Permeation current

The remainder of this section specifies some appUcations for coUoidal soUds, Hquids, and gases. Whereas some appUcations of coUoidal materials have been mentioned earlier regarding specific phenomena, other examples wiU iUustrate how coUoids affect many technologicaUy important systems ia a positive manner and demonstrate the broad range of appUcations that permeates current synthetic materials. 

Fig. 8.38 Hydrogen permeation current as a function of applied potential, showing effect of change in reaction mechanism below —1200 mV (SCE) (after Obuzor )...

Assuming that the exiting hydrogen atoms are oxidized sufficiently fast, the concentration of absorbed hydrogen direcdy beneath the exit surface may be taken as zero. Using Faraday s law, the steady-state hydrogen flux across the membrane is described by the steady-state hydrogen permeation current ... 

In other words, the analysis of Iyer et al. differs from the analysis of Bockris et al. by predicting a linear relation between the steady-state hydrogen permeation current and (lobs - instead of iiQ. Figure 27... 

Figure 27. Plots of the permeation current density vs. the square root of the Tafel reaction current density [Eq. (33)], recorded in ace-tate/acetic acid buffers at pH = 6 and 30°C, using carbon steel membranes of 1.0 and 0.5 mm thickness. ...

Hence, by measuring the permeation current, it is possible to study the diffusion coefficient of the hydrogen inside the metal. 

There is a simple way of experimentally demonstrating that stretching a metal leads to increased hydrogen absorption (McBreen, 1976). The permeation currents measured by the membrane technique should show an increase when the membrane is stretched. This is, in fact, what happens (Fig. 12.75). 

Experiment shows that when the hydrogen concentration is enough to cause embrittlement, the permeation current builds up with time and then instead of stabilizing to a steady state as it normally does in the absence of a crack-initiation and propagation process, it drops down and only then becomes steady (Fig. 12.83). Thus, the fall in permeation current occurs at the onset of crack propagation and embrittlement. 

Fig. 12.83. When cracks are initiated and hydrogen starts accumulat-ing inside the cavities, the permeation current falls off.

In Eq. (8.28), jt is transition hydrogen permeation current density, (A/cm ) is initial hydrogen permeation current density, (A/cm ) joo is the steady state hydrogen permeation current density, (A/cm ) ... 

Fig. 8.7 Hydrogen permeation current-overpotential dependence obtained on an AiSi 4340 steel membrane in an electrolyte containing 0.5 iVI HCi04 and 0.25 M NaCi04 (pH 0.3) and the membrane thickness=0.3 mm [3]. Reproduced by the permission of The Electrochemical...

In Eq. (8.43), aj and 2 are transfer coefficients for reactions (8.41) and (8.42), is the cathodic current density and a = F/RT. If ai = 2. the hydrogen permeation current is directly proportional to the cathodic current. This expression is consistent with experiment [16] for atomic hydrogen permeation transients through a HY-130 steel membrane as a function of time for different appHed potentials and other experimental data [20-27]. 

A plateau (limiting current) is observed in the I-E curve. At this point, the permeation current is related to the thickness of the membrane according to Pick s law ... 

Fig. 8.20 Comparison of cathodic and permeation current densities for Zn-Ni-Cd, Zn-Ni, and steel [90]. Reprinted by permission of The Electrochemical Society.

Fig. 8.21 Permeation current density as a function of applied overpotential for Zn-Ni and Zn-Ni-P alloys [91]. NACE International 2000.

E8.1. Values of the hydrogen cathodic current density, and steady state hydrogen permeation current, joo. a function of the overpotential, ij, measured using the Devanathan-Stachurski technique are given in Table E8.1. 

Use the hydrogen atom direct mechanism [18,19] discussed in Ref [16] to explain why the steady state hydrogen permeation current density, Joo. is directly proportional to the cathodic current density, f, and is independent of the membrane thickness when the cathodic current density is small. 

The resistance to hydrogen permeation in Cd deposited on steel was evaluated by using the Devanathan-Stachurski technique. The hydrogen evolution current, and the permeation current,7oo. were measured as a ffinction of overpotential and are hsted in Table E8.3. 

E8.8. Plot the hydrogen surface coverage for Cd deposited on steel as a function of overpotential. Use the data for permeation current and hydrogen evolution current as a function of overpotential listed in Table E8.3. The hydrogen surface coverage for Cd deposited steel was calculated using Eq. (E8.5) [81]. 

Fig. E8.1 Plot of as a function of the hydrogen permeation current through an AISI 4340 steel membrane of 0.01 cm thick.

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