Significance and Use 5.1 Electrode potential is the reversible work that is required to transfer a unit of positive charge between the surface in question and a reference electrode through the electrolyte that is in contact with both electrodes. The sign of the electrode potential is determined by the Gibbs Stockholm Convention described in Practice G3. 5.2 The electrode potential of a surface is related to the Gibbs free energy of the oxidation/reduction reactions occurring at the surface in question compared to the Gibbs free energy of the reactions occurring on the reference electrode surface.4 5.3 Electrode potentials are used together with potential-pH (Pourbaix) diagrams to determine the corrosion products that would be in equilibrium with the environment and the electrode surface.5 5.4 Electrode potentials are used in the estimation of corrosion rates by several methods. One example is by means of Tafel line extrapolation, see Practices G3 and G102. Polarization resistance measurements are also determined using electrode potential measurements, see Test Method G59 and Guide G96. 5.5 Corrosion potential measurements are used to determine whether metal surfaces are passive in the environment in question, see Test Method C876. 5.6 Corrosion potential measurements are used in the evaluation of alloys to determine their resistance or susceptibility to various forms of localized corrosion, see Test Methods F746, F2129, G61, and G150. 5.7 Corrosion potentials are used to determine the metallurgical condition of some aluminum alloys, see Test Method G69. Similar measurements have been used with hot dipped galvanized steel to determine their ability to cathodically polarize steel. See Appendix X2. 5.8 Corrosion potentials are used to evaluate aluminum and magnesium alloys as sacrificial anodes for underground and immersion cathodic protection application, see Test Method G97 and NACE TM0190–2012. 5.9 Corrosion potentials are used to evaluate the galvanic performance of alloy pairs for use in seawater and other conductive electrolytes, see Test Method F3044, Guide G71, and Guide G82. 5.10 Electrode potential measurements are used to establish cathodic protection levels to troubleshoot cathodic protection systems and to confirm the performance of these systems in soils, concrete, and natural waters, see NACE TM0497, NACE TM0108, and NACE TM0109. 5.11 Electrode potential measurements are necessary for the determination of hydrogen overvoltage values in testing for hydrogen embrittlement and related issues with hydrogen cracking. See Appendix X3.Scope 1.1 This guide provides guidance on the measurement of electrode potentials in laboratory and field studies both for corrosion potentials and polarized potentials. 1.2 The values stated in SI units are to be regarded as standard. Any other units of measurements included in this standard are present because of their wide usage and acceptance. 1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
