ELECTRODEPOSITION OF N-SUBSTITUTED POLYPYRROLES ON IRON AND THE CIPL STRATEGY

Film forming electropolymerization of N-substituted pyrroles on iron as a substrate (and platinum for comparison) were studied. Electrolytes were 0.1 M oxalic acid or 0.1 M potassium nitrate in water (1a, 1b), the same in 60 vol.% acetonitrile (2a, 2b) and 0.1 M LiClO4 in MeCN (3a) and in propylene carbonate (3b). In a first stage, model monomers as pyrrole, N-methylpyrrole and N-cyanoethylpyrrole were employed. From purely aqueous electrolytes (1), polymer films were obtained with current efficiencies (CE) of 10-80%. Current efficiencies decreased with increasing size of the N-substituent. The passivation of iron in the presence of (1a) is attributed to the formation of an iron(II) oxalate interlayer (chronopotentiometry, XPS and SEM). The competing anodic oxidation of oxalate was considered. Passivation of the polymer layer itself was observed in (1b) and even more in (2b). The partial aqueous electrolyte was successful up to 60 vol.% MeCN, but with 80 vol. % passivation of iron failed. The aprotic electrolyte (3b) was suitable, but not (3a). Passivation of iron occurred even in the absence of a monomer. Novel molecules were synthesized with N-substituents, which have at least a methylene group as conjugation barrier and diphenylsulfone or benzylsulfide components. The former could be electropolymerized to a black film from (3b) on iron and platinum, but not the latter. The failure is discussed in terms of nucleophilic interactions of the -S- group with the radical cation intermediate. The results assist in a practical approximation to the CIPL principle (corrosion inhibiting polymer layers). It has pronounced advantages over monomeric inhibitors.