The Alkaloid and Non-alkaloid fractions of the ethanolic extracts from Costus afer (AECA and NAECA) were comparatively studied for their inhibitive properties on the corrosion of mild steel in 5 M HCl solution using Gravimetric and Gasometric techniques. The results revealed that both extracts inhibited the corrosion of mild steel in HCl solution following the order: NAECA > AECA. The maximum inhibition efficiencies obtained were 49.2 % for AECA and 86.4% for NAECA at an inhibitor concentration of 5 g/L. Inhibition efficiency was found to increase with increase in concentration and increase in temperature. The adsorption of both extracts on the mild steel was found to be consistent with Langmuir adsorption isotherm.Physical adsorption mechanism was proposed for the adsorption of the components of the extracts on the mild steel surface.

ABSTRACT:The inhibition efficiency and adsorption characteristics of alkaloids, flavonoids and tannins extracts of Mangifera indica leaves in controlling corrosion of Zinc sheet in 5 M hydrogentetraoxosulphate (VI) acid solution has been evaluated by gasometric method. The formulation consisting of 2 g/L of extract offers 96.2%, 85.3% and 70.6% inhibition efficiencies (alkaloids, flavonoids and tannins respectively) at 30 0 C to Zinc metal immersed in 5 M hydrogentetraoxosulphate (VI) acid solution. Judging from the trend in inhibition efficiency against temperature, it can be deduced that the adsorption process was physisoption being that corrosion rate increased while inhibition efficiency decreased with temperature. Adsorption mechanism followed the Langmuir adsorption isotherm being that the correlation coefficient was found to be approximately unity i.e. 0.9237. © JASEM http://dx.doi.org/10.4314/jasem.v19i1.19 IntroductionTo be realistic, corrosion can only be reduced significantly or delayed to a reasonable extent. Controlling of the pH or ion concentration of the solution or controlling the metal solution interface had a number of ways to achieve the reduction in the rate of corrosion over the years. This is achieved through the addition of small quantity of chemicals called inhibitors, that either encourage film formation or form a barrier like layer on the metal surface and by so doing stop or slow down the rate of metal decomposition (Obot et al., 2011).