fundamental principle
The electrochemical potentials of different phases on the surface of metal samples are different, forming many micro-potentials, which will cause uneven dissolution in chemical solutions. In the process of dissolution, an oxide film will be produced on the surface of the sample. Due to the thin mucosa, the dissolution and expansion speed of the metal on the protruding part of the sample surface is slow, and the polished surface is smooth, but small undulations are formed, which can not meet the very ideal requirements. When observed with a microscope at medium and low magnification, this tiny fluctuation is generally within the vertical resolution of the objective lens, and very clear tissues can still be observed.