Multiscale characterization of pitting corrosion and application to an aluminum alloy

This paper documents a novel method for characterizing pitting corrosion da mage in structural materials such as Al 2024-T3. Specimens of such alloys a re corroded in a controlled environment and the pits' geometry is captured digitally using white light interference microscopy. The digital data are t hen processed with wavelet-based analysis, thus making possible a multi-res olution description of the geometrical features. The analysis reveals sever al interesting features of the pits that are similar for all the experiment al data analyzed herein, and independent of the process followed for creati ng them (time the material is exposed to corroding environment, concentrati on of the corroding agent, surface area exposed to the agent, etc.). The fi rst property identified as common to all pits is their geometrical scaling with a (Hurst) exponent of 0.63 +/- 0.12. Furthermore, the ratio omega of t he surface area of the pit as represented at coarse scales through the wave let representation, over the area of its intersection with the plane at zer o depth is found to be 1.17 +/- 0.07 consistently. The ratio of the total s urface area over its intersection is found to be 1.6 +/- 0.2. Either one of these ratios together with the Hurst exponent provide sufficient informati on for obtaining a pit's geometry from images capturing its two-dimensional shape only, a capability important for efficient characterization. Additio nally, such a characterization is paramount for rigorously addressing fatig ue crack initiation and propagation. (C) 2000 Editions scientifiques et med icales Elsevier SAS.