The effect of ~,~-polymethylenediamines NH2-(CH2)n-NH2 (n ~ 2-12) on the corrosion of iron in deaerated 6N HCI at 25 ~ was investigated by polarization measurements and colorimetric analysis of solution. The adsorption of these inhibitors at the metal/solution interface was monitored by measurement of the double layer capacitance using the single pulse method. 1,3-propanediamine was found to be a better inhibitor than ethylenediamine, but no further improvement in per cent inhibition resulted upon increasing the chain length from n ~ 3 to n z 8. Increased inhibitor efficiency for hydrocarbon chains longer than eight carbon atoms was attributed to the concomitant decreased solubility. On a relative solubility basis, the C6-diamine was more efficient than the Cl2-diamine, although both inhibitors produced 90% inhibition at a reduced concentration of 0.i. The double layer capacitance was approximately constant at 21 ~F/cm~ for the C2-through Cs-diamines, and alternated between 6 and 14 ~F/cm 2 for tbe C~-through Cl2-diamines. The constancy at 21 ~F/cm 2 suggets that diamines with up to 8 carbon atoms are adsorbed in the same configuration, probably the fiat position. The subsequent reduction and alternation in capacitance is believed due to a structuring of the adsorbate similar to that in the bulk where certain physical properties oscillate with carbon number. Colorimetric analysis of solutions with and without NH2-(CH2)4-NH.2 additions showed the dissolution rates to be higher than those measured by the polarization technique, possibly due to the "chunk" effect, in which dislodged grains of metal contribute to the total but not faradaic corrosion.

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The electrochemical reduction of carbon dioxide and formic acid has been studied in aqueous solutions on tin and indium electrodes using a-c impedance and photoemission techniques. Carbon dioxide reduction to formic acid (or the formate ion) proceeds with high current efficiencies (about 95%) although the overall power efficiency is low due to the high overpotential of the reaction. The reaction mechanism on tin and indium is similar to that postulated on mercury cathodes (17). The reaction rates are higher than on mercury and are affected by adsorption of intermediates and kinetics complications due to side reactions. Reduction of formic acid was only observed on tin cathodes at low current densities. The poor efficiency of this reaction is related to the formation of organometallic complexes on the electrode surface that accelerate the rate of hydrogen evolution.

The inhibitive effect of medium‐sized polymethyleneimines on the corrosion of iron in normalHCl solution is particularly good, and this effective inhibition is closely related to the angle of C‐N‐C bond or to strain in the ring as has been observed previously. The inhibition mechanism of the cyclic imines is developed further in this paper. Comparative studies on inhibition effectiveness of dimethylpolymethyleneammonium chlorides, N‐phenylpolymethylene‐imines, and the cyclic imines were carried out. Synergistic effects of iodide ion on the inhibition by cyclic imines and cyclic ammonium chlorides were also studied. Corrosion rate measurements on pure iron were made in normalHCl solution by polarization, hydrogen evolution, and weight loss. From the results, it was concluded that good inhibition of medium‐sized polymethyleneimine is caused by donation of the unshared π‐electron pair of its nitrogen atom to the metal.