Relationship between human perception and breakdown of passivity

By classifying hundreds of model rusted panels drawn by a computer into five dirtiness levels, it was found that relationship between human perception and passivity breakdown was characterized 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. Different typical trajectories were obtained for type 304 and 430 stainless steels in the same condition of cyclic corrosion test, which seemed to be originated from the nature of alloying elements.