Ação inibidora do extrato aquoso da semente de manona na corrosão do aço-carbono 1020 em HCl 1 mol L-¹

Abstract

Natural corrosion inhibitors are alternatives to the industrial use of synthetic inhibitors, which can be attributed to their efficient performance, renewable provenience and less toxicity. The purpose of this work was to investigate the inhibitive action of the lyophilized aqueous extract of castor beans on the surface of carbon steel 1020, in acidic media of HCl 1 mol L-1. Therefore, gravimetric and electrochemical analyses were fulfilled. Through the results of the mass loss test, it was found that the inhibition efficiency (IE) increased with the concentration of the extract and with the immersion time. For 2 h of immersion, IEs ranged from 89,3 to 94,1%, in concentration range from 100 to 1200 mg L-1. It was also tested 200 mg L-1 of high and low molecular weight fractions (HMWF 1 and LMWF), which presented IEs equal to 92,3 and 80,5%, respectively, indicating that substances of a molecular mass of less than 3 kDa were capable of inhibiting; however, macromolecules accomplished a better result to that function. The electrochemical impedance diagrams, in the presence of the extract, regardless of nature, showed a decrease from the double layer capacitance and an increase from the charge transfer resistance (RCT), which was attributed to the adsorption of molecules present in these extracts. IEs were calculated from RCT values and they ranged between 83,3 and 96,3%, for concentration range between 25 and 800 mg L-1. The Tafel extrapolation method exhibited the reduction in the anodic and cathodic current densities, where IEs ranged from 88,7 to 97,3%, in the range of 25 to 800 mg L-1. The linear polarization method displayed the increase of polarization resistance with the concentration, what caused IEs to range between 79,8 and 95,4%. The gravimetric and electrochemical results were supported by scanning electron microscopy and infrared analyses.