No description available.

No description available.

The purpose of this investigation was to determine the corrosion characteristics of Incoloy alloy 825 in 1.0 to 15.0 normal sulfuric and 0.5 to 3.0 normal hydrochloric acids at 25 to 60°C in both nitrogen-saturated and air-saturated acids, using potentiostatic polarization techniques. Anodic polarization curves were obtained by changing the potential of an Incoloy 825 test electrode and measuring the resulting current. In both acids, there is no difference in the polarization characteristics in air-or nitrogen-saturated solutions. In 1.0 to 15.0 normal sulfuric acid at 25 to 60°C, the alloy spontaneously passivates and does not exhibit a significant active region. In 0.5 normal hydrochloric acid at 25°C, Incoloy 825 is spontaneously passive, but is active at higher concentrations. At 25°C, as the hydrochloric acid concentration increases from 1.0 to 3.0 normal,the critical current density increases from 260 to 5900 microamperes per square centimeter. At 40°C, as the acid concentration increases from 0.5 to 2.0 normal, the critical current density increases from 25 to 28,000 microamperes per square centimeter. In 1.0 normal hydrochloric acid at 25°C, the corrosion rate calculated by weight loss measurements for potentiostatically controlled Incoloy 825 agrees closely with the corrosion rate calculated from polarization current densities, and indicates that the elements dissolve in the proportions present in the alloy. Potentiostatically passivated Incoloy 825 in 1.0 normal hydrochloric acid at 25°C does not exhibit a stable passive condition, reverting to the active state in less than 12 hours. / M.S.

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.

No description available.

Stress corrosion cracking (SCC) from the external surface of a buried pipeline is a serious matter and can cause significant economic and environmental losses. Despite of many research works which have been done on the understanding of crack initiation and propagation mechanisms, these mechanisms are still being debated. This research studied the crack growth behaviour of different pipeline steels including two types of X65, one X52 and one X80 pipeline steels in near-neutral pH environments. Crack growth behaviour of all steels has been found to be consistent with that of true corrosion fatigue. Crack growth rates were correlated with (K)2Kmax/f0.1. It was revealed that cracking behaviour of pipeline steels in near neutral pH environments is material dependent. Highest crack growth rate was seen in the steel which highest amount of hydrogen atoms could be generated and stored in its microstructure to contribute in cracking procedure due to hydrogen embrittlement effect. / Materials Engineering

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

abstract: AA 7XXX alloys are used extensively in aircraft and naval structures due to their excellent strength to weight ratio. These alloys are often exposed to harsh corrosive environments and mechanical stresses that can compromise their reliability in service. They are also coupled with fasteners that are composed of different materials such as Titanium alloys. Such dissimilar metal contact facilitates galvanic and crevice corrosion, which can further reduce their lifetimes. Despite decades of research in the area, the confluence of mechanical, microstructural, and electrochemical aspects of damage is still unclear. Traditionally, 2D and destructive methods have often been employed to study the corrosion and cracking behavior in these systems which can be severely limiting and lead to inaccurate conclusions. This dissertation is aimed at comprehensively studying the corrosion and cracking behavior of these systems using time-dependent 3D microstructural characterization, as well as correlative microscopy. The microstructural evolution of corrosion in AA 7075 was studied using a combination of potentiodynamic polarization, X-ray Computed Tomography (XCT) and Transmission X-ray Microscopy (TXM). In both experiments, a strong emphasis was placed on studying localized corrosion attack at constituent particles and intergranular corrosion. With an understanding of the alloy’s corrosion behavior, a dissimilar alloy couple comprising AA 7075 / Ti-6Al-4V was then investigated. Ex situ and in situ x-ray microtomography was used extensively to investigate the evolution of pitting corrosion and corrosion fatigue in AA 7075 plates fastened separately with Ti-6Al-4V screws and rivets. The 4D tomography combined with the extensive fractography yielded valuable information pertaining the preferred sites of pit initiation, crack initiation and growth in these complex geometries. The use of correlative microscopy-based methodologies yielded multimodal characterization results that provided a unique and seminal insight on corrosion mechanisms in these materials. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020

