The inhibition effect of the cocoa bean shell powder (CBSP) on SAE 1008 carbon steel was studied in acidic medium (HCl 0.5 mol L-1) by electrochemical techniques and gravimetric measurements. Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM) were used to the chemical and morphological characterization. The gravimetric and EIS results showed that the corrosion rate decreases with increasing inhibitor concentration, reaching the maximum value of 97.92% in 1.77 g L-1 inhibitor (highest concentration studied). Even at the lowest concentration (0.44 g L-1) good corrosion inhibition efficiency (96.03%) was observed. The polarization curves indicated that the CBSP acts as a mixed type inhibitor. The inhibitory molecules adsorption followed the Langmuir isotherm model. The inhibitor economic evaluation showed that the use of CBSP is 48-616 times cheaper than traditional inhibitors. The results showed that CBSP is an efficient corrosion inhibitor for carbon steel and its production is financially attractive.

ABSTRACT Carbon steel is used in various applications because of its excellent weldability, good mechanical resistance and low cost. However, its application becomes restricted due to its low corrosion resistance in neutral and acidic media, and it is necessary to use processes to retard the corrosive mechanism, such as the use of inhibitors. Many of the effective synthetic inhibitors used are associated with high cost and some have a high toxicity. In recent years, studies of corrosion inhibitors obtained from natural products have been increasing because they are environmentally sustainable and presents low-cost. Therefore, in the present study, different concentrations of the cocoa shell powder (Theobroma cacao) and hydroalcoholic extract were evaluated as a corrosion inhibitors of SAE 1008 carbon steel in 0.5 mol.L-1 sodium chloride solution. The inhibitory action of the cocoa residue was studied through gravimetric technique and electrochemical impedance spectroscopy (EIS). For the chemical characterization of the inhibitor, the Fourier Transform Infrared Spectroscopy (FTIR) was used. The EIS results showed that the corrosion inhibition efficiency of the steel decreased with the inhibitor concentration, reaching maximum values of 55.97% and 72.93%, for cocoa shell powder and hydroalcoholic extract, respectively. The gravimetric data confirmed the electrochemical results and through the study of the adsorption mechanism it was verified that the adsorbed inhibitor molecules follow the Flory-Huggins isotherm model, for both forms of the inhibitor. For all the concentrations evaluated, the cocoa shell powder did not represent an efficient corrosion inhibitor. In contrast, for the hydroalcoholic extract, at the concentration of 0.44 g.L-1, a good performance was verified as a corrosion inhibitor of SAE 1008 carbon steel in 0.5 mol.L-1 sodium chloride solution.