On colors of stainless-steel surfaces polished with magnetic abrasives

Multiple colors visible to the naked eye across the internal surfaces of stainless-steel 304 tubes polished with a magnetic abrasive finishing (MAF) process were investigated at varying levels of magnification. The colors were found to result from microscopic colored features of four different hues — green, blue, red, and yellow — having irregular sizes and shapes. Spectral analysis of their dispersion indicates that these colored features appear along the lay marks generated during MAF. Surface characterization employing energy dispersive spectroscopy and instrumented indentation testing indicates the presence of films of non-stochiometric chromium oxide and nickel oxide in the colored regions. Variations in the momentum and the density of the abrasive particles interacting with the surface during the MAF process, together with the chemical composition and morphology of the surface, provide thermodynamic conditions favorable for a non-uniform growth of the oxides film. Local color is dependent on the local composition and thickness of the oxides film.