An experimental survey of rust preventives in water II. The screening of organic inhibitors

AbstractThe methods described in Part I1 were applied to the screening of over 400 possible inhibitors, mainly organic, in non‐acid aqueous solutions containing dissolved air, and to supporting polarisation experiments.Inorganic anions tested included sodium azide which inhibits rusting although it forms a soluble complex with ferric ions and does not precipitate ferrous ions. Some heavy metal complexes, such as potassium ferrocyanide and a ferric EDTA complex, were inhibitors.The carboxylate anion favoured inhibition although this tendency is modified by the hydrocarbon residue and substituents. Thus in the series of sodium salts of fatty acids, the inhibitor effectiveness was maximum with a molecule containing 8—10 carbon atoms depending on the type of test.Results for amines indicate that inhibition is due to the neutral amine molecule; only with strong amines could inhibition be due partly or mainly to the hydroxyl ion. This conclusion is supported by the lack of inhibition ability of aliphatic amino‐acids of comparatively low molecular weight, since aqueous solutions of these exist mainly as zwitter ions. Sodium lauroyl sarcosinate, which is an amide‐type derivative of sarcosine with a large hydrocarbon part, has however good inhibiting properties except in presence of halide ions.Most non‐basic nitrogen compounds were ineffective, even including many derivatives of hydrazine which itself is used as an inhibitor and oxygen scavenger in boiler feed water. Maleic hydrazide and its derivatives were however exceptionally effective, although only moderately so in presence of halide. The nitro group favoured inhibition ability with water‐soluble compounds; the nitroso group favoured inhibition unless, as appeared usual, the molecule complexes iron; and the azo group appeared ineffective.Some but not all organic derivatives of phosphoric acid inhibited the rusting of steel, whilst two phosphinic oxides were ineffective.The sulphonate group resembles sulphate in that it opposes an inhibition tendency. Unlike halide ions, covalent halogen assists inhibition when present in the meta‐ or para‐, but not in the ortho‐position to the carboxylate group in benzoate.Polarisation curves showed most inhibitors to polarise anodic dissolution rather than cathodic oxygen reduction.