Studies on Amine-Type Corrosion Inhibitors (33rd Report)

Adsorption of Mercaptan on Cu-Surface by Displacing the Pre-Adsorbed Water

Abstract

In our previous reports, we concluded that adsorption of high molecular-weight mercaptan on Cu-surface could be made up by elevating the temperature of filming solution, or by adding suitable amount of stronger electron-accepting substances such as ether, alcohol, acid, ester, and amine. At first these substances pulled adsorbed water off and then mercaptan could be adsorbed on the site where the adsorbed water had been removed. These twostep processes could be explained by following equations:
1) M: O-H-H→heatM+O-H-H→+RSHM: S-H-R+H₂O
2) M: O-H-H→+R′-O-R′M+O-H…O-R′-R′→+RSHM: S-H-R+O-H-H…O-R′-R′
M: metal, R, R′: alkyl radical
In our experiments test coupons were always filmed in xylene solution of a corrosion inhibitor preliminarily, and the filmed coupons were dipped in corrosive media to get the efficiency of the inhibitor. And in the case of the equation 2) we found that higher inhibition efficiency could be obtained only when proton-accepting substance in suitable dose, and its excess amount caused lower inhibition.
In this report, we studied on the relation between the amount of proton-accepting substance and mercaptan. And results were summarized as follows:
1) Mercaptan could be adsorbed only when ether, as a proton-accepting substance, removed the adsorbed water by its hydrogen-bonding with the water.
2) When excess amount of ether in the filming solution, the ether was preferentially adsorbed on Cu-surface and mercaptan could not be adsorbed, as it occupied the place where mercaptan to be adsorbed. As the ether was more easily desorbed at lower temperature, ether film on the surface could not protect the metal more effectively thann mercaptan film in the corrosive media.
3) Quantitative relations might be dependent not on the ratio of ether to mercaptan but on thee ratio of ether to the adsorbed water on Cu-surface.