## 2 Experimental
### Electrochemical Measurements  
The test apparatus shown in Figure 3b consists of a 3-electrode setup, where WE is the steel sample under test with 1 cm\({}^{2}\) exposed area, graphite electrode as CE, and Ag/AgCl electrode as RE. For each case, an open circuit potential (OCP) was applied for 1 h to make the system stable. Linear polarization rate (LPR), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) were conducted for each case with the help of Gamry Ref 600 Potentiostat. LPR measurements were conducted at a potential range of \(\pm 20\) mV at a scan rate of 0.1 mV/s. Potentiodynamic polarization measurements were performed by sweeping voltage from \(-1200\) mV to 800 mV vs. OCP at a scan rate of 1 mV/s, by which the Tafel constants were determined. The obtained curves were then fitted using Gamry Echem Analyst software, which uses the Stern-Geary equation as given in Equation (1), where \(R_{p}\) is the polarization resistance and \(I_{corr}\) is the corrosion current density.  
\[R_{p}=\frac{B}{I_{corr}} \tag{1}\]  
\(B\) is the constant, which is calculated from Equation (2) where \(\beta_{a}\) and \(\beta_{b}\) are the anodic and cathodic constants.  
\[B=\frac{\beta_{a}\,\beta_{b}}{2.3\,\left(\beta_{a}+\beta_{b}\right)} \tag{2}\]  
Figure 2: (a) Schematic diagram of the flow Loop, (b) flow direction and specimen location, (c) actual installed flow loop.  
The corrosion rate was then calculated from Equation (3), where \(K\) is a constant consistent with the units for corrosion rate, \(E_{w}\) is the equivalent weight, \(D\) is the density, and \(A\) is the area of the test sample.  
\[Corrosion\ Rate=\frac{I_{corr}\ K\ E_{w}}{D\ A} \tag{3}\]  
In the electrochemical impedance spectroscopy (EIS) method, an alternating current is applied to the electrode system with an amplitude in the range of 5 to 20 mV and the response to the disturbance is measured. EIS works in the frequency domain and the interface is correlations between electrochemical system parameters and characteristic impedance elements, such as the capacitance of the electrode surface and the polarization resistance and the inductance. The most common method to represent and analyze the impedance of the electrode under investigation is using equivalent circuit modelling. The electrochemical processes in the corrosion system can be considered equivalent to an electric circuit that consists of elements such as resistance, capacitors, inductors, etc. For the corrosion system, it is assumed that the electrochemical reactions occur with relatively slow charge transfer, such as the rate determining step to evaluate the impedance. The impedance is expressed in terms of a magnitude, Z. The impedance relates the sinusoidal disturbance potential and the sinusoidal current response. A Nyquist plot, which represents impedance vectors (Z') real and imaginary part (Z'') at different measured frequencies, are obtained. In this case, the EIS measurements were carried out by applying a sinusoidal voltage excitation of 5 mV in a frequency range of 100 KHz to 10 mHz for 10 points per  
Figure 3: Electrochemical test apparatus (**a**) CAD design (**b**) on flow loop (**c**) with magnets.  
decade. The data were then fitted with the help of an equivalent circuit using Gamry Echem Analyst software.