Evaluation methods for rusting behavior of stainless steels in architectural applications

The evaluation methods for the degree of rusting on stainless steels in architectural applications are reviewed. Conventionally, the atmospheric corrosion resistance of stainless steels has been ranked based on so-called rating number methods, which is either the evaluation of the percentage of area rusted, or the comparison of the specimen surface with the standard panels having different levels of dirtiness due to rusting. However, it is impossible to extract information about the corrosion mechanism that is contained in the rust form by using these methods. On the other hand, by showing relationship between the percentage of area rusted and the number of the rust spots on the xy-coordinate, the change in rust morphology can be characterized from the inclination of the locus of each datum point. Also, it can be known the quantitative dirtiness of specimen from the relationship between the logarithm of area percentage and the logarithm of the number of rust. Furthermore, it is mentioned about the application of digital image processing to measure the rust morphology.