Evaluation of fungal biomass developed from cocoa by-product as a substrate with corrosion inhibitor for carbon steel

The fungal biomass of cocoa bean shell (FBCS) obtained by solid-state fermentation (SSF) with Penicillium roqueforti was used as a natural corrosion inhibitor of carbon steel in an acidic solution with 0.5 mol.L-1 HCl. The evaluation of the corrosive process was carried out using gravimetric, electrochemical, and surface analysis techniques. The gravimetric test data were used to calculate the corrosion rate, current density, and efficiency. Electrochemical measurements provided the polarization curves and the electrochemical impedance spectroscopy (EIS). Finally, the surface analysis was performed using an optical microscope. From Tafel's extrapolation, the largest deviation was noticed at 69.9 mV, confirming that FBCS acted as a partially cathodic inhibitor. Inhibitor molecules were physically adsorbed on the carbon steel surface and the adsorption mechanism followed the Langmuir isotherm model. The results of the EIS confirmed the gravimetric results. FBCS inhibited the acid corrosion of carbon steel AISI 1008 at a concentration range from 0.44 g.L-1 to 1.77 g.L-1, and the higher the concentration, the higher the inhibition rate. The maximum corrosion inhibition efficiency was 93%. The EIS by time results showed that the FBCS should not be applied for very long periods of immersion in the electrolyte. The maximum inhibition efficiency was verified within 12 hours of immersion (95.96%). Therefore, this study encourages the use of FBCS as inhibitor due to good inhibit efficiency and to be environmental friendly, as well as the use of others news materials, such as biomass resulting from the fermentation of other residues, proposed in an unprecedented way in this work.