rupture strains measured in pressurized water at 288 °C increase from about 8×10−4 to 2×10−3 mm/mm. Corrosion kinetics are parabolic; the corrosion rate first increases and then decreases as Cr increases from 5 to 39 pct. These observations are qualitatively consistent with a rupture-dissolution SCC mechanism. However, parametric modeling of the SCC growth process, applying available creep, oxide rupture strain, and corrosion kinetics data, indicates that the rupture-dissolution mechanism accounts for only a fraction of the effect of Cr on SCC resistance.

Fatigue reliability of deteriorating prestressed concrete bridges due to stress corrosion cracking

In this paper, an analysis framework is presented to develop a relationship between fatigue reliability in a prestressed concrete bridge and the progress of stress corrosion cracking (SCC) in prestressing steel. The fatigue limit state uses a cumulative damage model for prestressing steel wires, which is a function of both stress range and minimum stress. The SCC model takes into account varying degrees of material susceptibility, stress regimes, and environmental conditions and is structured around three stages: initiation of micro-cracking, propagation, and macro-crack instability using linear elastic fracture mechanics. The framework is an overall time-dependent analysis of the safety against fatigue of a prestressed concrete bridge. It involves a stochastic analysis of the evolution of prestressing wire corrosion as a function of time and a time-dependent probabilistic analysis of the fatigue reliability of the prestressed concrete bridge suffering a certain degree of deterioration. The uncertainties involved in the fatigue model, the SCC model, and traffic actions are considered. The updating of uncertainties is simplified by considering a limited number of classes representative of the severity of SCC exposure. The framework is applied to three deteriorating highway bridges.Key words: fatigue analysis, prestressing strands, stress corrosion cracking, reliability assessment, prestressed concrete bridges, deterioration.