2.3. Coloring Curve

Coloring potential-time curves is shown in Figure 1b. Point B is a point where coloring starts, and a stain starts to form. The potential corresponding to point B is the coloring initiating potential (E_B), and point C is the coloring point between points B and D. At point C, it is a specific form of color. The potential corresponding to point C is the coloring potential(E_C), and the point D is the end point of coloring, ΔE is the chemical potential, expressed as E_B − E_C = ΔE. The color of stainless steel is changed from brown → blue → yellow → blue-black → black → fuchsia → prasinous [21]. Each color corresponds to a different coloring potential when the coloring liquid composition and the substrate composition are constant [22]. In order to obtain the ideal SSAC, first choose the appropriate SSAC color, black or blue black between blue and fuchsia, the control area is more sensitive, so the control of the conditions such as composition and coloring time strict.