**5.1. Secondary electron microscope**

Most grain boundary characters can be observed in low-vacuum secondary electron microscope (LV-SEM). In the backscattered electrons (BSE) mode, Z-contrast can assist phase identification. SEM/BSE hardly observe oxide scale and steel substrate without etching, because two parts of oxides and steel hardly to etching both using the same etchant. That is because polishing and etching for sample preparation can bring out the grain boundaries to more easily delineate individual grains. SEM/EDS and scanning transmission electron microscopy (STEM)/EELS help analyse the chemical species present and, combined with elemental mapping, can provide a distribution of the different chemistries in a spot, line or area. Sometimes, SEM can couple focus ion beam (FIB) to observe the grain characters along the cutting surface when preparing for TEM samples.
