Inhibition, corrosion Langmuir isotherm

If one assumes that the blank corrosion rate dcorr) sp ssents the total number of "active sites" on the surface of a corroding metal and that the inhibited corrosion rate (fi p ) represents the total number of active sites minus the inhibited sites, then equation 15 is an expression for the fraction of the surface which is covered by inhibitor molecules. Thus, as explained previously, 0 stands for the fractional coverage of the surface with inhibitor and is, of course, equal to the percent protection as defined by equation 15. The dependence of coverage 0 on the concentration of the inhibitor, within the assumptions and restrictions made previously, is given by the adsorption isotherm of a particular inhibitor. A typical adsorption isotherm was derived by Langmuir and is given in equation 19 ... 

Corrosion and inhibition phenomena often involve the adsorption of reaction intermediates or dissolved species. The Langmuir isotherm generally serves as a starting point in the modeling of these phenomena as will be discussed in later chapters of this book. 

Assume, as has been proposed in the past, that inhibition follows some basic law of adsorption of the inhibitor on the metal surface, or more realistically on a corrosion product covered surface, and that the degree of protection is somehow related to the degree of coverage. The simplest of adsorption isotherms is the Langmuir isotherm, which states that the degree of coverage is related to the concentration as follows ... 

This assumption by no means implies a factual relationship, but is used as a working hypothesis to test whether corrosion inhibition might fulfill the assumptions underlying the Langmuir isotherm. 

Electrochemical impedance spectroscopy was used to determine the effect of isomers of 2,5-bis( -pyridyl)-l,3,4-thiadiazole 36 (n 2 or 3) on the corrosion of mild steel in perchloric acid solution <2002MI197>. The inhibition efficiency was structure dependent and the 3-pyridyl gave better inhibition than the 2-pyridyl. X-ray photoelectron spectroscopy helped establish the 3-pyridyl thiadiazoles mode of action toward corrosion. Adsorption of the 3-pyridyl on the mild steel surface in 1M HCIO4 follows the Langmuir adsorption isotherm model and the surface analysis showed corrosion inhibition by the 3-pyridyl derivative is due to the formation of chemisorbed film on the steel surface. 

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