Environmentally Assisted Cracking Research Articles

In Situ Micro Raman Spectroscopy for Characterization of Oxide Film Formed on the New Surface and for Measurements of the Stress of Oxide Film Formed on 304L Stainless Steel

Environmentally assisted cracking (EAC) consists of two distinct events viz., i) crack initiation and ii) crack propagation. On a smooth surface, the EAC initiates by the rupture or by the degradation of the surface film due to the combined action of stress and an electrochemical reaction of the materials with the environment. The mechanical properties of the surface oxide films are also important considerations when determining the susceptibility to EAC. In this research, Micro Raman Spectroscopy(MRS) was applied for in-situ oxides characterization and for in-situ measurements of the stress in oxide film formed on the surface of 304L stainless steel during the scratching electrode and the slow strain rate test (SSRT), respectively. The passive oxide film growth formed on the bare surface was continuously monitored by MRS as a function of time. For stress measurements, Cr2O3 was focused on and Raman shift at Cr2O3 peak of Raman spectrum was measured continuously. The strain rate was 8.2×10-7 /sec. . In the initial stage of SSRT, the Raman shift of surface film decreased gradually with strain. At 5% strain, the Raman shift of surface film increased rapidly to around the initial value of Raman shift. It is considered that the surface film was ruptured at this time. At 5% strain, the shift value of Raman peak of Cr2O3 reached to 5cm-1. This value (5cm-1) corresponds to 1.2GPa which value of tensile stress is calculated from reference data. These characteristics of oxide film will be implemented into the theoretical formulation of EAC and their implication to EAC growth rate will be discussed.

3D-FEM Simulation for EAC Crack Growth Evaluation and its Implication to Specimen Size Effects on Crack Growth Behavior

The primary objective of this study is to develop a quantitative model to predict the effects of materials, environment and mechanics such as loading configuration on environmentally-assisted cracking (EAC) of stainless steels in high-temperature water. It has basically been accepted that crack propagation in oxygenated high temperature water is controlled by a slip-dissolution and/or deformation-oxidation mechanism. According to this mechanism, the crack-tip strain rate is an extremely important mechanical parameter for determining the crack growth rates. Based upon a formulation obtained by combining Faraday’s equation with an elastic plastic analysis of the strain singularity at a growing crack-tip in work hardening materials, a theoretical formulation of crack-tip strain rate has been derived for plane strain and plane stress conditions. The FEM analysis for 3D crack growth can be compared to the theoretical 2D analysis. In this paper, we first make a CCP (Center Crack Plane) model, and performed a 3-dimensional Finite Element Analysis (3D-FEA) to evaluate the crack-tip stain rate paying attention to the element mesh size and to the loading history. After optimization these parameters, the calculated crack-tip strain distribution, including its logarithmic singularity, was founded to agree well with the theoretical distribution. The significance of the crack-tip strain rate upon the crack-tip strain distribution and crack growth rate was demonstrated. The specimen size effects on crack growth rates were discussed from this point of crack-tip strain distribution. Finally, we focused on the importance of crack-tip strain rate as a unique mechanical parameter that controls the crack growth rate.

Environmentally assisted cracking behavior of single and dual phase stainless steels in hot chloride solutions

The environmentally assisted cracking (EAC) behavior of AISI 316L austenitic stainless steel (SS), AISI 430 ferritic stainless steel and 22% Cr duplex stainless steel (DSS) in hot chloride solutions were investigated. Slow strain rate testing (SSRT) technique was employed to evaluate the susceptibility to EAC in 3.5 wt.% NaCl solution at 80 °C and in 40 wt.% CaCl 2 solution at 100 °C. The experimental results showed that the resistances to localized corrosion and EAC were strongly dependent on the microstructure, chemical composition and the nature of the environment involved. 430 SS was found less resistant to pitting corrosion and EAC in both chloride solutions. The SSRT results indicated that 316L SS and 22% Cr DSS became susceptible to EAC in hot 40 wt.% CaCl 2 solution. Selective dissolution was found participating in the initiation and growth processes in EAC of 22% Cr DSS. Besides, intergranular cracking was found in 316L austenitic SS while pitting enhanced EAC occurred in 430 ferritic SS in 40 wt.% CaCl 2 solution at 100 °C. The effect of strain rate on the susceptibility to EAC in hot CaCl 2 solution was more pronounced in 316L austenitic SS and 22% Cr DSS than that in 430 ferritic SS.

