Abstract
Studies on the corrosion behaviour of mild steel electrodes in inhibited hydrochloric acid are described. Conventional weight loss measurements show that a maximum concentration of 140 ppm of tannin from Rhizophora racemosa is required to achieve 72% corrosion inhibition. Similar concentration of tannin: H3PO4 in ratio 1:1 gave 61% inhibition efficiency, whereas efficiency obtained for phosphoric acid as inhibitor in the same environment was 55%. Corrosion rates obtained over six hours of exposure in 1M HCl solution at inhibitor concentrations of 140 ppm are 2 mA/cm2, 2.4 mA/cm2, 2.6 mA/cm2 and 6 mA/cm2 for tannin, tannin/H3PO4 and H3PO4-inhibited and uninhibited specimens respectively. Natural atmospheric exposure studies revealed that specimens treated in H3PO4 resisted corrosion for three weeks, while tannin treated specimens suffered corrosion attack after one week of exposure tests.
Highlights
Rhizophora racemosa is in abundance in the Mangrove forests of southern Nigeria
Similar concentration of tannin: H3PO4 in ratio 1:1 gave 61% inhibition efficiency, whereas efficiency obtained for phosphoric acid as inhibitor in the same environment was 55%
General Observations To the naked eye, untreated mild steel specimens showed some red-brown patches of rust on the first day of exposure to the atmosphere. This is expected as the corrosion product of iron exposed to moist air, ferrous hydroxide is further oxidised to the hydrated oxide during exposure. Those specimens treated with tannin, tannin/phosphoric acid and phosphoric acid, retained the colourations imparted on them by the respective inhibitor solutions
Summary
Rhizophora racemosa is in abundance in the Mangrove forests of southern Nigeria. The bark of its stem is rich in tannins which can be described as any group of naturally occurring phenolic compounds. Especially mild steel, on the other hand are largely used in acidic media in most industries including oil/gas exploration and ancillary activities During such activities, inhibited hydrochloric acid is widely used in pickling, descaling and stimulation of oil wells in order to increase oil and gas flow. It is not uncommon to use either tripolyphosphates or phosphoric acid [11] and/or a mixture of either with other inhibitors to inhibit the corrosion of mild steel especially in the petrochemical industry. In such cases synergistic effects where improved corrosion inhibitive efficiency is observed, lower costs implications of using either of the two are the primary motives. The present investigation is directed at the evaluation of tannin from Rhizophora racemosa, phosphoric acid and a combination of the two as inhibitors for the corrosion of mild steel in hydrochloric acid
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