For this work, a hydroalcoholic extract was prepared from the aerial parts of the Malvaviscus arboreus (Malvaceae) species, popularly known as hibisco-colibri in Brazil; and then, investigated by infrared and NMR spectroscopy. A class of special metabolites containing a porphyrin skeleton and pyrrole units was determined by typical absorption spectra, mainly 1H and 13C NMR, thus indicating the capability of showing anti-corrosion effect. Therefore, the M. arboreus extract was examined as a corrosion inhibitor for AISI 1020 carbon steel in H2SO4 1 mol L-1, using a mass loss (gravimetric) technique. Results showed good corrosion inhibition efficiency, at both time and temperature variation. The maximum efficiency observed at 500 ppm was 97.5%, at 3 h, 24 h and 48 h. The surface morphology was analyzed by scanning electron microscopy (SEM), through which a higher homogeneity was observed in the presence of the M. arboreus extract. The hydroalcoholic extract of hibisco-colibri can be considered an efficient green agent for preventing corrosion.

In this work, two β-enaminoesters, EN1 and EN2, were synthesized, characterized and their anticorrosion effects on American Iron and Steel Institute (AISI) 1020 carbon steel in 0.5 mol L-1 HCl were investigated using gravimetric and electrochemical methods. The results revealed that both compounds inhibit corrosion. Good correlations were observed between results obtained for gravimetric and electrochemical methods. EN1 exhibited highest efficiency of 98% after 24 h of immersion in HCl solution. The adsorption of compounds on steel surface followed the modified Langmuir adsorption isotherm. Surface morphology of carbon steel was examined using scanning electron microscopy showing that the metal surface was protected by EN1 as inhibitor of corrosion.

Polymers have received considerable attention for replacing synthetic inorganic and organic anti-corrosion inhibitors since they are not as toxic for the environment, besides being cheaper. Polyvinylpyrrolidone (PVP) has been studied due to its applicability, solubility, very low toxicity, cost and efficiency in corrosion inhibition against metals, such as zinc, aluminum, stainless steel, and others. Its anti-corrosion activity with 40000 g mol-1 was tested against AISI 1020 carbon steel in 0.5 mol L-1 HCl, and evaluated by electrochemical techniques: Potentiodynamic Polarization (PP), Linear Polarization Resistance (LPR), and Electrochemical Impedance Spectroscopy (EIS). The maximum efficiency was 90% and 89% in EIS and LPR, respectively, at concentration of 1.0 x 10-7 mol L-1. The adsorption mechanism was coherent with Langmuir isotherm model. The morphology of the corroding carbon steel surface, in the presence and absence of the PVP, was visualized using Scanning Electron Microscopy (SEM).