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An important factor preventing a more widespread acceptance of magnesium alloys in the chemical industry is its susceptibility to various forms of corrosion. Stress-corrosion cracking, occurring under conditions of residual fabrication stresses or high external stresses, is a severe localized corrosion type which can bring about complete failure in most wrought magnesium alloys. Previous investigations concerned with stress-corrosion cracking of magnesium alloys have dealt with its occurrence only in the sheet metal. The investigation involved accelerated laboratory stress-corrosion cracking tests on a commercial extrusion. Standard tension test specimens of magnesium alloy FS-1 extruded rectangular bar were subjected to bending in constant deformation by loading them transversely as end-supported center-loaded beams. The apparatus was constructed of angle irons for end supports and steel bars for an adjustable center load. Maximum tensile strains were measured with electric strain gages and corresponding stresses were determined from the tensile stress-strain diagram for the alloy. A corrosive medium of 35 grams per liter of sodium chloride and 20 grams per liter of potassium chromate was placed in small sections of glass tubing sealed to the specimen surface at the region of maximum tensile stress. The effects of the magnitude of tensile stress and pH on time to failure were investigated and the stress corrosion cracking limits (critical stresses) were determined at various pH values from 0.5 to 11.85. The apparatus was satisfactory for laboratory testing and the basic design was recommended to be adopted as a standard. The effect of creep was corrected to some extent, since creep rates were very high. The minimum critical stress for the alloy was found to be 27,600 psi, or 125 per cent of the tensile yield strength, at a pH of 3.0-3.5. Values of pH between 6.0 and 8.5 have no appreciable effect on the critical stress, although pH values below 3.0 and above 8.5 increase the stress-corrosion limit. From pH 6.0 to 3.5 critical stress is decreased from 29,300 psi to 27,600 psi. Creep was evident at strains of 3985 microinches and higher. / Master of Science

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by Lo, Chi Ho Chester. / Title also in Chinese characters. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1994. / Includes bibliographical references (leaves 159-163). / Acknowledgements --- p.i / Abstract --- p.ii / Table of Contents --- p.iv / List of Figures --- p.vii / List of Tables --- p.xii / Chapter Chapter One --- Introduction --- p.1 / Chapter Chapter Two --- Domain Theory --- p.8 / Chapter 2.1 --- Energies in Magnetic Domain Structure --- p.8 / Chapter 2.2 --- Domains in Iron and Nickel --- p.13 / Chapter 2.3 --- Magnetization Process --- p.15 / Chapter 2.4 --- Effect of Applied Stress --- p.19 / Chapter Chapter Three --- Magnetoacoustic Emission --- p.25 / Chapter 3.1 --- Models of MAE --- p.25 / Chapter 3.1.1 --- Discontinuous Wall Motion --- p.25 / Chapter 3.1.2 --- Displacement Model --- p.28 / Chapter 3.1.3 --- DW Creation and Annihilation --- p.30 / Chapter 3.1.4 --- Combined Model of MAE --- p.31 / Chapter 3.2 --- MAE and Magnetic Induction --- p.34 / Chapter 3.2.1 --- Eddy Current Shielding --- p.34 / Chapter 3.2.2 --- Magnetic Reluctance Calculation --- p.35 / Chapter Chapter Four --- Experiments / Chapter 4.1 --- Instrumentation --- p.40 / Chapter 4.1.1 --- Introduction --- p.40 / Chapter 4.1.2 --- Basic Setup --- p.41 / Chapter 4.1.3 --- Arrangement for Stress Measurement --- p.46 / Chapter 4.1.4 --- Specimen Preparation --- p.43 / Chapter 4.2 --- Methodology / Chapter 4.2.1 --- The Fundamental Study of MAE --- p.53 / Chapter 4.2.1.1 --- Effects of Demagnetizing and Stray Fields on MAE --- p.53 / Chapter 4.2.1.2 --- Dependence of MAE on Frequency of Applied Field --- p.55 / Chapter 4.2.1.3 --- Dependence of MAE on Specimen Thickness and Width --- p.55 / Chapter 4.2.2 --- Stress Measurement --- p.58 / Chapter 4.2.2.1 --- Effect of Uniaxial Stress on MAE --- p.58 / Chapter 4.2.2.2 --- Effect of Biaxial Stresses on MAE --- p.58 / Chapter 4.2.3 --- Defect