Double Layer Capacitance of Iron and Corrosion Inhibition with Polymethylene Diamines

Abstract

The effect of α,ω‐polymethylenediamines on the corrosion of iron in deaerated 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 . 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 C₁₂‐diamine, although both inhibitors produced 90% inhibition at a reduced concentration of 0.1. The double layer capacitance was approximately constant at 21 µF/cm² for the C₂‐ through C₈‐diamines, and alternated between 6 and 14 µF/cm² for the C₉‐ through C₁₂‐diamines. The constancy at 21 µF/cm² 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 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.