Inhibition by Adsorption

Tafel plot of a corrosion system with (a) inhibition of the cathodic reaction of a redox system, shift of Er to negative potentials (b) inhibition of the anodic metal dissolution, shift of the rest potential Er to more positive values and (c) inhibition of both partial reactions, Er remains at its value. 

Due to an increasing surface coverage 9 of the adsorbed inhibitor, the inhibition efficiency increases with the inhibitor concentration c. 9 changes in the simplest case according to a Langmuir isotherm given by Equation 1.172. 

The Frumkin isotherm assumes a linear decrease of AG j with 0, which introduces an exponential term with 0 as shown in Equation 1.173. The Freundlich isotherm uses the 

Assuming the conditions of the Langmuir isotherm, corrosion current densities and ic i refer to the surface fractions without and with inhibitor (0 = 1), which add to the total corrosion current density ic according to Equation 1.175. 

Rearrangement yields Equation 1.176, which allows calculating 0 from corrosion current densities. 

---

PROBABLE FATE photolysis, too slow to be important, vapor-phase reaction with hydroxyl radicals has a half-life of 12.9 days to 3.1 months oxidation not important hydrolysis not important volatilization slow volatilization distributes PCB s globally, but is inhibited by adsorption, significant volatilization from soil surfaces rapid volatilization from water in the absence of adsorption, half-life of 2 months-1 yr in typical water bodies sorption PCB s are... 

Petersen [85] used some of the data of Austin and Walker [107] to show how the first-order criterion would not be correct here the data are recalculated on the basis of Eq. 3.6.C-7. This reaction appears to be very strongly inhibited by adsorption of the product, carbon monoxide, which leads to large deviations from first-order beliavior. The rate equation, in concentration units, was of the standard adsorption type. 

Rate formulations can further include inhibition terms in the denominator, accounting for the effect of coverage of active surface sites by adsorbed species, decreasing the activity of the catalyst. These terms are often adopted from [22] and can also consider inhibition by adsorption on separated catalytic active sites. The inhibition coefficients are calculated according to Eq. (22.2) ... 

Another explanation for the promoter effects could involve the subsurface sites. This explanation is that whereas surface adsorption is inhibited by adsorption site blockers, subsurface adsorption would still occur and could even be promoted [120a], but no direct experimental evidence exists. 

On contact with a metal surface, the inhibitor vapor condenses and is hydrolyzed by any moisture present to liberate nitrite, benzoate, or bicarbonate ions. Since ample oxygen is present, nitrite, and benzoate ions are capable of passivating steel as they do in aqueous solution. The mechanism for carbonate may not be the same, and here the organic amine portion of the VPI may serve to aid inhibition by adsorption and by providing alkalinity. 

Growth Inhibition by Adsorption of AFGP Molecules on the Interface... 

---