Corrosion Inhibition by Naturally Occurring Substances: VII. The Effect of Aqueous Extracts of Some Leaves and Fruit-Peels on the Corrosion of Steel,Al,Zn and Cu in Acids

Abstract

The inhibitive effects of aqueous extracts of Opuntia ficus indica and Aloe eru (leaves) and of Orange, Mango and Pomegranate (fruit-peels) on the corrosion of mild steel, aluminium, zinc and copper in HCl and H₂SO₄ solutions have been investigated by means of weight loss and polarisation measurements. The extracts retard the dissolution reactions to an extent dependent on the metal used, the concentration of the additive and the type, concentration and temperature of the attacking acid. The additives provide adequate protection to steel in 5% HCl at 25°c and in 10% HCl at 25° and 40°c. In the presence of a sufficient concentration of the extracts in 5% HCl at 25°c, the inhibitive efficiency towards steel decreases in the order: Mango (82%), Orange and Aloe eru (80%), Opuntia f.i. (75%), Pomegranate (65%). The order of effiCiency differs for the different metals, but extract of Mango peels is still the most effectivefor Al (82%) and Zn (80%). The most effective extract for Cu is that of Pomegranate fruit-shells (73%). The extracts are generally more effective in HCl than in H₂SO₄

Polarisation measurements indicate that all the extracts increase the polarisation of the cathodic reaction of all the four metals tested and the anodic polarisation of steel and Zn, while Orange and Aloe have no effect on the anodic reaction of Al and only Pomegranate has any effect on the anodic polarisation of Cu. The results suggest that the extracts generally act as mixed inhibitors. The mechanism of inhibition has been discussed.