**2. Surface treatment of texturing stainless steel substrate**

#### **2.1 Electro-polishing process**

In this study, electrochemical processing was used to achieve sub-micro texturing stainless steel substrate base on the fundamental electrochemical reaction items as (1)-(3). Anode chemical reaction:

$$\text{Fe}^{2+} + 2\text{(OH)} \rightleftharpoons \text{Fe(OH)} \text{or } \text{Fe(OH)}\_{3} \tag{1}$$

$$\text{OH} \vdash \text{O}\_2 \uparrow \text{H}\_2 \text{ (Parastic reaction)}\tag{2}$$

Cathode chemical reaction:

$$2\text{H}^\* \rightarrow \text{H}\_2\uparrow\tag{3}$$

The electro-polishing system is shown in Fig. 1. The important parameters are as follows:


The clamp was used to hold the anode and cathode plates. The anode and cathode plates were separated by Teflon with thickness of 1 cm. Fig. 2 shows the optical microscopy (OM) images of 304 SS substrate with and without EP process. The average surface roughness (Ra) of 304 SS substrate increased from 0.045 μm to 0.197 μm after the EP process with current density of 1A/cm2 in 10 min.

Fig. 1. Experimental set-up of the EP process.
