Heterocyclic compounds as corrosion inhibitors for mild steel in hydrochloric acid medium — correlation between electronic structure and inhibition efficiency

Relationships between electronic parameters of aromatic corrosion inhibitors and inhibition efficiency have been investigated. Our earlier results obtained for corrosion inhibitors of mild-steel demonstrated that correlation exists between inhibition efficiency and the energy band gap (DE) between the energy of the highest occupied molecular orbital (Ehomo) and the lowest unoccupied molecular orbital (ELUMO). It was found that efficiency (related to 0.001 M inhibitor concentration) is optimal at DE @ 1.3 beta. These investigations have been extended to include new classes of molecules, namely oxadiazole, thiadiazole, and triazole derivatives. The new results have modified the conclusions drawn from earlier investigations: efficiency depends, besides DE, on the conformation of the molecules, viz. planar (unsaturated) molecules are more efficient than partially saturated (and therefore twisted) derivatives.