D.c. techniques

If protection by paints or varnish films is due to their ability to restrict the penetration of corrosive ions, then it follows that resistance measurements should form the basis of the prediction of their behaviour. In 1948 Bacon eta/. measured the resistance of over 300 paint systems immersed in seawater using a d.c. technique, and concluded that for good performance coatings should have a resistance in excess of 10 0cm Coatings having resistances in the range 10 -10 0cm were found to be unreliable, and those of lower resistance behaved poorly. 

In addition to the rather indirect a.c. techniques discussed above, a more direct technique for the electrical characterisation of the depletion region is available even though this is, primarily a d.c. technique, it is included here since the information is frequently highly complementary. In this technique, ohmic contacts are made to the working surface of the semiconductor electrode and then shielded from the electrolyte as shown in Fig. 36 they are then used as probes to measure the surface conductivity. Ideally,... 

Fig. 1.69 Determination of specific conductivity of a PSSA-PVDF membrane using a four-probe apparatus and d.c. techniques. Membrane was tested at room temperature and was fully hydrated.

The early period with works by Gunther-Schulze (1920), Weigel (1923) and by Rabinowitsch Wood (1933) . These investigations were made by d.c. techniques under application of very high voltages (70-500 V). Whether metal ions or protons (or hydroxyl ions) were the migrating species was a matter of debate in this period. 

Formerly, conductivity was frequently measured by d.c. techniques using either reversible electrodes (e.g. H, Pd, H MoGj, H Ti) and low voltages or just by electrolysing the sample at voltages above the decomposition potential. The former often suffers from electrode polarization effects, whereas the latter yields reliable results only if proton conductivity is the rate determining step for the overall current. 

A variety of methods has been used to measure the Hall voltage which is frequently not more than 1 or 2 juV. The d.c. technique gives, with suitable precautions, reliable results although for high resistance materials (a < 10f2" cm ) one of the a.c. methods is to be preferred. This is because the off-set voltages which arise because of (inevitable) probe misalignments, are difficult to eliminate in the d.c. technique. 

Petersen, D. C., Techniques of Safety Management, 2nd edn, McGraw-Hill, Kogakusha, USA... 

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