Many of the synthetic corrosion inhibitors being used currently, are toxic and therefore research for developing environmentally benign corrosion inhibitors is of great importance. Inhibitors such as natural polysaccharides of plant origin are popular due to its non-toxic nature. Here, the toxicity of a natural polysaccharide, guar gum, and its modified form, cationic guar gum was studied and their inhibitory effects were investigated by gravimetric, electrochemical and surface morphology studies. Both the inhibitors are found to be effective and their adsorption on metal is in accordance with the Langmuir adsorption isotherm. Experimental results are in consonance with the results of quantum chemical calculations by density functional theory.