Effect on critical current density

Table 10.31 Effect on critical current density and Flade potential of chromium content for iron-chromium alloys in lOwt.% sulphuric acid (after West )...

Table 10.32 Effect on critical current density and passivation potential on alloying nickel with chromium in In and IOn H2SO4 both containing 0-5N K2SO4 (after Myers, Beck and Fontana")...

TABLE 12.4 Effect on Critical Current Density and Passivation Potential of Chromium Content ... 

Because these variables have a very pronounced effect on the current density required to produce and also maintain passivity, it is necessary to know the exact operating conditions of the electrolyte before designing a system of anodic protection. In the paper and pulp industry a current of 4(KX) A was required for 3 min to passivate the steel surfaces after passivation with thiosulphates etc. in the black liquor the current was reduced to 2 7(X) A for 12 min and then only 600 A was necessary for the remainder of the process . From an economic aspect, it is normal, in the first instance, to consider anodically protecting a cheap metal or alloy, such as mild steel. If this is not satisfactory, the alloying of mild steel with a small percentage of a more passive metal, such as chromium, molybdenum or nickel, may decrease both the critical and passivation current densities to a sufficiently low value. It is fortunate that the effect of these alloying additions can be determined by laboratory experiments before application on an industrial scale is undertaken. 

Table 10.34 Effect of concentration of sulphuric acid at 24°C on corrosion rate and critical current density of stainless steel (after Sudbury, Riggs and Shock )...

In reality, this behavior is only observed in the limit of small jg. At currents o 1 A cm-2 that are relevant for fuel cell operation, the electro-osmotic coupling between proton and water fluxes causes nonuniform water distributions in PEMs, which lead to nonlinear effects in r/p M- These deviations result in a critical current density, p at which the increase in r/pp j causes the cell voltage to decrease dramatically. It is thus crucial to develop membrane models that can predicton the basis of experimental data on structure and transport properties. 

For the usual dc measurement the constant dc current source should be capable of providing currents in the range 0.1-10 mA for a typical bar of 1 mm square cross-section, 1 cm length, and a resistivity at 100 K of 50 pOhm-cm the voltage measured for a 1 mA current source would be 1 / V. Since even for a typical low value of the critical current density, 100 A/cm2, the measurement current would be 1000 times less and thus have essentially no effect on the measurement. However, the measurement of 1 / V to a precision of 1% already requires care to assure that noise and thermal voltages are reduced well below this value. Currents of similar value are used for measurements in thin films. 

Fig. 4. Effect of temperature on the critical current density required to start electropolishing silicon in 5% HF.

The effect of viscosity on the critical current density required to start electropolishing a horizontal silicon electrode in 5% HF solutions at 25° C is presented on a log-log plot in Fig. 6. Glycerine was used to increase the solution viscosity. 

A very recent fundamental investigation of the anode effect is that of Vogt [32] who showed that the effect of alumina concentration on the critical current density (above which an anode effect occurs) can be explained by the alumina mass transfer, fluid dynamics of gas release and wettability of the anode. 

Figure 6 accentuates the effect of the width of the psd. The three parameteriza-tions with s = 0.15,0.3, 0.6, labeled as (1), (2), (3) in Table 1 and Fig. 5 are used in Fig. 6. All other parameters of the calculation are specified in Table 2. The width of the psd decreases upon increasing s. The first moments of the psds are, respectively, r = 14.9, 3.0, 1.2 nm. In Fig. 6, yp and U are scaled to Jm ccr, defined in Eq. (19). Table 1 indicates that the permeability of the saturated membrane, K(ws), varies strongly with s. However, the effect of the psd width on membrane performance is accumulated in the scaling parameter Jm, as shown in Fig. 6. Here, the critical current densities are found at ypc/7m 0.66, 0.65, 0.67, practically independent of the psd with this scaling to Jm.

The effect on PEFC performance of 10-100 ppm CO in the anode feed stream is very substantial when Pt catalyst is used in the anode. It can be seen from Fig. 32 that the effect of CO can be qualitatively described in terms of some critical current density of value which drops with the level of CO in the anode feed stream. When current demand is below the critical current level, the PEFC maintains practically CO-free cell performance, whereas above that critical current level, cell performance drops sharply as the cell voltage falls well under the corresponding CO-free level. The explanation for this behavior was provided by a model that considered the interfacial kinetics at the PEFC anode in the presence of low levels of CO [42,66, 67]. 

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