Effect of Tannic Acid on Corrosion of Mild Steel in Boiler System

The effect of tannic acid on the corrosion of mild steel in boiler systems was investigated by physicochemical methods using an autoclave and a test boiler. Tannic acid was found to be a good oxygen scavenger at pH 11 as well as hydrazine and sodium sulphite. Tannic acid inhibited corrosion of steel (i) at room temperature and atmospheric pressure and (ii) at high temperature and pressure. The maximum inhibition efficiency was about 97%. The inhibiting behavior of tannic acid at pH 7 was different from that at pH 11. At pH 7, tannic acid acted as an adsorption-type inhibitor and suppressed catholic reaction of corrosion of steel. At pH 11, tannic acid inhibited corrosion of steel not only as an effective oxygen scavenger but also an effective Schikorr reaction promoter (formation of Fe3O4). Tannic acid is believed to be an effective inhibitor in boiler systems.