Effectiveness of furfural as an organic inhibitor in 5 percent hydrochloric acid which is used in acid pickling of steel was evaluated. Lungmuir, Frumkin and Flory-Huggins isotherms [1, 2] were used to determine the parameters of effective adsorption, free energy and entropy of adsorption. Furfural indicated an inhibition less than 17 percent in concentrations of 0.001 mole per liter for a dura...