No description available.

No description available.

No description available.

The following work divides into two parts: a: a study of the effect of stress on the inter-granular stress assisted corrosion attack of sensitised 20% Cr, 25% Ni, 0.7% Nb in HNO₃ environments. This problem was suggested by the C.E.G.B. and relates to the potential corrosion problems of AGR fuel cladding during storage after use. The aim of this work was therefore to determine how metallurgical condition, test potential and mechanical test variables affect corrosion behaviour. Low frequency cyclic loading offers a way to investigate the stress corrosion of systems at realistic stress levels and strain rates found in practice. b: an investigation into the effect of a low frequency cyclic stress on the process of stress assisted corrosion. The aim of this work was to gain information on the effect of stress cycling on the process of stress assisted corrosion attack. Tensile specimens were subjected to static loads both alone and with superimposed low frequency (10^-⁴ to 10^-² Hz) saw-tooth stress cycles. Cycling was carried out potentiostatically in HNO₃ environments, at below yield stress levels and ambient temperatures. Different frequencies, cyclic amplitudes and levels of background tensile stress were used. Irrespective of loading conditions the optimum potential for accelerated stress assisted corrosion attack was found to be -200mV (SCE). The results of tests showed that test potential, cycle frequency, cycle amplitude and level of background stress strongly affect rates of attack. Grain boundary penetration rates were found to increase as frequency decreased and as peak stress and stress amplitude increased. Different kinetics of penetration were seen for cyclic and static loading. Increase of penetration depth with time for cyclic loading experiments was found to vary with (time)^0.5 whilst that for static loading experiments increases linearly with time. A number of reasons are discussed to explain the difference in observations between cyclic and static penetration rates. Such reasons included the difficulty of ion transport down narrow paths, blunting of the penetration front, the possibility of local strain induced martensite transformation leading to hydrogen embrittlement and plastic strain enhanced dissolution resulting during cyclic loading. The anomalous effects observed during cyclic loading (such as "strain softening") were examined for tensile specimens cycled under a range of mechanical conditions. It was found that the extent of plastic strain increased for higher stress and larger cycle amplitudes. The process of thermal sensitisation of 20 wt% Cr, 25 wt% Ni, 0.7 wt% Nb stainless steel in three different material starting conditions (bar, "reworked bar" and tube) was investigated. Both Cr depletion and impurity segregation are discussed as mechanisms of sensitisaton. An attempt was made to correlate response in chemical and electrochemical tests with both microanalytical (STEM/EDX) observations on the shape of Cr depletion profiles and with analytical modelling. The collector plate model was found to describe AEM measured Cr depletion profiles well.

Thesis (M.S.)--Ohio University, August, 1991. / Title from PDF t.p.

Thesis (doctoral)--Helsinki University of Technology, 2004. / Includes bibliographical references (p. 192-206). Also available on the World Wide Web.

Thesis (Ph.D)--Materials Science and Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Singh, Preet M.; Committee Member: Carter, W. Brent; Committee Member: Gokhale, Arun, M.; Committee Member: Neu, Richard; Committee Member: Sanders, Thomas H., Jr.. Part of the SMARTech Electronic Thesis and Dissertation Collection.

The stress corrosion behaviour of precipitation hardened Al-9Mg, Al-22Zn and Al-3Mg-6Zn alloys has been studied in aqueous environments and ethanol. The stress corrosion susceptibility defined as the reciprocal of failure time has been investigated as a function of alloy-environment system, isothermal aging treatment, microstructure, applied tensile stress, and temperature using smooth and notched specimens. Constant load tests, load-relaxation tests and tensile tests in different environments have been used to evaluate the stress corrosion characteristics of aluminum alloys. A limited study of Mg-9Al has also been carried out in aqueous environments. The process of stress corrosion generally consisted of three parts: 1) A slow initiation stage 2) a rapid propagation stage 3) mechanical fracture due to tensile overload. With a few exceptions, the initiation time was greater than the propagation time. The crack initiation and propagation rates were stress and thermally activated and could be expressed by a general equation of the form Rate = [formula omitted] where α is the applied tensile stress, Q is the apparent activation energy of the rate controlling process and A(0) and n are constants for a given alloy-environment system. The apparent activation energy of the rate controlling process was different in the two environments. It also changed between initiation and propagation stages. The aluminum alloys when ranked in order of increasing susceptibility were: 1) Al-3Mg-6Zn, 2) Al-9Mg, 3) Al-22Zn. The alloys which were given heat treatments correlating to the presence of coherent or partially coherent phases, were found to be most prone to stress corrosion cracking. The environments placed in an order of increasing aggressiveness were dessicant-dried air, double distilled water, ethanol, ambient air, deionized water and NaCl/K₂CrO₄solution. The ductility of susceptible aluminum alloys was found to be significantly decreased by NaCl/K₂CrO₄and deionized water at low strain rates and enhanced by dessicant-dried air. Fractography showed the cracking to be intergranular in aluminum alloys and transgranular in the Mg-Al alloy. The stress corrosion surface was characterised by a rough or corroded appearance while the mechanically fractured surface exhibited slip steps and dimples caused by void formation. The hydrogen mechanism of cracking was examined in light of hydrogen charging experiments and other evidence and was found to be unsatisfactory. Models involving either dissolution or deformation alone were also inadequate in explaining the present results. Therefore a new model was postulated which involves the generation of a continuous path of chemical heterogeneity by shearing and link up of coherent precipitates followed by their dissolution. The rate controlling step in the deformation process is believed to change during the transition from initiation to propagation. The postulated model is consistent with the present results but its further development must await better knowledge of the kinetics of dissolution of precipitates and that of deformation processes at the crack tip. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Circumferentially notched rods of three types of austenitic stainless steel were stress corroded under freely corroding conditions at their yield stress in boiling 154°C MgCl2, and boiling MgCl2 with additions of HCl, CoCl2, and FeCl3. Alloy types 304, 316, and 310 were chosen because of their known different stress corrosion susceptibility. The corrosion products formed on the stress corrosion fracture surface were analyzed by electron diffraction and energy dispersive x-ray analysis. Electron diffraction of corrosion products, both in situ and stripped from the fracture surface, showed the corrosion product was a spinel oxide in all cases. Qualitative x-ray analysis of corrosion products, in situ and stripped from the fracture surface, indicated the corrosion product formed on all alloys was enriched in chromium and contained lesser amounts of the elements iron, nickel, silicon, molybdenum, magnesium, phosphorous and chlorine. Observations led to the conclusion that the corrosion product formed in the cracks of the various alloys was similar, being predominantly a chromium enriched oxide spinel with possible traces of metal chlorides or a corundum type oxide. The presence of the spinel oxide was consistent with anticipated E-pH equilibrium within the crack. However, it was felt the variation in s.c.c. behaviour between the different alloys could not be adequately accounted for in terms of the composition of the oxide. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

No description available.