アミン類を主剤とする防食剤の研究 (第34報)

銅表面吸着水とメルカプタンの交替吸着 (その3)

抄録

In our previous reports, we postulated that water which was strongly adsorbed on metal surface acted an important role for corrosion inhibition of organic substances. Firstly, adsorbed water departed from the metal surface, and secondly, corrosion inhibitors were adsorbed on the site which desorbed the water.
Mercaptan whose functional atom of the adsorbent group is S-atom belonging to the 3rd period of the periodic table was lacking in proton-accepting ability. Because of this poor ability the adsorbed water could not be removed by mercaptan. It was necessary to remove the adsorbed water from the surface by other means to make mercaptan film on Cu surface. This could be done by elevating the temperature of filming solution (1), or by adding suitable amounts of strong proton-accepting substances, such as ether, alcohol, acid, ester, and amine to filming solution (2), as shown by following equations:
M: O-H-Hheat→M+O-H-H+RSH→M:S-H-R+O-H-H (1)
M:O-H-H+R′OR′→M:O-H-H…O-R′-R′+RSH→M:S-H-R+O-H-H…O-R′-R′ (2)
M: metal surface
In this report, assuming that mercaptan could be adsorbed on Cu surface in accordance with the equation (2), we carried out experiments using dibutyl ether as proton-accepting substances and dodecyl mercaptan as inhibitors.
Ether, whose functional atom of adsorbent group was O belonging to the 2nd period of the periodic table, could remove adsorbed water by its hydrogen-bonding with water, and then could be adsorbed by donating an unshared pair of electrons to the electron-poor metallic surface resulted by desorbed water. But mercaptan was lacking proton-accepting ability as compared with elements in the 2nd period of the periodic table. So, mercaptan could not donate unshared pairs of electrons to hydrogen atom of adsorbed water molecule, therefore, it couldnot remove the water from the metal surface by hydrogen-bonding.
Accordingly, mercaptan could act as a filming agent by the aid of strong proton-accepting substances, as following equations:
M:O-H-H+R′OR′→M:O-H-H…O-R′-R′+RSH→M:S-H-R+O-H-H…O-R′-R′
It could be concluded that amounts of proton accepting substances required for removing adsorbed water might be equal to the amounts of adsorbed water on metal surface. If excess was added in filming solution, ether was preferentially adsorbed on metal surface and mercaptan could not be adsorbed, as ether occupied the place where mercaptan to be adsorbed. As ether was more easily desorbed at lower temperature, it could not protect the metal more effectively than mercaptan in corrosive media.