2.4.1. Potentiodynamic Polarization

All electrochemical tests were performed using an Autolab potentiostat (Metrohm, PGSTAT101, Herisau, Switzerland) equipped with Nova Software (Metrohm, Version 1.0, Herisau, Switzerland). The experimental set up of an electrochemical cell kit with a three-electrode assembly is depicted in Figure 2. The working electrode is the mounted mild steel coupon with 0.79 cm² of exposed surface area to the electrolyte. A platinum mesh was used as the counter electrode, while a standard Ag/AgCl electrode was used as the reference electrode. The cell was filled with 300 mL of corrosive solution, 1 M HCl solution with and without the addition of EFB-H at varying dosages. After the electrochemical cell has been set up, the system was allowed to stabilize and reach a steady open circuit potential for 30 min [22,23]. This step is crucial to ensure an accurate measurement as the corrosion reaction fluctuates at the initial stage [24]. After the system has stabilized, the potentiodynamic polarization measurement was performed with polarization sweep within ±0.1 V of the open circuit potential at a scan rate of 1 mVs⁻¹ [25]. Each measurement was repeated three times, and only the average values were reported. New sets of corrosive solutions and mild steels were used for each repetition as the polarization measurement is a destructive technique that alters the electrode surface [26]. After completing the measurement, the data was plotted into a Tafel curve, and the electrochemical parameters such as the corrosion potential, Tafel slopes, corrosion current, corrosion rate and inhibition efficiency were computed. The corrosion potential can be computed from the lowest point of the current density in the Tafel plot, whereas the slope can be obtained by merely measuring the slope of each linear segment of the curve. As the corrosion current is known, the corrosion rate and the inhibition efficiency can be calculated using the following equations:

where C_R is the corrosion rate, k is a unit conversion factor (0.13), d is the density of mild steel, EW is the equivalent weight of iron, IE_PDP is the inhibition efficiency derived from PDP method and icorrinh is the corrosion current density of inhibited system.

Schematic diagram of electrochemical test.