Conditions for passivation

Different views exist as to the reasons for selective dissolution of the asperities. According to older concepts, convection of the liquid is hindered in the solution layers filling recesses hence, reaction products will accumulate there and raise the concentration and viscosity in these layers. Both factors tend to lower a metal s anodic dissolution rate relative to that at raised points. According to other concepts, a surface condition close to passive arises during electropolishing. In this case, the conditions for passivation of the metal at raised points differ from those in recesses. 

Equation (4.7) corresponds to the potential variation of a metal electrode of the second kind as a function of pH. The Flade potential is used to evaluate the conditions for passive film formation and to determine the stabihty of the passive film. The reversible Flade potential of three important engineering materials is approximately +0.63 V for iron, +0.2 V for nickel, and —0.2 V for chromium [7,8]. The negative value of the Flade potential for chromium (—0.2 V) indicates that chromium has favorable Gibbs free-energy for the formation of passive oxide film on its surface. The oxide film is formed at much lower potentials than in other engineering materials. 

By immersion of a piece of iron in 70% (about 15 mol/1) nitric acid, spontaneous passivation is obtained (case I). With an HNO3 concentration = 20% (about 4 mol/1), the condition for passivity is that the iron is passive from the beginning (due to previous exposure in a more concentrated nitric acid, for example) (case II). Table 5.1 gives some information about the conditions for iron in 0.1 N = 0.1 mol/1 HNO3 (case III). 

Laycock and coworkers developed simulations of pit growth on stainless steel at constant applied potential. The present discussion focuses on their second paper, in which the model was substantially improved by the extension of the simulation domain to include the solution outside the pit cavity. The pit growth model incorporated a hypothesis of the critical condition for passivation within the pit, and the resulting development of the pit shape was evaluated. The metal surface was assumed to be passivated at metal ion concentrations lower than a critical value the dissolution rate increased with metal ion concentration above the critical value. These concentration-dependent dissolution kinetics were closely related to results of experimental studies of artificial pits, and the criterion for passivation was supported by measurements of actual pit growth. It should be mentioned that description of mass transport in the model did not conform to the dilute solution theory as outlined above, since a current continuity equation was used which included contributions from migration but not diffusion hence, the potential gradient in the pit was probably overestimated. However, inaccuracies in the prediction of potential were likely compensated by adjustment of empirical dissolution kinetic parameters in the model. 

The CBD diagram can provide various lands of information about the performance of an aUoy/medium system. The technique can be used for a direc t calculation of the corrosion rate as well as for indicating the conditions of passivity and tendency of the metal to suffer local pitting and crevice attack. 

If the metal is exposed to highly oxidizing conditions in the complete absence of water, a violent reaction may occur (for example, in completely dry chlorine). In this case, 0.015% water is added as the minimum for passivation of titanium. 

EMGRESP is overly conservative for passive gas dispersion applications. No time-varying releases may be modeled. Dense gas dispersion may be computed for only "instantaneous" releases conditions. 

The theoretical aspects of molybdenum s corrosion behaviour are complex and there is as yet no clear cut, generally applicable picture. There are, however, a large number of literature references which include data on polarisation, passivation and potential of molybdenum under widely assorted conditions. The electrode potential of molybdenum depends on its surface condition. For example, some tests showed an of -t-0-66V when the molybdenum was passivated by treatment with concentrated chromic acid and —0-74 V after activation by cathodic treatment in sodium hydroxide. 

The simplest and most thoroughly studied solutions are those based on phosphoric acid at low temperatures (<35°C) which alone can fulfil all three requirements of acid solvent, film former (as metal phosphate) and diffusion agent by virtue of its viscosity. Thus copper and its main alloys of brasses and bronzes can be very effectively electropolished in 60-70% orthophos-phoric acid with the temperature maintained below 35°C under other conditions copper passivates or dissolves freely under mass transfer controlled conditions, but by varying the conditions appropriately polishing can be continued under mild agitation. An annotated polarisation curve is given in Fig. 11.7 readers are referred to recent studies for more detailed 2ispects " . 

Potentiodynamic polarisation The characteristics of passive/active conditions for metals can be readily defined using this technique ". Details for laboratory application can be found in ASTM Standard G5 (latest revision). Application in plant is easily performed as portable equipment (potentiostat) is available from several manufacturers, with some models incorporating built-in computer facilities. 

Passivation is defined as the state where even though a metal electrode fulfills the thermodynamic condition for dissolution (solution composition, electrode potential, etc.), a corrosive reaction scarcely proceeds. 

Figure 20. Pit-dissolution current density pit radius and ion concentration buildup AC in the pit electrolyte corresponding to the critical condition for growing pits on 18Cr-8Ni stainless steel to passivate at different repassivation potentials, EK, in 0.5 kmol m 3 H2S04 + 0.5 kmol m-3 NaCl during cathodic potential sweep at different sweep rates.7 (From N. Sato, J. Electrochem. Soc. 129,261,1982, Fig. 1. Reproduced by permission of The Electrochemical Society, Inc.)...

As shown in Fig. 22, since the dissolved metal ions are locally enriched near the surface, the fluctuation in concentration takes a positive value. In this fluctuation process, the passive film does not provide the absolute condition for protecting the substrate dissolution because, as shown in the preceding section, a breakdown in local passivity prior to... 

These authors constructed Pourbaix diagrams for the MnTe, ZnTe, Cdi , Mn i Te, and Cdi cZn cTe systems and argued that the related analysis is an effective approach to determine conditions for selective etching, chemical polishing, passivation, and self-metallization of ZnTe, MnTe, and their solid solutions. 

Monnard and Deamer (2001) carried out further studies, using DMPC liposomes, to determine their properties under conditions of passive diffusion of dissolved molecules. The passage across the lipid bilayer is a precondition for the intake of nutrient substances via the vesicle envelope. The experiments showed that even polar molecules can enter the interior of the liposomes oligonucleotides, however, cannot cross the lipid bilayer of DMPC vesicles. 

Some laboratories have found an alternative to the short-term cultures by using cell lines other than Caco-2 cells. The most popular of these is Madin-Darby canine kidney (MDCK) cells, an epithelial cell line from the dog kidney. MDCK cells have been suggested to perform as well as Caco-2 cells in studies of passive drug permeability [56]. These cells have also been used to optimise the conditions for studies of low-solubility drugs [53]. However, as noted previously, the active transport processes of this cell line can be quite different to those of Caco-2 cells [28-30], Another cell line that only requires short-term culture is 2/4/A1, which is a conditionally immortalised rat intestinal epithelial cell line [86]. The 2/4/A1 cell line is discussed in Section 4.3.2.2 below. 

The following protocol for passive adsorption is based on methods reported for use with hydrophobic polymeric particles, such as polystyrene latex beads or copolymers of the same. Other polymer particle types also may be used in this process, provided they have the necessary hydrophobic character to promote adsorption. For particular proteins, conditions may need to be optimized to take into consideration maximal protein stability and activity after adsorption. Some proteins may undergo extensive denaturation after immobilization onto hydrophobic surfaces therefore, covalent methods of coupling onto more hydrophilic particle surfaces may be a better choice for maintaining native protein structure and long-term stability. 

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