Juan José Santana

Application of AC-SECM in Corrosion Science: Local Visualisation of Inhibitor Films on Active Metals for Corrosion Protection†

The suitability of frequency-dependent alternating-current scanning electrochemical microscopy (4D AC-SECM) for investigation of thin passivating layers covering the surface of corrosion-inhibited metals has been demonstrated. Inhibition of copper corrosion by benzotriazole (BTAH) and methylbenzotriazole (MBTAH), which are effective inhibitors for this metal under many environmental conditions, was investigated. Strong dependencies were found for the AC z-approach curves with both the duration of the inhibitor treatment and the frequency of the AC excitation signal applied in AC-SECM. Both negative and positive feedback behaviours were observed in the AC approach curves for untreated copper and for Cu/BTAH and Cu/MBTAH samples. Negative feedback behaviour occurred in the low-frequency range, whereas a positive feedback effect was observed at higher frequencies. A threshold frequency related to the passage from negative to positive regimes could be determined in each case. The threshold frequency for inhibitor-modified samples was found always to be significantly higher than for the untreated metal, because the inhibitor film provides electrical insulation for the surface. Moreover, the threshold frequency increased with increasing surface coverage by the inhibitor. 4D AC-SECM was successfully applied to visualizing spatially resolved differences in local electrochemical activity between inhibitor-free and inhibitor-covered areas of the sample.

Local Electrochemical Impedance Spectroscopy Investigation of Corrosion Inhibitor Films on Copper

In this work, the applicability of localized electrochemical impedance spectroscopy (LEIS) to characterize metal-corrosion inhibitor systems has been investigated. Copper corrosion inhibition by benzotriazole (BTA) and 2-mercaptobenzimidazole (MBI) has been studied, and compared to the electrochemical behaviour of the non inhibited metal. A rather wide frequency range of the AC potential signal was employed, namely 50 mHz ≤ f ≤ 65 kHz. The measurements were conducted for different probe-substrate distances in order to explore the effect of the different component of the localized impedance. Nyquist diagrams exhibit inductive loops in the high frequency range, and capacitive semicircles at intermediate and low frequencies for all the systems investigated, though they were affected in a different extent by the variation of the probe-substrate distance depending on the surface condition of the metal samples, this effect being more notorious for distances smaller than the dimensions of the bi-electrode probe.