Passivity adsorption mechanism

Silicates. For many years, siUcates have been used to inhibit aqueous corrosion, particularly in potable water systems. Probably due to the complexity of siUcate chemistry, their mechanism of inhibition has not yet been firmly estabUshed. They are nonoxidizing and require oxygen to inhibit corrosion, so they are not passivators in the classical sense. Yet they do not form visible precipitates on the metal surface. They appear to inhibit by an adsorption mechanism. It is thought that siUca and iron corrosion products interact. However, recent work indicates that this interaction may not be necessary. SiUcates are slow-acting inhibitors in some cases, 2 or 3 weeks may be required to estabUsh protection fully. It is beheved that the polysiUcate ions or coUoidal siUca are the active species and these are formed slowly from monosilicic acid, which is the predorninant species in water at the pH levels maintained in cooling systems. 

Several observations discussed in the following sections support either the film breaking or the adsorption mechanism for the nonstationary or stationary conditions of the passive film. 

Several experimental results support the adsorption mechanism for stationary conditions of the passive layer. Even the stationary passive current density depends on the composition of the electrolyte. For iron in 0.5 M H2SO4, the passive current density is 7 pA cm , whereas less than lpAcm is detected in 1 M HCIO4. From these observations, a catalysis for the transfer of Fe + from the passive layer to the electrolyte by S04 ions was concluded [55, 56]. Similarly, the dissolution Ni + from passive nickel and nickel base alloys is accelerated by organic acids hke formic acid and leads to a removal of NiO from the passive layer [57]. Additions of citrate to the electrolyte cause the thinning of passive layers on stainless steel and increase its Cr content [58]. Apparently Fe and Ni ions are complexed at the surface of the passive film, which causes an enhancement of their dissolution into the electrolyte. It should be mentioned that the dissolution of Cr " " apparently is not catalyzed by these anions and remains... 

For stationary conditions, the passive layer is more effectively attacked via the adsorption mechanism. Strong support is provided by the increased dissolution in the passive state and the observed breakdown of passivity of iron by fluoride ruiming through three stages. XPS studies... 

Three main mechanisms are discussed by most authors for the processes leading to breakdown of passivity the penetration mechanism, the film-breaking mechanism, and the adsorption mechanism [4]. Figure 7.2a through c presents diagrams for their explanation. 

The effect of a local increase of the potential drop at the oxide surface is similar to the situation discussed for the adsorption mechanism of Section 7.4.3 and Figure 7.2c. Apparently, the defects lead to locally enhanced dissolution, however, with still maintained passivity (Figure 7.10a). Statistical fluctuations may cause even an intermediate localized activation, which, however, will not last due to the self-healing effect of passive surfaces. However, they cause local depressions at the specimen surface. Temporary local breakdown should therefore be accepted also for electrolytes free of aggressive anions. This concept is consistent with an increased general corrosion rate during the early stages of passivation that will decrease with time when the film structure becomes more crystalline with a related decrease of its defects. 

If halides like Cb are present they will compete at the surface of the passive layer with OH ions. Chemisorbed halides may help to remove locally the passive layer due to their complexing properties similar to the already discussed adsorption mechanism. 

In view of the fact that there are two opposing views on the mechanism of passivity it is not surprising that a similar situation prevails concerning the mechanism of breakdown of passivity. The solid film theory of passivity and breakdown of passivity is dealt with in some detail in Section 1.5, so that it is appropriate here to discuss briefly the views based on the adsorption theory. 

With regard to the anodic dissolution under film-free conditions in which the metal does not exhibit passivity, and neglecting the accompanying cathodic process, it is now generally accepted that the mechanism of active dissolution for many metals results from hydroxyl ion adsorption " , and the sequence of steps for iron are as follows ... 

---