Break-in effect

This suggestion was confirmed by an experiment in which a Pd wire was exposed to a CO/Oz reaction mixture without any previous surface cleaning (173). The stationary rate of C02 formation as a function of temperature (as shown in Fig. 58) is at first rather low, but increases steeply above a certain activation temperature. In any further runs the catalyst exhibited its normal activity, as is known from atomically clean surfaces. Auger spectroscopy demonstrated that at the beginning the surface was heavily contaminated by S and C, whereas these elements were completely absent after the sample had reached its full activity. Thus the well-known break-in effect of the catalyst (210) can in this case be simply explained by the removal of the inhibitors by surface reaction with one of the reactants, and the above-stated assumption is confirmed. 

Only cation exchange, which can be enforced by incorporation of immobile anions in the polymer during the synthesis, e.g., polystyrene sulfonate. In this case a strong change of Aw accompanies the frequency change A/and the simple Sauerbrey equation cannot be applied. Moreover, values of A/ and Aw do not return to the initial values. Such behavior can be explained by a break-in effect. ... 

The importance of polymer swelling and solvation was clearly demonstrated in the break-in and memory effects observed in many electroactive polymer systems. Break-in effects in tetracyanoquinodimethane... 

All the ion and solvation changes discussed so far in this section relate to chemically reversible processes. Anecdotally, it is widely accepted in the redox polymer modified electrode literature that the first redox cycle of a newly deposited film is atypical. Although the reasons and processes are chemically rather different, this is reminiscent of the first cycle effect discussed in Sect. 2.7.3.6 for Prussian Blue films. PVF provides a typical example of this first cycle, or break-in , effect. The initial EQCM response to PVF oxidation in water (after its deposition from an organic solvent, typically dichloromethane) is quite different from that of a previously cycled film [132]. The result is most clearly demonstrated by considering the mass flux - or, better stiU, the difference between the total mass flux and the elec-tron/ion flux-as a function of apphed potential or film charge. Such plots show a once-only pulse of solvent into a new film this solvent (typically ca. five solvent molecules per ferrocene redox site) is retained ( trapped ) within the film thereafter and provides the baseline upon which subsequent redox-driven solvation... 

The expansion and contraction of the polymer netwoik in conjunction with the sorption/desorption of solvent molecules and ions can be described in terms of mechanical work. This mechanical contribution should be considered in the calculation of the equilibrium electrode potential (see Chap. 5). The deformation coupled to the redox reaction is elastic in nature. A plastic deformation occurs when a neutral, diy film is immersed in electrolyte solution and electrolyzed. It has been observed for a range of neutral polymer films freshly deposited on metal substrates by solvent evaporation techniques that several potential sweeps are required for the films to become fully electroactive [2,19,126,195,196]. This phenomenon has been referred as the break-in effect (Fig. 6.19). 

Fig. 6.19a,b Break-in effect as observed in cyclic voltammetric and simultaneous EQCM measurements performed with a poly(tetracyanoquinodimethane) electrode. F = 7 x 10 molcm . Electrolyte 2.5 M LiCl. Sweep rate 6 mV s . a Consecutive cyclic voltammograms b simultaneously obtained EQCM frequency curves. (Reproduced from [195] with the permission of Elsevier Ltd.)... 

Hillman and Bruckenstein have used this type of analysis to discuss break-in effects, charge and mass trapping, structural evolution with redox cycling, kinetic decoupling of ion and solvent transfer, and variations in apparent charge transport rate and formal potential with experimental time scales. The reader is referred to the original paper for further details. 

A high value of the BET constant c is a useful preliminary indication of the presence of microporosity, but it does not enable one to estimate the micropore volume itself, that is in effect to break down the composite isotherm (iii) into its components (i) and (ii). 

Trialkylammonium salts, such as lidocaine hydrochloride, are titrated in an aqueous solution containing a surfactant. The presence of the surfactant increases the trialkylammonium salt s K , giving a titration curve with a more pronounced break. The effect of adding an immiscible organic solvent, such as methylene chloride or toluene, also is demonstrated. 

What of the corrosion resistance of new turbine-blade alloys like DS eutectics Well, an alloy like NiaAl-NisNb loses 0.05 mm of metal from its surface in 48 hours at the anticipated operating temperature of 1155°C for such alloys. This is obviously not a good performance, and coatings will be required before these materials are suitable for application. At lower oxidation rates, a more insidious effect takes place - preferential attack of one of the phases, with penetration along interphase boundaries. Obviously this type of attack, occurring under a break in the coating, can easily lead to fatigue failure and raises another problem in the use of DS eutectics. 

Pits occur as small areas of localized corrosion and vary in size, frequency of occurrence, and depth. Rapid penetration of the metal may occur, leading to metal perforation. Pits are often initiated because of inhomogeneity of the metal surface, deposits on the surface, or breaks in a passive film. The intensity of attack is related to the ratio of cathode area to anode ai ea (pit site), as well as the effect of the environment. Halide ions such as chlorides often stimulate pitting corrosion. Once a pit starts, a concentration-cell is developed since the base of the pit is less accessible to oxygen. 

What are the implications of the effects of the various halide leaving groups Because the halogen with the weakest bond to carbon reacts fastest. Ingold concluded that the carbon-halogen bond breaks in the rate-detennining step. The weaker the carbon-halogen bond, the easier it breaks. 

In Eq. (8-35), Afvap is the molar energy of vaporization, and AH p is the molar heat of vaporization. In effect, -it is a measure of the energy required to break some of the solvent-solvent forces, whereas ced is a measure of the energy required to... 

The aggressive ions either break down the protective films or prevent their formation and, in effect, increase the corrosion rate. In presence of chloride and sulfate ions the corrosion attack is more localized and, as a result, causes deep pitting. Inhibitive ions, on the other hand, tend to limit the attack and decrease the corrosion rate by forming protective films. The film is similiar to adherent carbonate-containing rust, which polarizes the anodic areas. When aggressive and inhibitive ions are present together, the aggressive ions, if present in sufficient quantities, interfere with the deposition of the protective layers. 

The most commonly observed effect of current flow is the development of alkaline conditions at the cathode. On bare metal this alkaline zone may exist only at the metal surface and may often reach pH values of 10 to 12. When the soil solution contains appreciable calcium or magnesium these cations usually form a layer of carbonate or hydroxide at the cathodic area. On coated lines the cations usually move to holidays or breaks in the coating. On failing asphalt or asphalt mastic type coatings, masses of precipitated calcium and magnesium often form nodules or tubercles several centimetres in diameter. 

Inhibitors can also lead to the co-called polarity-reversal effects. In corrosive environments the zinc coating on galvanised steel acts sacrificially in preventing the corrosion of any exposed steel. However, in the presence of sodium benzoate or sodium nitrite steel exposed at breaks in the zinc coating may corrode quite readily. 

The Schwerdtfeger polarisation break and the polarisation resistance methods have been studied by Jones and Lowe " in relation to their effectiveness in evaluating corrosion rates of buried metals. A Holler bridge circuit was used to remove IR contributions during the measurement of the polarised potential. Jones and Lowe, on the basis of their studies of buried steel and aluminium specimens, concluded that the polarisation resistance was the most useful, and that the polarisation break had the serious limitation that it was difficult to identify the breaks in the curve. 

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