Detection --- p.60 / Chapter 4.2.3.1 --- Nickel --- p.60 / Chapter 4.2.3.2 --- Mild Steel --- p.61 / Chapter Chapter Five --- Results and Discussion / Chapter 5.1 --- Effects of Demagnetizing and Stray Fields on MAE --- p.63 / Chapter 5.1.1 --- MAE Profiles --- p.63 / Chapter 5.1.2 --- Magnetic Reluctance Calculation --- p.68 / Chapter 5.1.3 --- Effect of Annealing --- p.74 / Chapter 5.1.3.1 --- Experimental Results --- p.74 / Chapter 5.1.3.2 --- Discussion --- p.77 / Chapter 5.1.3.3 --- Magnetic Reluctance Calculation --- p.78 / Chapter 5.2 --- Dependence of MAE on Frequency of Applied Field --- p.83 / Chapter 5.2.1 --- Experimental Results --- p.83 / Chapter 5.2.2 --- Theoretical Consideration --- p.88 / Chapter 5.3 --- Dependence of MAE on Specimen Thickness and Width --- p.96 / Chapter 5.3.1 --- Experimental Results --- p.96 / Chapter 5.3.2 --- Theoretical Consideration --- p.99 / Chapter 5.4 --- Effects of Uniaxial and Biaxial Stresses on MAE --- p.107 / Chapter 5.4.1 --- Effect of Uniaxial Stress --- p.107 / Chapter 5.4.1.1 --- Experimental Results --- p.107 / Chapter 5.4.1.2 --- Discussion --- p.116 / Chapter 5.4.2 --- Effect of Biaxial Stresses --- p.120 / Chapter 5.4.2.1 --- Study on Mild Steel Specimen --- p.120 / Chapter 5.4.2.2 --- Study on Nickel Specimen --- p.132 / Chapter 5.5 --- Defect Detection by MAE --- p.137 / Chapter 5.5.1 --- Study on Nickel Specimen --- p.137 / Chapter 5.5.1.1 --- Experimental Results --- p.137 / Chapter 5.5.1.2 --- Discussion --- p.140 / Chapter 5.5.2 --- Study on Mild Steel Specimen --- p.142 / Chapter 5.5.2.1 --- Experimental Results --- p.142 / Chapter 5.5.2.2 --- Discussion --- p.151 / Chapter Chapter Six --- Conclusions and Suggestions for Further Studies --- p.153 / References --- p.159

Duplex stainless steel (DSS) is a dual-phase material with approximately equal volume amount of austenite and ferrite. It has both great mechanical properties (good ductility and high tensile/fatigue strength) and excellent corrosion resistance due to the mixture of the two phases. Cyclic loadings with high stress level and low frequency are experienced by many structures. However, the existing study on corrosion fatigue (CF) study of various metallic materials has mainly concentrated on relatively high frequency range. No systematic study has been done to understand the ultra-low frequency (10-5 Hz) cyclic loading effect on stress corrosion cracking (SCC) of DSSs. In this study, the ultra-low frequency cyclic loading effect on SCC of DSS 2205 was studied in acidified sodium chloride and caustic white liquor (WL) solutions. The research work focused on the environmental effect on SCC of DSS 2205, the cyclic stress effect on strain accumulation behavior of DSS 2205, and the combined environmental and cyclic stress effect on the stress corrosion crack initiation of DSS 2205 in the above environments. Potentiodynamic polarization tests were performed to investigate the electrochemical behavior of DSS 2205 in acidic NaCl solution. Series of slow strain rate tests (SSRTs) at different applied potential values were conducted to reveal the optimum applied potential value for SCC to happen. Room temperature static and cyclic creep tests were performed in air to illustrate the strain accumulation effect of cyclic stresses. Test results showed that cyclic loading could enhance strain accumulation in DSS 2205 compared to static loading. Moreover, the strain accumulation behavior of DSS 2205 was found to be controlled by the two phases of DSS 2205 with different crystal structures. The B.C.C. ferrite phase enhanced strain accumulation due to extensive cross-slips of the dislocations, whereas the F.C.C. austenite phase resisted strain accumulation due to cyclic strain hardening. Cyclic SSRTs were performed under the conditions that SCC occurs in sodium chloride and WL solutions. Test results show that cyclic stress facilitated crack initiations in DSS 2205. Stress corrosion cracks initiated from the intermetallic precipitates in acidic chloride environment, and the cracks initiated from austenite phase in WL environment. Cold-working has been found to retard the crack initiations induced by cyclic stresses.

