Corrosion inhibition of carbon steel in the near neutral media by blends of tannin and calcium gluconate

The efficiency of tannin/calcium gluconate blends, as corrosion inhibitors for carbon steel in near neutral water media, have been studied. Synergistic behaviour of two components blend of different concentrations has been demonstrated in solutions prepared with distilled and a tap water. The anodic character of the inhibitor was recognised by the shift of corrosion potential to more positive values and a decrease of the passivation current on potentiodynamic polarisation curves. The cathodic inhibition was also recognised by slowing down the cathodic current of the polarisation run. Soluble iron tannate chelate has been observed as a dark blue flux flowing down from the metal surface into solution of tannin in the tap water, resulting in a corrosion process. Tannin decreases the pH of tap water. Calcium gluconate added in tannin solutions increases the pH maintaining it in the near neutral region enabling the precipitation of a low soluble protective iron tannate and metal gluconate. © WILEY-VCH Verlag GmbH.