Microstructural characterization of aluminum alloys using Weck's reagent, part II: Coloring mechanism

This paper is the second part of the study focusing on the color metallography using Weck's reagent for Al alloys. Following the first part of the study which demonstrated a wide application of Weck's reagent, this paper investigates the coloring mechanism of color etching with Weck's reagent in detail by various characterizations of the specimen surface after etching. The results show that after the Al specimen is etched with Weck's reagent, a thin film consisting of Mn oxide is formed on the surface with different morphologies and thicknesses from location to location, which causes different colors observed by optical microscopy correspondingly. Further investigations show that the film is characterized by a relatively smooth surface but a rough interface with the substrate. Such structure may cause interference of light reflected from different sites. The growth of film is strongly influenced by the solute micro-segregation in the Al phase. Film starts to form first on the area with higher solute (Ti) concentration. As the etching time increases, film also grows on other areas. 12 s etching yields to the best color contrast because the difference in the film morphology and thickness is the most obvious at this time.