Material Issues of Blanket Systems for Fusion Reactors-Compatibility with Cooling Water-

Environmental assisted cracking (EAC) is one of the material issues for the reactor core components of light water power reactors(LWRs). Much experience and knowledge have been obtained about the EAC in the LWR field. They will be useful to prevent the EAC of water-cooled blanket systems of fusion reactors. For the austenitic stainless steels and the reduced-activation ferritic/martensitic steels, they clarifies that the EAC in a water-cooled blanket does not seem to be acritical issue. However, some uncertainties about influences on water temperatures, water chemistries and stress conditions may affect on the EAC. Considerations and further investigations elucidating the uncertainties are discussed.

Fracture mirror of optical fibers

Optical fiber specimens with and without polymer surface coating (stripped) were tested under tensile and two-point bending. Their fracture surfaces were evaluated. A distinctive fracture mirror region can be seen at the fracture origin of every stripped fiber failed under stress. Using the known relationship equation between the fracture stress and mirror radius, the fracture mirror constants for the stripped optical fibers subjected to tension and bending were determined to be 1.3 and 9.4 MN m 3/2, respectively. However, the fracture mirror region of as-received optical fiber (with polymer coating) cannot be found from the fracture surface. The fracture hackle marks directing to a fracture initiation flaw are observed. The mirror region formation in bare glass fiber may shed a light for the mechanism of environmental assisted crack growth.

Environment-Assisted Cracking Behavior of Friction Stir Welded AA 2195

Environment-Assisted Cracking Behavior of Friction Stir Welded AA 2195

Fatigue Crack Growth Of SA508 C1.3 Pressure Vessel SteelIn High-temperature Argon And Water Environments

Effects of environment on the fatigue crack growth rate were studied. The hydrogen-charged SA508 C1.3 was tested at 288OC to determine the effect of hydrogen on environmentally assisted cracking (EAC) in high-temperature water. The growth rate with the H-charged specimen was enhanced by 2-3 times that of the as-received specimen under the same environment and loading conditions. Then the fracture surface revealed a striationless cleavage-like fracture showing few secondary cracks and striations. The fracture path of the enhanced crack growth was related to banitic lath boundaries. The data reflected that hydrogen in steel can actually be involved in the occurrence of the environmentally assisted cracking.

923 Ti および Ti 合金の環境脆化マップ

Titanium and titanium-aluminum alloys suffer degradation of mechanical properties by hydrogen entering from wet environments, so called, environment-assisted cracking (EAC). In order to make sure the environmental conditions for the mechanical degradation of the materials (i.e., to construct environmental degradation map), EAC behavior of Ti, Ti-6Al-4V, and TiAl was investigated under controlling electrochemical potential as well as pH of the solution. In the cases of TiAl and Ti-6Al-4V, the severest EAC was found under low pH and around -1.0 V_<SHE>, whereas the conditions of low pH and potential lower than -1.0 V_<SHE> made the EAC moderate. Ti demonstrated no EAC susceptibility under any test conditions, but showed the largest hydrogen entry and hydride formation under the low pH and around -1.0 V_<SHE>. The potential-pH region where the severest EAC was found was in good agreement with the Ti ion stable region in the potential-pH diagram for Ti/H_2O system.

