Using a previously-described procedure, the author has studied the corrosion velocity of steel in dilute acetic acid, in the presence of an oil phase, and in the presence and absence of an organic inhibitor. Values of s, the fractional surface covered by adsorbed inhibitor, are calculated from the experiments for each inhibitor concentration c. The Langmuir equation, in the form l/s = (W)(llc) -+ 1, is solved graphically by p!otting the observed values of (l/s) against (l/c).
It is suggested that the value of the Langmuir constant 6, found in this manner, can be used for the over-all evaluation of an organic corrosion inhibitor.
I. THE LANGMUIR
EQUATION
Investigators of corrosion phenomena are quite generally in agreement that polar organic corrosion inhibitors exert their inhibiting properties through adsorption processes (1, z).~
As to whether van der Waals forces, electrostatic forces, or chemisorption, are controlling, are subjects for some difference of opinion. While the role of adsorption is firmly established as a basis for inhibition phenomena, and corrosion inhibitors are frequently referred to as "strongly adsorbed" or "weakly adsorbed" (3), a means for measuring the actual extent of adsorption in corrosion reactions was not devised until recently (4) when a procedure was described for the determination of the fractional surface area covered by adsorbed inhibitor, and some of these measurements were reported.
The next step in such a study might well be to relate the degree of inhibition to the concentration of inhibitor in the solution.
A logical way to do this would seem to be to express the fractional surface covered by adsorbed inhibitor as a mathematical fun&ion of the inhibitor concentration.
A number of investigators have studied the adsorption of gases and solutes on surfaces, and have given equations to express the relation between the amount of adsorption and the partial pressure of adsorbate in the gas phase, or the concentration of the adsorbate in the liquid phase.
Of all these equations, the most useful have been the Freundlich (5) equation and the Langmuir (6) equation.
An excellent resume of these equations is given by Brunauer (7), who believes the Langmuir equation "is perhaps the most important single equation in the field of adsorption."