INHIBITORS - AN OLD REMEDY FOR A NEW CHALLENGE

This lecture initially presents the results of systematic research performed to interpret the mechanisms of corrosion inhibitors in various environments. The behavior of several series of S-containing and N-containing compounds has been investigated by radiochemical, electrochemical, gravimetric, and micrographical techniques. In particular, by using substances labeled with C-14, S-35, and H-3, experimental evidence was obtained about the adsorption of the inhibitors onto the metal surface. Information was also obtained about the irreversibility of the adsorption and its dependence on the time and the potential. The recording of anodic and cathodic polarization curves, polarization resistance data, and electrochemical impedance spectroscopy (EIS) diagrams brought about the definition of the inhibitor effect on the partial electrochemical reactions and the existence of relationships between the molecular structure of the additives and their inhibiting efficiencies. Since a certain level of knowledge had been reached about the inhibition mechanisms of iron and mild steel acid corrosion, attention was paid to the particular problem of duplex steel inhibition in hot concentrated hydrochloric acid solutions. Satisfactory results have been obtained with binary or ternary mixtures of inhibitors that can form thick protective surface layers by reduction and polymerization reactions. Quaternary ammonium salts, alkynols, and trans-cinnamaldehyde were found to be efficient inhibitors. The synergistic effect of potassium iodide was also examined. Finally, the possibility of suggesting reliable inhibitors to prevent or to block localized corrosion phenomena is outlined. Examples are discussed concerning galvanic corrosion, pitting, dezincification, intergranular corrosion of sensitized austenitic stainless steels in acid solutions, and stress corrosion cracking of austenitic stainless steels, both in acidic chloride solutions and in boiling magnesium chloride solutions.