A new experimental method is presented to investigate the mechanism of rust formation on a low alloy steel and to study the role of added copper on the mechanism.
The neutralization reaction between sodium hydroxide and ferrous-ferric salt solutions was employed for the experiment. In some cases, copper salts were added to the latter solutions. The reaction was carried out in a quasi-static way through an ion exchange membrane as a diaphragm.
The amount of sodium hydroxide consumed by the reaction was determined. The products deposited on the membrane were identified by X-ray diffration. These products were named “pseudo-rust”.
The results obtained are as follows
(1) Using cation exchange membrane did not form “pseudo-rust”. By using an anion exchange membrane, however, “pseudo-rust” was favorably formed which was found to have anion permeability.
(2) The permeability of anions through the “pseudo-rust” was decreased by the presence of copper in the rust.
(3) When ferrous-cupric sulphate solution was added to a critical concentration, checked deposition of metallic copper was observed on the salt solution side of the anion exchange membrane.
(4) Using ferrous chloride solution resulted in the inclusion of metallic iron in the “pseudo-rust”.

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In our previous reports, it was presumed that there were two types of water molecules adsorbed on metal surface. One was adsorbed by donating an electron-pair of its oxygen atom to metal and the other by hydrogen-bridging with metal. When the former was desorbed from the metal surface because of being attracted with electron-donating inhibitors, organic inhibitors would be immediately adsorbed by sharing an electron-pair with metal on the site where the former was desorbed.
In this report, the latter type of water was concerned. To verify existence of the latter type of water, some preliminary experiments were carried out by means of examining the adsorption of inhibitors with and without an active hydrogen atom on metal oxide. These experiments were anodic polarization curve measurements and others to confirm the influence of inhibitors on the induction period of passive film breakdown which would be observed on nickel in 1N H₂SO₄ by adding some amounts of KCl. Infrared absorption spectrum was also adopted to determine the amounts of inhibitors adsorbed on nickel oxide.
The induction period of breakdown for electrochemically prepared nickel oxide was prolonged by addition of inhibitors with active hydrogen atom, such as, n-octylamine, butyl alcohol, and butyric acid. No prolongation in the induction period was, however, detected by inhibitors without active hydrogen atoms, such as N, N-dimethyloctylamine, and dibutyl ether.
These showed that inhibitors were adsorbed by two ways, one by electron donation and the other by protonation. The adsorption of inhibitors by protonation suggested the possibility of the water which was hydrogen-bridged with metal surface.

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