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A photometric study was made of t h e refl ection of plane pol ari zed li ght no rmalh' incident in a metallographic microscope upon specim ens of tin, aluminum, and monel with variou s surface treatments. A high degree of correlation was found in most c9,se~ between the extin ction position and the projection of the optic axis or of the cubic axis making the greatest angle with the surface normal. Moreo ver, a fourth power sine relationship was obser ved between the change of intensity and t he angle between t.he s urface normal and the optic ax is in t in. The inte nsity correlation in the case of the cubic metals was not so clear, but a sli ght indication of position d epend ence was obse rv ed for a luminum . The res ults indicate that the optically anisotropic effects obser ved with cubic m etals can be caused both by an isotropic fi lm s a nd by oriented surface conto urs, but that the latte r is the source of t hose effects that are correlated \\'i t h t he crystall ograph ic ori e ntation of t he grain.

In this paper, we describe the preliminary stages of the development of a mathematical model of the evolution of the solution chemistry within a corroding crevice on passive stainless steel. It is based on a formulation by Oldfield and Sutton [1], but models the physical and chemical processes which determine the crevice solution in a more rigorous manner. The model will eventually be used to assess whether a ‘critical solution composition’, which results in the depassivation of the crevice and the onset of localised corrosion, is attainable for a range of repository conditions, steel types and canister designs etc. We also describe experiments that provide input data in the form of passive currents for this model. Preliminary sensitivity tests with the model have indicated a need for accurate thermodynamic data for the chemical equilibria constants (particularly those for the chromium reactions). These tests also suggest that there are certain critical relationships between various parameters in the system (such as crevice dimensions, the composition of solution outside the crevice and the passive current) that mark different behaviour in the evolution of the solution composition. Further experiments will be performed, as part of this work, both to validate the predictions of the model and to determine whether the predicted compositions of the crevice solutions are sufficiently aggressive to initiate crevice corrosion.

This paper describes the use of time-lapse video techniques for in situ observation during controlled and free corrosion of high-strength Al-Zn-Mg-Cu alloys 7075 and 7150 and two Al-Zn binary alloys. Direct observation of corroding 7075 in various tempers shows that, as a precursor to stable pit formation, a deformable gelatinous layer of corrosion product develops. Gas is evolved at the metal/hydrated oxide interface, which distorts the gel layer at potentials below those expected for pitting. During stress corrosion cracking of 7150 T6, time-lapse video images confirm that initial drying after total immersion of 0.6M NaCl solution leads to an acceleration in the apparent rate of crack growth from 5 × 10-7 m/s to 9 × 10-6 m/s. This rate is sustained for only a few minutes but could help explain the aggressive nature of alternate immersion testing.