On aesthetic degradation of stainless steel

Since stainless steels were used as architectural materials, their aesthetic degradation in the atmosphere has been one of the important field of study. By classifying hundreds of model rusted panels drawn by a computer into five dirtiness levels by human sense, it was found that aesthetic degradation was expressed as a function of spot density and its area ratio. This method was applied to evaluate the evolution of rust on stained stainless steels, and the following became clarified 1) At the initial stage of corrosion, number of small pits increases with exposure time. 2) In the middle stage, area of stained spots grows. and, 3) At the last stage, expansion of rusty dots resulted in merging of the stains. The behavior could be expressed on the spot density vs. area ratio chart in such a way that a point moves from the origin to upper right direction in a linear manner at the initial stage, horizontally right in the middle, and then down right at the last. These trends were confirmed to hold true in the cases of exposure test to actual marine atmosphere. Comparison of the trajectories on the spot density v. s. area ratio chart for the exposure test case with those for the cyclic corrosion tests, revealed that the lowering of drying temperature causes inadequacy as an acceleration test.