Dissimilar metal joining was performed with the main goal being maximization of the strength of the joined samples, but because of some potential applications of the dissimilar joints, analyzing their corrosion behavior also becomes crucial. Starting with materials that initially have suitable corrosion resistance, ensuring that the laser processing does not diminish this property is necessary. Conversely, the laser shock peening processing was implemented with a complete focus on improving the corrosion behavior of the workpiece. Thus, many commonalities occur between these two manufacturing processes, and this thesis goes on to analyze the thermal and mechanical influence of laser processing on materials’ corrosion resistances. Brittle intermetallic phases can form at the interfaces of dissimilar metal joints. A process called autogenous laser brazing has been explored as a method to minimize the brittle intermetallic formation and therefore increase the fracture strength of joints. In particular, joining of nickel titanium to stainless steel wires is performed with a cup/cone interfacial geometry. This geometry provides beneficial mechanical effects at the interface to increase the fracture strength and also enables high-speed rotation of the wires during irradiation, providing temperature uniformity throughout the depth of the wires. Energy dispersive X-ray spectroscopy, tensile testing, and a numerical thermal modelling are used for the analysis. The material pair of nickel titanium and stainless steel have many applications in implantable medical devices, owing to nickel titanium’s special properties of shape memory and superelasticity. In order for an implantable medical device to be used in the body, it must be ensured that upon exposure to body fluid it does not corrode in harmful ways. The effect that laser autogenous brazing has on the biocompatibility of dissimilar joined nickel titanium to stainless steel samples is thus explored. While initially both of these materials are considered to be biocompatible on their own, the laser treatment may change much of the behavior. Thermally induced changes in the oxide layers, grain refinement, and galvanic effects all influence the biocompatibility. Nickel release rate, polarization, hemolysis, and cytotoxicity tests are used to help quantify the changes and ascertain the biocompatibility of the joints. To directly exert a beneficial influence on materials’ corrosion properties laser shock peening (LSP) is performed, with a particular focus on the stress corrosion cracking (SCC) behavior. Resulting from the combination of an applied load on a susceptible material exposed to a corrosive environment, SCC can cause sudden material failure. Stainless steel, high strength steel, and brass are subjected to LSP and their differing corrosion responses are determined via cathodic charging, hardness, mechanical U-bend, Kelvin Probe Force Microscopy, and SEM imaging. A description accounting for the differing behavior of each material is provided as well as considerations for improving the effectiveness of the process. SCC can occur by several different physical processes, and to fully encapsulate the ways in which LSP provides mitigation, the interaction of microstructure changes induced by LSP on SCC mechanisms is determined. Hydrogen absorbed from the corrosive environment can cause phase changes to the material. Cathodic charging and subsequent X-ray diffraction is used to analyze the phase change of sample with and without LSP processing. Lattice dislocations play an important role, and transmission electron microscopy helps to aid in the analysis. A finite element model providing spatially resolved dislocation densities from LSP processing is performed.

Environmentally Assisted cracking (EAC) is a very critical materials science problem that concerns many technological areas such as petrochemical engineering, aerospace operations and nuclear power generation, in which cracking or sudden failure of materials may happen at stress far below the tensile strength. This type of corrosion is initiated at the microscopic level and is complicated due to the combination of chemistry (reaction caused by corrosive agents) and mechanics (varying load). As EAC is generally related to the segregation of impurity elements to defects (mainly grain boundaries), the symptoms of risk may not be apparent from the exterior of the metal components: hence EAC remains latent and gives no sign of warning until the failure occurs. Due to its intricate nature, conducting experiments on this phenomenon involves difficulties and requires much effort. In this work, we employed advanced molecular simulation techniques to study EAC in order to give insight into its atomistic behavior. First, Density-Functional Theory (DFT) method was used to investigate the fundamental processes and mechanism of EAC-related issues at the nanoscale level, with two case studies concerning the stress corrosion in iron and hydrogen embrittlement in palladium. When segregating to the grain boundary (GB) of iron, different impurity elements such as sulfur, phosphorus and nitrogen raise corrosion failures in a variety of ways. Hydrogen atoms, due to their mobility and small atomic size, are able to form high occupation at crystal defects, but show different interactions to vacancy and GB. Then, we used the classical Molecular Dynamics (MD) method to gain an understanding of the dynamic response of materials to mechanical load and the effects of temperature, strain and extreme conditions (high pressure shock compression) on structural properties. The MD simulations show that hydrogen maintains the highest localization at grain boundaries in the vicinity of ambient temperatures, and grain boundaries are the preferred nucleation sites for dislocations and voids. This computational work, using DFT and MD techniques, is expected to contribute to the better understanding on chemistry and mechanisms of complex environment-assisted cracking phenomenon at a fundamental level in order to beneficially complement conventional laboratory approaches.

Corrosion and stress corrosion cracking of structural components cost an estimated $300 billion annually in the United States alone and are a safety concern for a number of industries using hot alkaline environments. These process environments may contain different amounts of sulfide and chloride; however, the combined role of these ions on the stress corrosion cracking of duplex stainless steels, which are widely used because of their generally reliable performance, had never been studied. This study shows that chlorides in sulfide-containing caustic environments actually have a significant influence on the performance of these alloys. A mechanism for stress corrosion cracking of duplex stainless steels in hot alkaline environments in the presence of sulfide and/or chloride was proposed. Microstructural and environmental aspects were studied using mechanical, electrochemical, and film characterization techniques. The results showed that selective corrosion of the austenite phase depended on percent sulfidity, alkalinity, and chloride content. Chlorides enhanced crack initiation and coalescence along the austenite/ferrite phase boundaries. Unstable passivity of duplex stainless steels in hot alkaline-sulfide environments was due to anion adsorption on the surface leading to defective film formation. Chlorides and sulfide available at the electrolyte/film surface reduced the charge transfer resistance and shifted the response of the films to lower frequencies indicating the films became more defective. The surface films consisted of an outer, discontinuous layer, and an inner, barrier layer. Fe, Mo, and Mn were selectively dissolved in alkaline and alkaline-sulfide environments. The onset of stress corrosion cracking was related to the extent of selective dissolution and was consistent with a film breakdown and repair mechanism similar to slip-step dissolution. Recommendations for reducing the susceptibility of duplex stainless steels to stress corrosion cracking in sulfide-containing caustic environments include reducing the chloride to hydroxide ratio and alloying with less Mo and Mn. The results will impact the petrochemical, pulp and paper, and other process industries as new duplex grades can be developed with optimal compositions and environments can be controlled to extend equipment life.

