Adsorption saturation, 252 competing ions

In the ion-exchange method, the cations C+ or the anions A" tend to saturate the first adsorption sites so that most of the compound adsorbs near the pore mouth, and a large concentration gradient develops within the pellet pore. There are several ways to obtain a more uniform distribution (1) a large supply of compound is used so as to saturate every adsorption site, (2) the support may be left for a long time in contact with the solution, and (3) competing ions are added to the solution, which will adsorb on the same sites as the ions of the metal precursor. This is the subject of the method discussed next. 

The corrosion process can be inhibited by the addition of phosphate or polyphosphate ions [344], inorganic inhibitors as, for example, chromate ions [336], adsorbed alcohols [345], adsorbed amines, competing with anions for adsorption sites [339,] as well as saturated linear aliphatic mono-carboxylate anions, CH3(CH2)n-2COO , n = 7 — 11, [24]. In the latter case, the formation of the passive layer requires Pb oxidation to Pb + by dissolved oxygen and then precipitation of hardly soluble lead carboxylate on the metal surface. The corrosion protection can also be related to the hydrophobic character of carboxylate anions, which reduce the wetting of the metal surface. 

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