アミン類を主剤とする防食剤の研究 (第39報)

金属表面上の吸着水の二型 (その2)

抄録

As corrosion inhibitors for metal, ethyl alcohol, diethyl ether, cetyl mercaptan, and methylcetyl sulfide were studied in aq. HCl solution.
Ethyl alcohol and cetyl mercaptan have two abilities, proton-donating and proton-accepting (electron-donating) (group I), while diethyl ether and methylcetyl sulfide have one, proton-accepting (group II).
Organic oxygen compounds are easily adsorbed on metal by donating electrons of their oxygen atoms at room temperature, while organic sulfur compounds are hardly adsorbed by electron donation of their sulfur atoms at this temperature.
Differences in adsorption behaviors were studied between I and II by means of corrosion test and corrosion potential measurement with Cu, Ni, and Fe coupons in acid media.
i Group I was always more effective for corrosion nhibition than group II in all conditions tested.
We considered that adsorption of organic corrosion inhibitors on metal might be brought about not only by electron donation of their functional atoms but also by their proton donation.
In our previous reports, we proposed the mechanism that inhibitors were adsorbed on metal by displacing the water molecules which were originally adsorbed by donating electron pairs of their oxygen atoms. In this paper another type of water adsorption was proposed. This water should be hydrogen-bridged to metal surface by accepting electrons from the surface. A metal can have two types of water molecules, which coexist on its surface.
Proton donation of inhibitors was closely concerned with the hydrogen-bridged water, and inhibitors might be adsorbed by combining it and/or displacing it, as following manners shown schematically,
M: H-O-H…H-SR or M: H-SR
At this time it is hard to say which, but we suppose that the distributional proportion of the two types of water might vary according to the characteristics of metal.