Fatigue Degradation of Nanometer-Scale Silicon Dioxide Reaction Layers on Silicon Structural Films

AbstractAlthough bulk silicon is ostensibly immune to cyclic fatigue and environmentally-assisted cracking, the thin film form of the material exhibits significantly different behavior. Such silicon thin films are used in small-scale structural applications, including microelectromechanical systems (MEMS), and display ‘metal-like’ stress-life (S/N) fatigue behavior in room temperature air environments. Fatigue lives in excess of 1011 cycles have been observed at high frequency (∼40 kHz), fully-reversed stress amplitudes as low as half the fracture strength using surface micromachined, resonant-loaded, fatigue characterization structures. Recent experiments have clarified the origin of the susceptibility of thin film silicon to fatigue failure. Stress-life fatigue, transmission electron microscopy, infrared microscopy, and numerical models have been used to establish that the mechanism of the apparent fatigue failure of thin-film silicon involves sequential oxidation and environmentally-assisted crack growth solely within the nanometerscale silica layer on the surface of the silicon, via a process that we term ‘reaction-layer fatigue’. Only thin films are susceptible to such a failure mechanism because the critical crack size for catastrophic failure of the entire silicon structure can be exceeded by a crack solely within the surface oxide layer. The growth of the oxide layer and the environmentally-assisted initiation of cracks under cyclic loading conditions are discussed in detail. Furthermore, the importance of interfacial fracture mechanics solutions and the synergism of the oxidation and cracking processes are described. Finally, the successful mitigation of reaction-layer fatigue with monolayer coatings is shown.

High-cycle fatigue of micron-scale polycrystalline silicon films: fracture mechanics analyses of the role of the silica/silicon interface

It is known that micron-scale polycrystalline silicon thin films can fail in room air under high frequency (40kHz) cyclic loading at fully-reversed stress amplitudes as low as half the fracture strength, with fatigue lives in excess of 1011 cycles. This behavior has been attributed to the sequential oxidation of the silicon and environmentally-assisted crack growth solely within the SiO2 surface layer. This ‘reaction-layer fatigue’ mechanism is only significant in thin films where the critical crack size for catastrophic failure can be reached by a crack growing within the oxide layer. In this study, the importance of the bimaterial (e.g., Si/SiO2) interface to reaction-layer fatigue is investigated, and the critical geometry and stress ranges where the mechanism is a viable failure mode are established.

A reaction-layer mechanism for the delayed failure of micron-scale polycrystalline silicon structural films subjected to high-cycle fatigue loading

A study has been made to discern the mechanisms for the delayed failure of 2-μm thick structural films of n +-type, polycrystalline silicon under high-cycle fatigue loading conditions. Such polycrystalline silicon films are used in small-scale structural applications including microelectromechanical systems (MEMS) and are known to display ‘metal-like’ stress-life ( S/N) fatigue behavior in room temperature air environments. Previously, fatigue lives in excess of 10 11 cycles have been observed at high frequency (~40 kHz), fully-reversed stress amplitudes as low as half the fracture strength using a surface micromachined, resonant-loaded, fatigue characterization structure. In this work the accumulation of fatigue-induced oxidation and cracking of the native SiO 2 of the polycrystalline silicon was established using transmission electron and infrared microscopy and correlated with experimentally observed changes in specimen compliance using numerical models. These results were used to establish that the mechanism of the apparent fatigue failure of thin-film silicon involves sequential oxidation and environmentally-assisted crack growth solely within the native SiO 2 layer. This ‘reaction-layer fatigue’ mechanism is only significant in thin films where the critical crack size for catastrophic failure can be reached by a crack growing within the oxide layer. It is shown that the susceptibility of thin-film silicon to such failures can be suppressed by the use of alkene-based monolayer coatings that prevent the formation of the native oxide.

Environment-assisted cracking of gamma titanium aluminide in aqueous sulfate solutions

Environment-assisted cracking (EAC) of γ-TiAl has been investigated from electrochemical viewpoint. The TiAl was forged at 1373 K and annealed at 1073 K for 7.2 ks. The specimen whose potential was controlled was tested in aqueous sulfate solutions of various pHs at room temperature by using a slow strain rate tensile testing apparatus. The experimental EAC results evaluated by the maximum stress was discussed on the basis of the potential-pH diagram of Al and Ti/water system. It was revealed that the diagram of Ti/water system can predict the EAC susceptibility of TiAl, although the diagram of Al/water system shows almost no relation to the susceptibility. The results suggest that the EAC of TiAl is controlled by the stability of Ti oxide, hydride, and ion on the solution/substrate interface, which affect the entry of hydrogen into the material.

