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

Over the last few years, TEM has become a powerful technique to study cracks and specially crack tips. However, the number of publications including TEM results has not grown as it was expected. The main reason for this might be difficulties in the sample preparation. In this work we present a novel FIB sample preparation technique which has proved to be an ideal tool for preparing cross sectional samples containing crack tips. The morphology of intergranular stress corrosion cracking (IGSCC) has been investigated in Alloy 600 subjected to constant load and slow strain rate tests in simulated primary circuit pressurized water reactor conditions. Cracks were observed to nucleate at high-angle grain boundaries and propagate to depths of a few tens of micrometer along such boundaries, still in the initiation stage. Electron diffraction, energy dispersive x-ray (EDX) and electron energy loss spectroscopy (EELS) have been used to identify the different corrosion products and precipitates. Elemental mapping was employed to reveal changes in composition in the crack tip area. Major observations at cracks and grain boundaries include: the presence of different oxides in different locations, differences in grain boundary oxides and open crack/free surface oxides. These observations suggest that IGSCC involves oxygen diffusion through a porous oxide region along grain boundaries to the bare metal. This is a novel concept that offers an alternative to previous mechanisms proposed in the literature e.g. H embrittlement, slip-dissolution, etc., for which no supporting evidence has been found.

Stress-corrosion cracking of austenitic alloys in an oxygen-free carbon dioxide environment containing chloride ions was studied under static conditions. Stiffness and X-ray measurements supported results obtained from SEM photomicrographs indicating that the CT specimens loaded to a stress intensity of 22 ksi-in^.5 were not susceptible to SCC in this environment. These alloys were also evaluated for their SCC resistance in boiling MgCl₂ and NaCl solutions. Results of this study indicated that alloys containing higher nickel contents were more resistant to chloride SCC. / Master of Science