Al-Zn-Mg-Cu (7xxx series) aluminium alloys are widely used for aircraft structures. It is difficult to obtain a combination of optimal strength and stress corrosion cracking (SCC) resistance for these alloys. It appears that SCC resistance of these alloys is related to grain boundary precipitate morphology. One technique to control the grain boundary precipitate morphology is to introduce a controlled cooling procedure referred to as High Temperature Pre-precipitation (HTPP) treatment following the solution heat treatment. There is need for a detailed study of the effect of HTPP on the properties of commercial Al-Zn-Mg-Cu alloys using different intermediate temperatures. In this thesis research, the results of ten HTPP processes applied to 7075 and 7050 commercial 7xxx series alloys are presented in terms of hardness, electrical conductivity, corrosion resistance, TEM analysis of grain boundary precipitate morphology and EDS analysis of solute concentration profile at the grain boundary. Results indicate that subsequent to HTPP processing, the 7050 alloy can be precipitation aged to a higher hardness compared to 7075; this result is associated with the modification of 7050 alloy by zirconium versus chromium in 7075 alloy. HTPP heat treatment achieves better SCC resistance compared to standard T6 temper. However, it does not appear that HTPP can achieve a combination of hardness, electrical conductivity and corrosion resistance superior to standard T6 and T7X tempers. / October 2014

No description available.

This research uses nonlinear Rayleigh surface waves to characterize stress corrosion cracking (SCC) damage in sensitized 304 Stainless Steel (304 SS). 304 SS is widely used in reactor pressure vessels and fuel pipelines, where a corrosive environment in combination with applied stress due to high internal pressures can cause SCC. SCC poses great risk to these structures as it initiates cracks late in the lifetime and often unexpectedly. The initiated microcracks grow and accumulate very quickly to form macroscopic cracks that lead to material failure. Welds and the nearby heat affected zones (HAZ) in the vessels and pipework are particularly affected by SCC as welding induces sensitization in the material. SCC damage results in microstructural changes such as dislocation movement and microcrack initiation that in the long term lead to reduced structural integrity and material failure. Therefore, the early detection of SCC is crucial to ensure safe operation. It has been shown that the microstructural changes caused by SCC can generate higher harmonic waves when excited harmonically. This research considers different levels of SCC damage induced in samples of sensitized 304 SS by applying stress to a specimen held in a corrosive medium (Sodium Thiosulfate). Nonlinear Rayleigh surface waves are introduced in the material and the fundamental and the second harmonic waves are measured. The nonlinearity parameter that relates the fundamental and the second harmonic amplitudes, is computed to quantify the SCC damage in each sample. The results obtained are used to demonstrate the feasibility of using nonlinear Rayleigh waves to characterize SCC damage.

The influence of steel metallurgy on rock bolt SCC was studied using a series of commercial carbon and low-alloyed steels. The chemical composition, their mechanical properties and the microstructures of these steels varied considerably in order to gather information for the discussion of the metallurgical influences under Hydrogen Embrittlement (HE) and Stress Corrosion Cracking (SCC) conditions. In order to understand the metallurgical influences on Rock Bolt SCC, an evaluation was carried out to fifteen commercial steels. The experiments reproduced the Stress Corrosion Cracking condition at which commercial rock bolts had failed in Australians mines. Due to the selected materials, stress and electrolyte condition it is expected that Hydrogen Embrittlement (HE) will affect the steel failure. The approach was to use the Linearly Increasing Stress Test (LIST) and exposing the sample to a dilute pH 2.1-sulphate solution, in accordance with prior studies. Stress Corrosion Cracking was evaluated by analysing the decrease in tensile strength, loss of ductility and fractography observed using Scanning Electron Microscopy (SEM). The initial series of test to the fifteen steels were performed at the free corrosion potential (f.c.p.) vs. Ag/AgCl. From this initial test only five steels (AISI 1008, AISI 4140, AISI 4145H, pipeline X-65 and X-70) did not show Stress Corrosion Cracking features. These five steel were tested in accordance with the Linearly Increased Stress Test (LIST) in the dilute pH 2.1 sulphate solution at different electronegative applied potential to minimum value of -1500mV. The experimental procedure tried to reproduce the Stress Corrosion Cracking condition to identify the most aggressive condition the steel is able to support before failing due to Stress Corrosion Cracking to then compare the theory of SCC and HE in low carbon and low alloy steel with the obtained experimental results. The investigation compared the well-known theory of SCC and HE in low carbon and low alloy steel with the obtained experimental results. Surprisingly, the experimental result did not always agree with the theory.

Thesis (Ph. D.)--Ohio State University, 2008. / Title from first page of PDF file. Includes bibliographical references (p. 219-235).

Thesis (M. Eng.), Memorial University of Newfoundland, 1999. / Bibliography: p. 204-205.

Thesis (M.Sc. in Applied Science(Metallurgical Engineering))--University of Pretoria, 2008. / Abstract in English. Includes bibliographical references (leaves 67-69).