Double Layer Capacitance of Iron and Corrosion Inhibition with Polymethylene Diamines

The effect of α,ω‐polymethylenediamines NH 2 ‐ ( CH 2 ) n ‐ NH 2 ( n = 2 – 12 ) on the corrosion of iron in deaerated 6 N HCl 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 = 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 C 6 ‐diamine was more efficient than the C 12 ‐diamine, although both inhibitors produced 90% inhibition at a reduced concentration of 0.1. The double layer capacitance was approximately constant at 21 µ F /cm 2 for the C 2 ‐ through C 8 ‐diamines, and alternated between 6 and 14 µ F /cm 2 for the C 9 ‐ through C 12 ‐diamines. The constancy at 21 µ F /cm 2 suggets that diamines with up to 8 carbon atoms are adsorbed in the same configuration, probably the flat 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 NH 2 ‐ ( CH 2 ) 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.