Spectro-electrochemical studies of the inhibition effect of 4, 7-diphenyl -1, 10-phenanthroline on the corrosion of 304 stainless steel

Abstract

The compound 4,7-diphenyl-1,10-phenanthroline (DPP) was selected to study the relationship between adsorption and inhibition on the corrosion of 304 stainless steel in 0·1 M HCl solution. The current vs. potential behaviour of the system was determined potentiostatically and the amount of the inhibitor adsorbed on the electrode surface was measured by the concentration depletion method using a modified dual-wavelength spectrophotometric method. The adsorption of the inhibitor was plotted against the applied potential (from − 0·7 V t o 0·0 V (SCE)), in the inhibitor concentration range of 5·0 × 10 −7 to 1·0 × 10 −5M. Five adsorption isotherms were tested for their fit to the experimental data. The Frumkin, Virial Coefficients, Hill-de Boer, Blomgren-Bockris and Conway-Barradas isotherms fit the experimental data with a correlation coefficient > 0·95 in the surface coverage range θ = 0 to θ = 0·5. At higher surface coverages deviations from the isotherms are found. Attractive interaction exists between the adsorbed species. The calculated mean adsorption constant for the five isotherms changes from (2·2 ± 0·4) × 10 5 at − 0·6 V to (1·2 ± 0·1) × 10 5 at − 0·2 V. The calculated standard free energy of adsorption, — δG° , has a mean value of 30 ·5± 4 kj mole −1 − 0·6 V and 28·4 ± 0·4 k mole −1 at − 0·2 V. Polarization curves at constant surface coverage with DPP, from θ = 0·1 to θ = 0·5, all have cathodic Tafel slopes identical to that in pure 0·1M HCl, 116 mV/decade, the current decreasing with increasing θ at constant potential. The inhibition efficiency is ca. 15 % at 1·02 × 10 −6M DPP concentration and ca. 65% at 1·28 × 10 −5M DPP concentration in the potential region of − 0· 5 V to − 0·7 V. At − 0·3 V the efficiency varies from 17% to 40% in the same DPP concentration range. Efficiencies were compared with the fractional surface coverage of DPP on the electrode surface. Generally, the percentage coverage is larger than the corresponding inhibition efficiency, but is similar in magnitude. It is proposed that the inhibition effect of DPP on both the anodic and the cathodic reaction is mainly due to a surface blocking effect.