Double Layer Capacitance of Iron and Corrosion Inhibition with Polymethylene Diamines

Abstract

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 polariza-tion measurements and colorimetric analysis of solution. The adsorption of these inhibitors at the metal/solution i terface was monitored by measurement of the double layer capacitance using the single pulse method. 1,3-propanedi-amine 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 be-tween 6 and 14 ~F/cm 2 for tbe C~- through Cl2-diamines. The constancy at