Duplex stainless steels (DSS) with roughly equal amount of austenite and ferrite phases are being used in industries such as petrochemical and pulp and paper mills. However, many DSS grades have been reported to undergo corrosion and stress corrosion cracking in some aggressive environments such as chlorides and sulfide-containing caustic solutions. Although stress corrosion cracking of duplex stainless steels in chloride solution has been investigated and well documented in the literature, SCC mechanisms for DSS in caustic solutions were unknown. Microstructural changes and environmental factors, such as pH of the solution, temperature, and resulting electrochemical potential also influence the SCC susceptibility of duplex stainless steels. In this study, the role of material and environmental parameters on corrosion and stress corrosion cracking of duplex stainless steels in caustic solutions were investigated. Results showed that the austenite phase in the DSS is more susceptible to crack initiation and propagation in caustic environment, which is different from that in the low pH chloride environment where the ferrite phase is the more susceptible phase. This study also showed that alloy composition and microstructural changes in duplex stainless steels due to different heat treatments could affect their SCC susceptibility. Moreover, corrosion rates and SCC susceptibility of DSS was found to increase with addition of sulfide to caustic solutions. Corrosion films on DSS indicated that the metal sulfide compounds formed along with oxides at the metal surface in the presence of sulfide containing caustic environments made the steel susceptible to SCC initiations. The overall results from this study helped in understanding the mechanism of SCC in caustic solutions. Favorable slip systems in the austenite phase of DSS favors slip-induced local film damage thereby initiating a stress corrosion crack. Repeated film repassivation and breaking, followed by crack tip dissolution results in crack propagation in the austenite phase of DSS alloys. Result from this study will have a significant impact in terms of identifying the alloy compositions, fabrication processes, microstructures, and environmental conditions that may be avoided to mitigate corrosion and stress corrosion cracking of DSS in caustic solutions.

This research investigates the mechanism of transgranular stress corrosion cracking (TGSCC) on fuel transmission pipelines. This research proposes that in near-neutral pH environments, hydrogen can be generated by the dissociation of carbonic acid and the reaction of metal ions with bicarbonate solutions, significantly increasing the available hydrogen for diffusion into the pipeline steel. This research has shown that TGSCC of pipeline steels is possible in simple groundwater solutions containing bicarbonate ions and carbon dioxide. Microstructural characterization coupled with hydrogen permeation indicates that the level of strain in the microstructure has the most influence on hydrogen diffusivity. Hydrogen accumulation occurs preferentially in at high energy discontinuous interfaces such as inclusion interfaces. It was determined that a stress concentration is required to facilitate sufficient hydrogen accumulation in the pipeline steel in order to initiate TGSCC. It was discovered that these stress concentrations develop from inclusions falling out of the pipeline surface. Slow strain rate tests found that TGSCC occurred in a wide range of compositions and temperatures as long as near-neutral conditions were maintained. Microcracks ahead of the crack tip provide evidence of hydrogen in these cracking processes. Morphology of these microcracks indicates that cracks propagate by the coalescence of microcracks with the main crack tip. Further research findings, scientific impact, and potential future work are also discussed.

The effect of crack tip strain rate (CTSR) on environmentally assisted cracking was studied for alloys AA-7075 (Al-Zn-Mg-Cu) and AA-8090 (Al-Li-Cu-Mg) in the artificially aged condition. Fatigue pre-cracked double cantilever beam (DCB) specimen were employed with the crack plane parallel to the rolling plane. The cracking behaviour under monotonic and cyclic loading conditions was investigated in aqueous sodium chloride solutions with and without additions of sodium chromate as a corrosion mhibitor. CTSR values were described in terms of K-rate ∆K/∆t (ie. dK/dt) as a measured average over the loading period of a fatigue cycle. This allowed a comparison with CTSR's of monotonically increasing load or constant load tests. At frequencies ≤1 Hz, the load was applied with a triangular wave form. A high frequency of 30 Hz was obtained by sinusoidal loading. Expressed as K-rate, CTSR values were varied over 7 orders of magnitude from 10⁵MPa√m/s to 10² MPa√m/s. Stress intensities investigated were mainly around region II values with respect to SCC K-log(da/dt) behaviour. At low K-rates, real time crack velocities (da/dt) measured under monotonic slow loading or constant load conditions were comparable to crack velocities obtained with cyclic loading experiments. As the K-rate was increased from low values, typical of constant load experiments, the real time crack velocities decreased. This was caused by plasticity induced crack growth retardation effects and a decrease in crack tip film rupture events during the unloading part of a cycle. The crack