Review of Corrosion Modes For Alloy 22 Regarding Lifetime Expectancy of Nuclear Waste Containers

ABSTRACTAlloy 22 (UNS N06022) was selected to fabricate the corrosion resistant outer barrier of a two-layer nuclear waste package container. This paper reviews the main corrosion degradation modes that are predicted for the outer layer of the container. Current results show that the containers would perform well under general corrosion, localized corrosion and environmentally assisted cracking (EAC). For example, the general corrosion rate is expected to be below 100 nm/year and the container is predicted to be outside the range of potential for localized corrosion and environmentally assisted cracking.

Interfacial Effects on the Premature Failure of Polycrystalline Silicon Structural Films

ABSTRACTAlthough bulk silicon is not known to be susceptible to cyclic fatigue, micron-scale structures made from mono and polycrystalline silicon films are vulnerable to degradation by fatigue in ambient air environments. Such silicon thin films are used in small-scale structural applications, including microelectromechanical systems (MEMS), and display “metal-like” stress-life (S/N) fatigue behavior in room temperature air environments. Previously, the authors have observed fatigue lives in excess of 1011 cycles at high frequency (∼40 kHz), fully-reversed stress amplitudes as low as half the fracture strength using a surface micromachined, resonant-loaded, fatigue characterization structures. Stress-life fatigue, transmission electron microscopy, infrared microscopy, and numerical models were used to establish that the mechanism of the fatigue failure of thin-film silicon involves the sequential oxidation and environmentally-assisted crack growth solely within the native silica layer, a process that we term “reaction-layer fatigue”. Only thin films are susceptible to such a failure mechanism because the critical crack size for catastrophic failure of the entire silicon structure can be exceeded by a crack solely within the native oxide layer. The importance of the interfacial geometry on the mechanics of the reaction-layer fatigue mechanism is described.

Environment-Assisted Cracking of Corrosion-Resistant Alloys in Oil and Gas Production Environments: A Review

Abstract The environment-assisted cracking (EAC) susceptibility of corrosion-resistant alloys (CRA) used in oil and gas production is reviewed. The alloy categories are limited to martensitic stain...

High-cycle fatigue and durability of polycrystalline silicon thin films in ambient air

Abstract To evaluate the long-term durability properties of materials for microelectromechanical systems (MEMS), the stress-life ( S / N ) cyclic fatigue behavior of a 2-μm thick polycrystalline silicon film was evaluated in laboratory air using an electrostatically actuated notched cantilever beam resonator. A total of 28 specimens were tested for failure under high frequency (∼40 kHz) cyclic loads with lives ranging from about 10 s to 34 days (3×10 5 to 1.2×10 11 cycles) over fully reversed, sinusoidal stress amplitudes varying from ∼2.0 to 4.0 GPa. The thin-film polycrystalline silicon cantilever beams exhibited a time-delayed failure that was accompanied by a continuous increase in the compliance of the specimen. This apparent cyclic fatigue effect resulted in an endurance strength, at greater than 10 9 cycles, of ∼2 GPa, i.e. roughly one-half of the (single cycle) fracture strength. Based on experimental and numerical results, the fatigue process is attributed to a novel mechanism involving the environmentally-assisted cracking of the surface oxide film (termed reaction-layer fatigue). These results provide the most comprehensive, high-cycle, endurance data for designers of polysilicon micromechanical components available to date.

Review on effect of cyclic loading on environmental assisted cracking of alloy 600 in typical nuclear coolant waters

Corrosion fatigue data for alloy 600 in simulated primary side, pressurised water reactor (PWR) coolant and in oxygenated pure water (OPW) have been reviewed. Higher crack growth rates occur in these environments than in air and the former show a dependence upon frequency of loading, ν, and R ratio. The d a/d N versus ΔK plots have a tendency to show plateau regions where d a/d N remains constant with increasing ΔK. The enhancements of crack growth over those in air due to cyclic loading are larger in OPW than in simulated PWR primary side water. If corrosion fatigue of alloy 600 was perceived of importance for reactor components, the existing data base will need to be extended and it would be necessary to generate more air fatigue data than is currently available. Conventional S– N fatigue and corrosion fatigue data from smooth surface specimens may also be required, collected taking due regard of the possible effects of R and ν mentioned above.

