Polarisation characteristics

The anode material must have a more or less constant operating potential over a range of current outputs. Consequently the anode must resist polarisation when current flows the polarisation characteristics must also be predictable. 

The extent of galvanic effects will be influenced by, in addition to the usual factors that affect corrosion of a single metal, the potential relationships of the metals involved, their polarisation characteristics, the relative areas of anode and cathode, and the internal and external resistances in the galvanic circuit (see Section 1.7). 

Some investigatorshave advocated a type of accelerated test in which the specimens are coupled in turn to a noble metal such as platinum in the corrosive environment and the currents generated in these galvanic couples are used as a measure of the relative corrosion resistance of the metals studied. This method has the defects of other electrolytic means of stimulating anodic corrosion, and, in addition, there is a further distortion of the normal corrosion reactions and processes by reason of the differences between the cathodic polarisation characteristics of the noble metal used as an artificial cathode and those of the cathodic surfaces of the metal in question when it is corroding normally. 

One should keep in mind that e. m. waves are transverse vector helds and are polarized. Strictly speaking, one has to consider the polarisation characteristics of the light throughout this treatment. Polarisation plays an important practical role as instrumental depolarisation can change the coherence charac-... 

The basic corrosion instrumentation requirement involves the measurement of potential difference. Currents are measured as the potential across a resistor (R ) as shown in Fig. 1.2, where the potential difference is again determined with an operational amplifier. More sophisticated measurements such as polarisation characteristics and zero resistance ammetry involve the use of potentiostats which again use operational amplifiers in a differential mode. The potentiostat is an instrument for maintaining the potential of an electrode under test at a fixed potential compared with a reference cell, and the basic circuit is similar to that for potential measurement with the earth return circuit broken to an auxiliary electrode in the electrochemical cell. Such a circuit would maintain the potential of the test electrode at the reference cell potential. This potential may be varied by inserting a variable potential source (V ) in the input circuit as shown in Fig. 1.3. The actual cell potential (V ) and the current required to polarise the test electrode to this potential may be measured using the basic circuits shown in Figs. 1.1 and 1.2 respectively. 

Fig. 7 Polarisation characteristics of sPEEK and sPEEK-ZrP (50 (Am) prepared by ion-exchange/precipitation with sPEEK membranes. The electrodes employed in these MEAs were standard ImgPtcm" E-TEK, Nafion dispersion was used at the mem-brane/electrode interface, and the cell was pressurised to 2.6barabs. sPEEK at 85 °C (A) sPEEK-ZrP at 85 °C ( ) and sPEEK-ZrP at 100°C ( )...

Other methods involve holding specimens in suitable fixtures so that they form the walls of channels through which the test solution can be passed at controlled rates of flow. Such devices have been used at the Harbor Island Test Station in North Carolina primarily for studying the electrode potential and polarisation characteristics of metals and alloys, but they are also suitable for observing effects of velocity on corrosion. This is illustrated in Fig. 19.5 in which the specimen and Pt electrode are of the same size and are placed parallel to one another in the holder. When required potentials are measured by inserting a capillary through the hole in the Pt it is then removed to avoid shielding effect. 

Obuekwe CO, Westlake DW, Plambeck JA, Cook FD (1981) Corrosion of mild steeUn cultures of ferric iron reducing bacterium isolated from crude oil, polarisation characteristics. CORROSION 37(8) 461 7... 

Assuming that each of the electron transfer stages follows the regularities of a delayed discharge, determine a theoretical value of a Tafel s slope of polarisation characteristics depending on a number of a delayed stage. 

---