propagation rate decreased until minimal crack advance increments per cycle were dictated by mechanical parameters acting on a hydrogen embrittled crack tip region. Under monotonic loading conditions region II crack velocities were not influenced by an increase in K-rate which was explained with a mass transport controlled cracking process. Tests with alloy 7075 at intermediate K-rates and a high R-ratio of 0.78 allowed a crack tunnelling mechanism to operate. This overcame the plasticity induced crack growth retardation and, therefore, cracks propagated at the same rates as during low K-rate tests where no retardation phenomena were encountered. Scanning electron microscope investigations revealed a striated intergranular fracture surface of alloy 7075 if tested at K-rates above the transition value to K-rate independent crack propagation rates. Individual striations could be matched on opposing fracture surfaces and the striation spacing corresponded to the average crack propagation increment per cycle. The striations, therefore, were formed as part of the crack advance during every fatigue cycle. At the lower K-rates no striations were present but micro tear ridges could be found on the intergranular fracture facets indicating that dissolution processes alone did not cause the intergranular crack advance. Alloy 8090 did not reveal significant changes in fractography over the entire K-rate range investigated, except at the highest K-rates where small interlocking steps could be detected on some opposing transgranular fracture surfaces. In general, however, the crack path at all K-rates was mainly intergranular with dimpled fracture facets. Alloy 8090 exhibited a high resistance to SCC with fatigue pre-cracked DCB specimen. Therefore, to obtain crack velocity values with low K-rate monotonic loading tests very long test durations would have been necessary. It is concluded that the transition from intergranular SCC to intergranular CF occurs at a critical K-rate. Below the critical K-rate crack velocities are not increased by cyclic loading. Instead crack growth retardation effects can result in lower real time crack velocities than those typical for constant load tests at comparable stress intensities but much lower K-rates. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Chloride-induced stress corrosion cracking (CISCC) has been identified as the main degradation mechanism for spent nuclear fuel dry storage canisters. This type of induced cracking is complex and depends on several factors, such as material composition, exposure temperature, relative humidity, applied tensile stress, and atmospheric salt concentration. An accelerated experiment was designed to simulate marine environments in a controlled fogging chamber to examine 304 and 304L stainless steel U-bend and welded U-bend samples. The samples were exposed to chloride rich and humid fogging in a corrosion chamber at 35℃ continuously for 4 weeks, 8 weeks, and 12 weeks. The same experiment was repeated at 50℃ for 4 weeks, 8 weeks, and 14 weeks to study the sensitivity of CISCC to temperature changes. A qualitative evaluation of optical micrographs from a 3D Surface Profiler was performed for 16 corroded samples and compared with 2 reference samples. Cracking was observed on 12 out of 16 samples exposed to 35℃ and 50℃ for durations ranging from 8 to 14 weeks. Likely cracking observations were noted on 4 out of 16 samples. A quantitative statistical analysis was also performed using surface profile depth (valley) data from corroded and reference samples. The quantitative analysis examined the effect of temperature, welding, exposure duration, and material composition. The quantitative results were compared with the qualitative results and literature published in CISCC. / Master of Science / Most nuclear power plants are currently using dry storage canisters (DSCs) which are made of a concrete vault and a stainless steel canister that houses the spent nuclear fuel (SNF) assemblies. Multiple conditions must be present simultaneously for chloride-induced stress corrosion cracking (CISCC) to develop, such as the presence of a susceptible alloy, high relative humidity, high temperature, high atmospheric salt concentrations, and applied tensile stresses. DSCs are typically made from 300-series austenitic stainless steels which are susceptible to this type of corrosion during long-term storage near marine environments. Therefore, understanding of the factors leading to CISCC is critically important for proper management and mitigation and to estimate the service life of DSCs for the safe long-term storage of SNF. An accelerated experiment was designed to examine the effects of temperature, exposure duration, and welding on pitting and cracking for 304 and 304L U-bend samples. The experimental results concluded that stainless-steel grades 304 and 304L are susceptible to CISCC when exposed for 8 weeks or longer to fogging at temperatures between 35℃ and 50℃, 95% relative humidity, and 5% salt concentration. This study also concluded that increasing exposure duration from 8 to 12 weeks or the temperature from 35℃ to 50℃ had no significant effect on the acceleration of CISCC. Also, unwelded samples were deemed more susceptible to CISCC than welded samples and the susceptibility of 304 and 304L grades were relatively similar.

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / by Ronald George Ballinger. / Sc.D.