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Measurement of E corr (corrosion potential) The experimental arrangement for the measurement of corrosion potential is shown in Figure 3.The potential of the metal electrode (working electrode) is measured with respect to a standard Calomel electrode, which is non-polarizable. The reference electrode is kept in a separate container and it is connected electrically with the working electrode placed in a container in contact with the electrolyte via a salt bridge. A high impedance voltmeter is connected between the working electrode and the reference electrode. The negative terminal of the voltmeter is connected to the working electrode and the positive to the reference electrode. The open circuit potential in the freely corroding state is shown by the voltmeter. The corrosion potential is also referred to as the open circuit potential as the metal surface corrodes freely. It is called mixed corrosion potential as it represents the compromise potential of the anodes and the cathodes. At the rest potential no drive force is applied.

Figure 2 an experimental arrangement for making corrosion potential measurement Measurement of i corr (corrosion rate) Arrangements for polarization measurements are shown in figure 2,

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Figure 3: The polarization circuit

Consider a freely corroding metal -+ +® zeMM z in an acid. The anodic partial process represented by intersects the cathodic partial process at Ecorr, the corrosion potential. The current corresponding to Ecorr is icorr, the current between the local anodes and cathodes cannot be obviously measured by conventional means, such as by placing an ammeter. At Ecorr (freely corroding potential), the current is icorr and the rate of forward process (if) is equal to the rate of reverse process if = ir what can, therefore, be done to measure the current? An experiment can be designed to measure the current as well as the potential. Impress a potential E1 in the noble direction. Actual (theoretical) and measured (experimental) curves are shown in Figure 4. The corrosion potential (Ecorr) at the intersection of the anodic and cathodic curves is shown in the actual diagram this value is measured experimentally and plotted in the measured diagram. The potential lines E1, E2, and E3 intersect the anodic curves at i1, i2, and i3. As the overvoltage is shifted to E3, the current becomes completely anodic, as shown by i3. These three values are plotted in the measured diagram. On connecting the three points, a measured anodic polarization curve is obtained. In a similar manner but in an opposite direction, a cathodic measured polarization curve is obtained on increasing the potential from Ecorr to E1,E2, and E3 in the negative direction (active potential direction), points i1c, i2c, corresponding to cathodic currents similarly obtained in the measured polarization curve. The points are connected and a measured cathodic

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polarization curve is obtained. In linear portions of the measured anodic and cathodic polarization curves are extrapolated to Ecorr the point of intersection yields, icorr ,the corrosion current obtained from the measured diagram corresponds accurately with the icorr in the actual diagram.

Figure 4 actual and measured polarization curves

In the previous discussion, it was shown that a net anodic current is generated when the potential is raised from E1 to E3 in the noble direction. As the potential is raised from E1 to E3, an excess of electrons is being generated because of oxidation. In anodic polarization, the auxiliary electrode is the cathode and the working electrode is the anode, whereas in cathodic polarization it is the reverse. With this arrangement, the metal can be anodically and cathodically polarized and the net current can be measured. Three -Electrode cell The electrochemical corrosion behavior of metal and alloys can be studied by generation polarization curves , which provide several

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electrochemical regions of diverse physical meaning .A custom design device is shown in figure 5

Figure 5: schematic polarization device

§ Working electrode WE is name given to the electrode being investigated , we use the term “working” rather than “anode” because we are not limited to investigations of anodic behavior alone; cathodic behavior can also be examined

§ Counter ( Auxiliary) Electrode AE :this electrode is present specifically to carry the current created in the circuit by investigation , it is not required for measurements of potential , usually rod of carbon is used ,or any material that will not introduce contaminating ions into electrolyte , platinum or gold can also be used

§ Reference Electrode RE, is present to provide a very stable datum against which the potential of working electrode can be measured.

§ Luggin capillary, It is a probe or tube filled with an electrolyte to provide an ionic conductive path through the soluble ionic salt (KCl). The Luggin capillary and salt bridge connecting the cell and

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the reference electrode (RE) do not carry the polarizing current, and it serves the purpose of reducing the ohmic resistance gradient through the electrolyte between the WE and AE. In fact, some of the ohmic potential (E = IR) is included in the polarized potential

§ Potentiostat: A manually operated potentiostat is a stepwise instrument and develop E vs. log (i). A commercially programmable potentiostat, together with an electrometer, logarithmic converter, and data acquisition device is an automated instrument that provides variability of continuous sweep (scan) over a desired potential range, for obtaining an entire polarization curve, including the cathodic and anodic regions.

§ Three –electrode cell is the standard laboratory apparatus for the quantitative investigation of corrosion properties of materials shown in figure 6 .It contain WE, AE and RE while the external circuit can be varied considerably ,the essential components are a current – measuring device , a potential measuring device and a source of potential .The current measuring device should be capable of reading micro amps at least .Source of potential (emf.) must drive working electrode to produce the desired cell reactions

Practical working electrode can be constructed in a variety ways, A simple method is to mount a small specimen in cold-setting resin figure 7 , electrical connection must be made to the specimen , and this can be done with solder or spot weld on reverse side before mounting .After mounting specimen are often ground and polished , as far metallographic examination .if this technique is used , the surface will be activated , in other words , passive films may have been either removed entirely or just changed from the as-received condition. This should always be borne in mind. Obviously if the original passive film is part of corrosion investigation, no pretreatment should be used. In fact this is the biggest single reason for discrepancies between lab and field data.

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Figure 6: commercially available electrochemical cell

Figure 7: simple laboratory mount in cold setting resin

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Although these specimen configurations are adequate for most purpose accurate work may require a more carefully designed method of mounting ,the design which is commercially available are illustrated in figure 8

Figure 8: disc specimen