Analysis of Crack Growth Kinetics and Crack Tip Strain Rates of Sensitized Type 304 Stainless Steel in Simulated Boiling Water Reactor Environment : Experiments and Theory(EAC2)

Three slow strain rate tests (SSRT) on one-inch thick-compact tension (1T-CT) specimens, which were fabricated from sensitized 304L stainless steel, were performed with different extension rates in a simulated boiling water reactor (BWR) environment, to derive the environmentally assisted cracking (EAC) growth kinetics. The crack growth kinetics, depending on the testing conditions, were also described theoretically by Shoji's model. Inverse analysis was performed by fitting the experimental data to the theoretical data, to determine the model parameters r_0, which represents a characteristic distance to define crack tip strain rate (CTSR), and m, which is a slope of the current decay curve and a function of crack tip water chemistry. Both r_0 and m can be determined using experimental crack growth data in the analysis. Theoretical CTSR of sensitized 304 stainless steel during constant load line displacement rate tests was calculated using Shoji's model and experimental data of P. Aaltonen et al. The calculation shows that dependences of experimental crack growth rate (CGR) on theoretical CTSR for tests under different loading conditions are quite consistent. The theoretical CTSR was also compared to that evaluated using Rice's model under the same experimental conditions. Differences between the two theoretical CTSRs were found and the causes are discussed.

Potential-pH region for environment-assisted cracking of gamma titanium aluminide

Environment-assisted cracking (EAC) of γ-TiAl has been investigated from electrochemical viewpoint. The material whose potential was controlled was tested in aqueous sulfate solutions of various pHs at room temperature by using a slow strain rate tensile testing apparatus. The TiAl forged at 1373 K and annealed at 1073K for 7.2 ks was used in the EAC test. It was found in the alkaline and the natural solution that maximum stress was independent of the potential in higher potential region, and decreased with decrease in the potential in the lower potential region. In acidic solution, the maximum stress changed with the potential as follows; the maximum stress was independent of the potential above −200 mV SHE, decreased with decrease in the potential between −200 and −1000 mV SHE, and increased with decrease in the potential below −1000 mV SHE. The experimental EAC results was discussed on the basis of the potential-pH diagram of Al and Ti/water system. It was revealed that the diagram of Ti/water system can predict the EAC susceptibility of TiAl, although the diagram of Al/water system shows almost no relation to the susceptibility. Therefore, the EAC of TiAl is controlled by the stability of Ti oxide, hydride, and ion on the solution/base material interface, which affect the entry of hydrogen into the material.

In-service electrochemical potential data in a deaerator feedwater storage vessel; the influence of a noble metal coating

The present paper describes the two separate test runs where the Electrochemical Potential, ECP, of two weldments, one in the normal oxidised condition and one coated with a layer of palladium, were monitored “in situ” in the storage vessel of a deaerator system in a 20 MW boiler. Although both the test runs, called gamma and delta, were dogged with abnormal operational conditions, viz. numerous off-load events occurred in one run and intermittent condenser leaks in the other, it was established that the Pd coated weld exhibited ECP levels which were consistently lower than the normal oxidised weld. Indeed the differences varied from 100 to 180 mV and averaged 125 mV. When the normal oxidised weld ECP data from the earlier test runs where little, or no, upset operational conditions occurred was compared to the present oxidised data sets it was evident that the initial conditions after going on-load were critical in determining the ECP levels attained for each test. If off-load events or oxygen leaks were prevalent during the initial 400 h, where the hysterisis effects were noticed, the subsequent ECP levels were approximately 200 mV more positive than those ECP levels attained from a test run where no upset conditions were present during the initial stages. Through the use of the beneficial effects arising from Pd coated welds, it was shown that the ECP values in real working deaerator storage vessel welds could be “conditioned”, i.e. lowered, to levels which were well below the critical ECP levels required to induce environmental assisted cracking, EAC, viz. E CRIT, during operation. Other locations in a boiler, viz. those that are water filled, which are prone to EAC can also be treated in a similar manner