Corrosion InhibitorsCorrelation Between Electronic Structure and Efficiency

• Published in: Corrosion (Houston, Tex.) Vol. 57; no. 1
• Main Authors: Lukovits, I., Kálmán, E., Zucchi, F.
• Format: Journal Article
• Language: English
• Published: NACE 01.01.2001; NACE International
• ISSN: 0010-9312; 1938-159X

ABSTRACTCorrosion inhibition efficiencies of heterocyclic, unsaturated (aromatic and nonaromatic) compounds (pyrimidines, benzothiazole derivatives, amino acids containing an aromatic part, pyridines, and quinolines) were correlated with quantum chemical indices of the respective molecules. Inhibition efficiencies were determined in acidic solutions containing 0.001 M or 0.01 M of the inhibitor. Quantum chemical calculations were made by using the Hückel method. The difference ( ?) between energy of the highest occupied and the lowest unoccupied molecular orbital was related to the inhibition efficiency (E) of the molecules. At values ? < 1.3 beta, efficiency increased with increasing values of ?. At values ? > 1.3 beta, efficiency (in 0.001 M concentration) tended to decrease with increasing values of ?. The optimal value of index ? may have been lower in solutions containing 0.01 M of the inhibitor. Results indicated that ? alone is insufficient to account for all variations in the experimental inhibition efficiency.INTRODUCTIONCorrosion inhibitors are bound to the metal/metaloxide surface (e.g., through chemisorption, physisorption, complexation, or precipitation) and prevent the access of oxygen to the cathode, prevent the diffusion of the hydrogen from the cathode, or inhibit (anodic inhibitors) the dissolution of the metal. The inhibition efficiency (E) depends on the parameters of the system (pH, temperature, duration, metal composition, etc.) and on the structure of the inhibitor molecule. Quantitative relationships between parameters characterizing the chemical structure and E of various types of compounds have been the topics of several treatises.1-4 Notably, Sastri and Perumareddi1 observed that, in a series of methyl pyridines (Figure 1), E in acidic solutions decreases with increasing values of the difference (?) between the energy of the highest occupied molecular orbital and the energy of the lowest unoccupied molecular orbital. In a recent study, inhibition efficiency of pyrimidine derivatives (Table 1) was investigated in acidic solutions.5 Again correlation between E of the inhibitor molecules and ? was demonstrated. Sastri and Perumareddi reported that the correlation coefficient between E and ? is negative,1 while investigations by the authors suggested that the correlation is positive. It was not clear whether, besides pyridines and pyrimidines, the observed relationship applies to other sets of nonsaturated compounds as well. In this study, several additional aromatic systems were considered, and it was found that a relationship between ? and the efficiency could be demonstrated in several sets of molecules. It seems that there is an optimal value of ? with respect to corrosion inhibition. The present investigations also revealed that index ? alone is not sufficient to account for all variation in