Polarisation Phenomena

The intensity of the coupling current, and thus the amount of galvanic corrosion, depends on several factors [4], including the nature of the electrolyte, polarisation phenomena, the ratio between the surfaces of the two metals, their distance, and 

Galvanic corrosion depends on both anode and cathode reactions. It can be slowed down by the polarisation of the anode or cathode surfaces. Polarisation is caused in particular by the accumulation of insoluble corrosion products on the anode surface. This is the case with alumina. 

On bolted aluminium-steel assemblies with unprotected contacts, an important accumulation of alumina is observed in the contact area after a few years of immersion in seawater. This accumulation is capable of deforming the aluminium sheets. This cataplasm may stop galvanic corrosion, at the price of a certain deformation of the aluminium sheets. 

It is obviously not appropriate to rely on possible anodic or cathodic polarisation phenomena in order to limit the damage caused by galvanic corrosion. Prevention is necessary at the assembly design stage. 

Among parts recovered from the wreck of the Titanic, a megaphone in the form of a sawn-off cone was found, crimped at each end with a wire of—probably originally galvanised—steel. After 70 years in seawater at great depth, only part of the aluminium sheet in contact with the wire had been consumed. 

---

The value of e" polarisation is related to the molecular polarisation phenomena such as dipole rotation (Debye model), space charge relaxation (Maxwell-Wagner model), hopping of confined charges [42,126,127]. The physical origin of this polarisation term is often ambiguous [42],... 

Thus, the evolution of the permittivities is due to relaxation of the conduction process, without any polarisation phenomena, and is described by the general physics of relaxation. The given results point out not only the capabilities, but also the limitations of using conductive polymers as materials for microwave absorbtion purposes. 

There is strong evidence in favour of the point of view 1. in the variants colloid anion + micro cation or colloid cation -f micro anion in the specific ion sequences for reversal of charge or for coacervation or flocculation. We remind the reader for example of the sequences Cs < Rb < K < Na < Li which occur with sulphate colloids and carboxyl colloids (p. 289). Here polarisation phenomena are still in the background and here the largest ion, that is to say, the least hydrated ion, is most suitable for reversal of charge or coacervation. This points strongly therefore to a direct contact between cation and ionised group of the colloid. 

The retained solutes can accumulate at the membrane surface where their concentration wiir gradually increase. Such a concentration build-up will generate a diffusive flow back to the bulk of the feed, but after a given period of time steady-state conditions will be established. The convective solute flow to the membrane surface will be balanced b> the solute flux through the membrane plus the diffusive flow from the membrane surface to the bulk (it should be remembered that only concentration polarisation phenomena are considered here with fouling being, excluded). A concentration profile has now been established in the boundary layer (see figure VII - 4). 

Concentration polarisation phenomena lead to an increase Of the solute concentration at the membrane surface. If the solute molecules are completely retained by the membrane, at steady-state conditions the convective flow of the solute molecules towards the membrane surface will be equal to the diffusive flow back to the bulk of the feed. Hence, at 100% rejection the average velocity of the solute molecules in the boundary layer will be zero. Because of the increased concentration, the boundary layer exens a hydrodynamic resistance on the permeating solvent molecules. The solvent flux can then be represented by a resistance model in which both the boundary layer resistance (R, ) and the membrane resistance (Rm) appear (assuming that no gelation occurs ). A schematic drawing of this resistance model is given in figure VII - 18. 

Although the driving forces, the. separation principle and the membranes are completely different in elecirodialysis from those in pressure-driven membrane processes, polarisation phenomena may severely aflect the separation efficiency. 

In comparison to isothermal membrane processes, little attention has been paid to date to polarisation phenomena in non-isothermal processes. In non-isothermal processes such as membrane distillation and thermo-osmosis, transport through the membrane Occurs when a temperature difference is applied across the membrane. Temperature polarisation will occur in both membrane processes although both differ considerably in membrane structure, separation principle and practical-application. In a similar manner to concentration polarisation in pressure-driven membrane processes, coupled heat and mass transfer contribute towards temperature polarisation. 

The performance of membrane operations is diminished by polarisation phenomena, although the extent to which these phenomena can occur differ considerably. Thus, in microfiltration and ultrafiltration the actual flux through the membrane can be only a fraction of the pure water flux, whereas in pervaporation the effect is less severe. 

With all polarisation phenomena (concentration, temperature polarisation), the flux at a finite time is always less than the original value. When steady state conditions have been attained a further decrease in flux will not be observed, i.e. the flux will become constant as a function of time. Polarisation phenomena are reversible processes, but in practice, a continuous decline in flux decline can often be observed. This is shown schematically in figure VH - 23. 

POLARISATION PHENOMENA AND MEMBRANE FOULING pressure drop over the cake are equal. 

The remaining problem areas in the ionic electrolyte field are mostly associated with electrode polarisation phenomena, although there is still a need to discover an alkali metal conductor with a room temperature conductivity better than beta alumina and approaching that of the silver salts. 

The EFISH experiments with conjugated polymers are to be performed carefully because of internal polarisation phenomena observed in these materials (Chollet et al. (1985) f Obhi et al. (I986)) and due to the creation of electron-hole pairs by incident photons through one or two photon processes. This polarisation cancels the applied external fields and results in a decrease (up to its vanishing) of SHG signal. 

For more details on this problem see Ch. IV/ 8a/ p. 168. In a remarkable investigation on mercury Frumkin showed that here the electrophoresis is complicated by polarisation phenomena at the mercury-w ater interface, which results in differences of interfacial tension between the mercury drops and the surrounding liquid. These differences in interfacial tension are the cause of liquid movements in the mercury drops, wliich may increase the elearophoretic velocity to such an extent, that practically the whole electrophoretic retardation is nullified and the electrophoresis is nearly described by the simple Stokes equation... 

---