The electrochemical measurement was carried out in order to determine the localized attack in the nickel-chromium (Ni-Cr) electroplates and also the microscopic study was applied for the examination of the corroded surface of the Ni-Cr coating in an attempt to elucidate the mechanism of the corrosion in the Ni-Cr electroplates.
The corrosion of the Ni-Cr electroplates at the potential region of the active state of nickel such as at -0.34- +0.1V vs. S. C. E. might correspond to the anodic dissolution of nickel through the pores or cracks in the chromium coating in Ni-Cr electroplates.
The corrosion pits of the Ni-Cr electroplates observed in acid sulfate solution are hemispherical in shape and the radii of the pits were found to increase with the rate of dissolution of the Ni-Cr electroplates.
It was found that the corrosion pits of the nickel layer were always in the sites of the cracks of the chromium and the pattern of the corrosion pits was a kind of reproduction of the cracks in chromium coating.
The dissolution of nickel along the crevice of the chromium coatings was found to be pronounced when the concentration of chloride ions was increased in the sulfate solution.
Incorporated sulfur in the bright nickel had a similar effect on the corrosion of Ni-Cr electroplates giving larger corrosion current in the sulfate solution.

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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.

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