## 2 Materials and Methods  
### Experimental Procedure  
All the experimental tests were conducted in a 3-electrode cell filled with 1 L of acidic solution prepared with deionized water and either valeric acid (C\({}_{5}\)H\({}_{10}\)O\({}_{2}\)) or acetic acid (CH\({}_{3}\)COOH) to obtain a pH equal to 3, 3.5, or 4. Increasing temperatures were chosen for the experiments, in particular, 20, 40, and 60 \({}^{\circ}\)C. The samples employed were cylindrical carbon steel coupons of 5 mm thickness and a 7 mm radius with the chemical composition given in Table 1, abraded with 320-grit silicon carbide paper and fit into a sample holder that exposed a 1 cm\({}^{2}\) surface. The acids chosen are two weak, carboxylic acids with their 3D molecular structures reported in Figure 1. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Fe** & **Mn** & **Cr** & **C** & **Si** & **Al** & **Ni** & **Cu** & **S** & **P** & **Mo** & **N** \\ \hline
97.81 & 1.07 & 0.73 & 0.18 & 0.05 & 0.05 & 0.04 & 0.03 & 0.01 & 0.01 & 0.001 & \textless{}0.001 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of carbon steel coupons in wt %. Figure 1: Three-dimensional molecular structures of valeric acid (**a**) and acetic acid (**b**). Two main tests were performed: potentiodynamic polarization tests and weight loss tests. Potentiodynamic polarization tests were used to assess the kinetic parameters of the corrosion process, namely, the Tafel slope and the exchange current density of the hydrogen evolution cathodic reaction and iron dissolution anodic reaction. The tests were performed in a standard cell [56] with a carbon steel working electrode, a Ag/AgCl/KClsat reference electrode, and an inert MMO-titanium counter electrode. The cell was placed on a hotplate and equipped with a temperature probe and a pH probe for continuous control throughout the experiment. A nitrogen insufflator was used to keep the environment deaerated thus maintaining hydrogen evolution as the main cathodic reaction. The electrochemical measurements were conducted via a Metrohm Autolab PGSTAT and a Luggin capillary was used to minimize the ohmic drop in the potential measurement. The solution was insufflated with nitrogen for 45 min before inserting the carbon steel sample, whose potential was monitored for 15 min to ensure its stability before starting the potentiodynamic measurements. The potential varied from \(-0.5\) V to \(+0.3\) V with respect to the open circuit potential Ecorr, and the scan rate used for the measurements was \(0.17\) mV/s. The Nova 2.1.7 software by Metrohm Autolab was used to fit the polarization curves and determine the kinetic parameters. The cathodic Tafel slope \(\mathrm{b_{H_{2}}}\) was calculated \(50\) mV below the open circuit potential, where the current is predominantly representative of the cathodic reaction. The cathodic exchange current density \(\mathrm{i_{0,H_{2}}}\) was determined as the meeting point between the cathodic Tafel line and its equilibrium potential. The anodic exchange current density \(\mathrm{i_{0,Fe}}\) was found as the intersection point between the anodic Tafel line and its equilibrium potential. A linear regression analysis of the overpotential with respect to the logarithm of the estimated \(\mathrm{i_{0,Fe}}\) near the Ecorr allowed us to derive the anodic Tafel slope \(\mathrm{b_{Fe}}\). Weight loss tests were performed to evaluate the uniform corrosion rates of the coupons. In particular, the samples were immersed in the acidic solution for 3 days, and then cleaned by sonication in a solution prepared with equal amounts of deionized water and 38% hydrochloric acid, with the addition of 3.5 g/L hexamethylene tetraamine that acts as a corrosion inhibitor. The procedure followed the instructions reported in ASTM G1-03 [57]. The samples were rinsed in deionized water, immersed in acetone, and then dried. The weight measurements were performed before and after each test, with a weight scale with a \(10^{-3}\) g accuracy.