The increased energy demand within South Africa has led to continued periods of load shedding. This has had an adverse impact on industry, quality of life and the economy as a whole. A larger requirement for production time, reduced downtime and an enlarged focus on health and safety have steered industry towards a paradigm shift in inspection and maintenance. These activities have progressed from a predominantly time-based (prescriptive) approach towards a risk-based approach. Generally accepted standards like BS EN 16991:2018 and API RP 580 give a comprehensive outline of the basic elements for developing, implementing and maintaining a risk-based inspection program. API RP 581 takes this outline one step further and contains the quantitative methods that support the minimum guidelines presented by API RP 580. Similarly, Kent W. Mühlbauer’s approach has developed a relative risk ranking model for petroleum and gas pipelines, which outlines a qualitative method for representing risk. None of these models are however directly applicable to predicting the failure of pressurised boiler equipment due to the mechanism of corrosion fatigue. API RP 580 / 581 was primarily developed for the oil and gas industry and have practical limitations when applied to pressurised equipment typically found in utilities. BS EN 16991:2018 supplies a framework for utilities, but doesn’t go into the specific detail of how to structure, formulate and apply a risk based management model. The methodology laid out by Kent W. Mühlbauer, while practical and easily implemented, was designed for oil and gas pipelines. A systematic methodology to evaluate the risk associated with specific failure mechanisms in boilers, such as corrosion fatigue, does not exist or is not readily available. A comprehensive risk-based predictive model, using aspects of the abovementioned standards and guides, was developed to demonstrate the predictability of corrosion fatigue in sub-critical boilers. Weightings were assigned to contributory causes to corrosion fatigue, which then allocated relative risk ranks to certain segments within a boiler. Operators and owners of boilers can derive benefit from this model by focusing inspection, maintenance and alteration activities on those equipment locations with the highest relative risk score. / Dissertation (MEng (Metallurgical Engineering))--University of Pretoria 2020. / Eskom Power Plant Engineering Institute (EPPEI) Supervisor: Mr. L. Reddy / Materials Science and Metallurgical Engineering / MEng (Metallurgical Engineering) / Unrestricted

