Self-passivation

The property of coatings to transfer metal substrates into a passive state (self-passivation) is realized during their formation or operation. 

Substrate passivation during coating formation is achieved by treating the surface being covered with special technological fluids, i.e. passivators. The passivation operation precedes the application of the polymer layer to the substrate. 

Passivators are inorganic substances possessing oxidative properties whose reaction products with metals form a passive film on the substrate surface, which shifts the corrosion potential of the substrate to the positive side by a few tens of volts. Like a depolarizer, the passivator generates a current on the anodic areas of the substrate of 1 i density, where i is the critical density of the passivation current. This means that the chemical composition of the passivating film on a metal substrate is the same whether the substrate is passivated by anodic polarization in an acid or is treated with solutions of chromates (CrO ), nitrates (NO ), molybdates (MoO ), tungstates (WO ), ferrates (FeO ) or pertechnates (TcOj). 

Passivator ions acting as oxidizers are adsorbed on the substrate and reduced easily, thus enlarging the cathode surface area. An optimum passivator solution concentration should exceed some critical value and, the higher the passivator concentration, the easier it is adsorbed, and the smaller the anodic areas on the substrate will be. This promotes an increase in anodic polarization and total passivation of the substrate. If the passivator concentration is lower than some critical value, it initiates local corrosion of the substrate. 

Self-passivation of metal substrates occurs in the course of application of a new generation of electronic pol3Tner coatings called organic metals, described in Sect. 3.2.2. 

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The use of the potentiostatic method has helped to show that the process of self-passivation is practically identical to that which occurs when the metal is made anodically passive by the application of an external current" . The polarisation curve usually observed is shown schematically in Fig. 19.37a. Without the use of a potentiostat, the active portion of the curve AB would make a sudden transition to the curve DE, e.g. along curve AFE or AFD, and observation of the part of the curve BCDE during anodic polarisation was not common until the potentiostat was used. 

AFM has been used in real time to image reacting surfaces in contact with a liquid phase [35]. A study ofthe self-passivating reactionbetween p-chloranil and an aromatic amine included monitoring the surface topography of the p-chloranil in contact with the amine in aqueous solution at 25-second intervals, Fig. 5.27. Under the conditions employed, passivation by the reaction product was shown to be complete in 5 minutes a concomitant hydroxide induced dissolution could also be monitored [7]. 

Fig, 8. (a) Schematic anodic polarization curve, (b) Polarization curves for self-passivation. 

Fig. 8(b) shows schematically the curves for the process of self-passivation X and Y represent two possible cathodic polarization curves for an electron acceptor present in the... 

Koskinen P, Malola S, Hakkinen H (2008) Self-passivating edge reconstruction of graphene. Phys Rev Lett 101 115502/115501-115504... 

Table 1. Self-Passivation Experiment Erosion Depths...

In addition to the numerous physical property enhancements reported for polymer nanocomposites, nanoscale dispersion of the inorganic particles has the potential to provide unique chemical properties, such as self-passivation and self-healing. Figure 3 summarizes the conceptual origin of the self-passivating/self-healing response of nanocomposite materials. 

As a final comment, the self-passivation response is not only advantageous for enhancing durability in service, but also provides unique opportunities for post-processing modification of nanocomposites. For example, pretreatment of the nanocomposite in oxygen plasma may enhance the materials resistance to thermal-oxidative or UV-electron environments. Alternatively, self-generation of the inorganic surface on commercial nanocomposites films may enhanced barrier properties before introduction to service. 

The tunable luminescence of the self-passivated organic QCNs and observation of interesting optoelectronic properties like up-conversion, have prompted the researchers to use them as replacement in OLEDs based on their inorganic counterparts. 

Typical corrosion, potential, and critical-current density behavior of Cr-Fe alloys is shown in Figs. 6.9-6.11. Note in Fig. 6.11 that /critical for passivation of Cr-Fe alloys at pH 7 reaches a minimum at about 12% Cr in the order of 2pA/ cm. This value is so low that corrosion currents in aerated aqueous media easily achieve or exceed this value, illustrating why >12% Cr-Fe alloys are self-passivating. In addition, the passive film becomes more stable with increasing chromium content of the alloy. 

A variety of polycrystalline substrates have been employed during CVD growth, including silicon and self-passivating metals such as niobium, molybdenum,... 

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