アミン類を主剤とする防食剤の研究 (第37報)

金属表面上の吸着水の二型

抄録

Several methods were adopted for the study of the adsorption mechanism of organic corrosion inhibitors. These methods included weight loss measurements of test coupons in corrosive media, gas chromatographic measurements of the inhibitor solutions to give amounts of consumption of the inhibitor, I. R. absorption spectroscopic measurements of the inhibitor solution which gave amounts of desorbed water from metal surface, and polarization curve measurements for inhibitor-filmed Cu electrodes in 5% aq. solution of HCl at 35°C.
Inhibitors were classified into two groups. One of which was proton-donating and proton-accepting (I), and the other, proton-accepting (II). Methanol or hexadecylamine was (I), while dimethyl-n-hexadecylamine or dimethyl-n-butylamine was (II). Hexadecylmercaptan was also used as a special inhibitor due to its weaker proton-accepting and stronger proton-donating properties at low temperatures.
Filmings of metal surface were made by placing metal in xylene solution of the inhibitor. Preliminarily filmed Cu-coupons were dipped in 5% aq. solution of HCl at 50°C and the degrees of corrosion inhibition were obtained from the weight losses of the coupons. Better inhibition was generally obtained for coupons filmed with (I) than for ones treated with (II). Only poor inhibition was observed for coupons filmed with mercaptan at low temperatures. It was found, however, that coupons filmed with mercaptan at 28°C and then refilmed with (II) gave better inhibition than those filmed only with (II). Furthermore, I. R. absorption spectra of the filming solution showed that contrary to (II) type of inhibitor mercaptan could not displace the adsorbed water on metal surface. Gas chromatogram of the solution of (I) showed that metal surface treated with (II) has some areas on which adsorption of (I) is still possible.
Judging from these experiments, there might be some areas where (II) could not be adsorbed but (I) or mercaptan could. In other words, there might be two kinds of active points on metal surface, one accommodating both (I) and (II), the other accommodating (I) only. We supposed that these phenomena were brought about from two types of adsorbed water, as illustrated as follows:
A. for (I) and (II) M: O-H-H
(metal accepting a lone pair of electrons from oxygen atom of H₂O)
B. for (I) M: H-O-H
(metal donating electrons to hydrogen atom of H₂O)
(I) and (II) could be adsorbed on A by displacing the water and (I) could be adsorbed on B by hydrogen-bridging with the water.