Les connecteurs clips utilisés pour assembler les tubes de riser pour le forage pétrolier offshore subissent un chargement cyclique dû aux vagues. 90% de la durée de service est passée en mode connecté, avec une contrainte moyenne élevée, alors que 10% est passé en mode déconnecté, avec une faible contrainte moyenne. Des calculs numériques montrent qu’un chargement cyclique de tension biaxiale en phase prévaut dans la zone critique de la structure. Les effets de contrainte moyenne et de biaxialité doivent tous deux être pris en compte pour un design approprié. Les critères de fatigue multiaxiale de la littérature sont basés sur des données de traction-torsion et ne discriminent pas bien l’influence de la tension biaxiale de celle d’une contrainte moyenne. Un des objectif de cette étude est donc de caractériser séparément ces deux effets.Pour étudier les effets de contrainte moyenne, des essais de fatigue uniaxiale ont été menés avec différents rapports R. Les durées de vie diminuent avec l'augmentation de R, et la limite d'endurance suit la parabole de Gerber. À faible contrainte moyenne et amplitude de contrainte élevée, les fissures s’amorcent en surface, tandis que pour des rapports R élevés et des amplitudes faibles, les fissures s’amorcent à partir de défauts internes ou coupant la surface. Cette transition est analysée à partir de calculs élasto-plastiques des champs de contrainte et déformation autour des défauts. Les fissures internes se propagent sous un faible ΔK indépendant de R, ce qui est attribué à la quasi absence d’effets de fermeture.Pour étudier l'effet de biaxialité, des essais cycliques de tension et pression interne combinées en diverses proportions ont été effectuées à rapport R fixe (0,25). Un taux de biaxialité modéré (B = 0,25 et 0,5) a un effet bénéfique, attribué à un retard de l'amorçage des fissures, alors que la tension équibiaxiale a un effet légèrement nuisible, attribué à un "pseudo effet de taille" (probabilité plus grande qu’une microfissure se propage le long de deux plans principaux équivalents, au lieu d’un seul).De facettes intergranulaires révélatrices de fragilisation par l'hydrogène ont été observées sur les surfaces de rupture. L’évolution de leur fraction avec ΔK et la biaxialité suggèrent une réduction de la vitesse de fissuration à B≤0.5, mais l'effet néfaste de la tension équibiaxiale ne peut être attribué à une accélération de la propagation.Plusieurs des critères de fatigue existants échouent à décrire toutes les données de cette étude. Les critères d'endurance avec un terme linéaire de contrainte moyenne ou de tension hydrostatique ne parviennent pas à prédire les variations de la limite d'endurance. Un nouveau critère de fatigue a été proposé sur la base de la parabole de Gerber. Il décrit bien les effets combinés d'une contrainte moyenne et d’un taux de biaxialité positif.Des essais biaxiaux ont également été effectués dans l’eau salée (3.5% NaCl) pour étudier l'influence de ce milieu sur les durées de vie en fatigue, en corrosion libre et avec une protection cathodique qui amplifie le dégagement d’hydrogène.En corrosion libre, l'eau salée réduit fortement les durées de vie et supprime la limite d'endurance. Cela est dû à la formation de piqûres de corrosion qui favorisent l’amorçage précoce et multiples de fissures. La tension équibiaxiale n’accentue pas l’effet nocif de l'eau salée, malgré des mécanismes de fissuration différents: décohésion fragile transgranulaires en tension uniaxiale, mais principalement intergranulaire en tension biaxiale.La protection cathodique annule l'effet néfaste de l'eau salée pour tous les taux de biaxialité, en dépit d'un net accroissement de la fragilisation par l’hydrogène des joints de grains. Les surfaces de rupture deviennent presque entièrement intergranulaire, tandis qu’à l'air, le taux de rupture intergranulaire ne dépasse pas 45%. / The clip connectors used to join the riser tubes for offshore oil drilling undergo cyclic loading due to sea waves. 90% of the service life is spent in the “connected mode” with a high mean stress and 10% in the “disconnected mode” with a lower mean stress. Finite element computations revealed in-phase biaxial tension in the critical areas of the clip connector along with high mean stresses. Thus, both the mean stress effect and the biaxiality effect need to be addressed for proper design of these structures. However, most of the multiaxial fatigue criteria are based on tension-torsion fatigue data and do not discriminate the influence of biaxial tension from that of a mean stress. This study investigates separately these two effects.For investigating the mean stress effect, uniaxial fatigue tests were run on Cr-Mo steel with various R ratios (σmin/σmax). The fatigue lives, as well as the slope of the S-N curves were found to decrease with increasing R, and the endurance limit to follow Gerber’s parabola. At low R ratios and thus relatively high stress ranges, fatigue cracks initiated from the surface, while for high R ratios, and thus low stress ranges, cracks initiated from internal or surface-cutting defects. This transition was analyzed based on elastic-plastic computations of stress-strain fields around the defects. The threshold for internal fatigue crack growth from defects was found to be quite low and independent from the R ratio. This was attributed to a nearly closure-free propagation.To investigate the effect of positive stress biaxiality, combined cyclic tension and internal pressure tests with various proportions of each loading were run on tubular specimens, at fixed R ratio (0.25). Moderate stress biaxialities (B= 0.25 and 0.5) had a beneficial effect on fatigue lives, attributed mainly to a retardation of crack initiation, while equibiaxial tension had a slightly detrimental effect, attributed to a “pseudo size effect” (higher probability for an incipient crack to grow along two possible planes, compared to a single one).Intergranular facets associated with temper and H2 embrittlement were observed on the fracture surfaces. The evolutions of their surface fraction with ΔK and load biaxiality suggested a possible reduction in crack growth rate at moderate biaxialities, but the detrimental effect of equibiaxial tension could not be explained in terms of crack growth rate.Several popular fatigue criteria failed to describe all fatigue data. Endurance criteria that include a linear mean stress term or contain a hydrostatic tension term fail to predict the variations of the endurance limit of this material with the R ratio and biaxiality ratio. Thus, a new fatigue criterion based on Gerber’s parabola was proposed. It captured the evolution of the endurance limit under the combined effects of positive mean stress and biaxiality.Similar tests were run to investigate the influence of salt water (3.5% NaCl) on fatigue lives under two types of test conditions: 1) free corrosion and 2) cathodic protection.In free corrosion, salt water strongly reduced the fatigue lives and suppressed the endurance limit. This was due to the formation of corrosion pits that favor early, multiple crack initiations. The detrimental effect of salt water was not enhanced by equibiaxial tension, which did not modify the size and density of corrosion pits. Fatigue lives in uniaxial and biaxial tension were nearly the same, although the crack growth mechanism was different: transgranular brittle decohesion in uniaxial loading and mostly intergranular in biaxial tension.Cathodic protection cancelled the detrimental effect of salt water for all biaxialities, in spite of a clear enhancement of H-induced embrittlement of the grain boundaries. The fracture surfaces were nearly fully intergranular, irrespective of load biaxialities, while in air the proportion of intergranular fracture was less than 45%.