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31 NACE INTERNATIONAL: VOL. 56, NO. 10 MATERIALS PERFORMANCE OCTOBER 2017 monly used wet references include silver/silver chloride and saturated calomel for laboratory use and por- table copper/copper sulfate for field use. Because wet electrodes require periodic electrolyte replenishment, they are not suitable for permanent installation. To overcome this limita- tion, references intended for perma- nent use have a gelling agent added to the electrolyte. Gelled references require no maintenance. However, they do have a fixed lifetime. Like wet electrodes, gelled electrodes are subject to dry-out and contamina- t i o n i f t h e y a r e n o t p r o p e rl y installed. Reference Potential The potential of a reference elec- trode is determined by the element metal and the active species concen- tration in the electrolyte. The form of the electrolyte, wet or gelled, has no effect on the reference potential. Fig- ure 1 shows the potentials of com- monly used reference electrodes. The potentials are shown relative to the saturated hydrogen electrode (SHE), which has been defined as the zero point on the potential scale. Note that there are two positions for the Ag/AgCl elect rode: 1) sat urated, which refers to a saturated potas- sium chloride (KCl) electrolyte; and 2) seawater, which refers to a dry electrode immersed in seawater. The reference potentials of t hese t wo electrodes are about 40 mV apart. The laboratory Ag/AgCl electrode, not shown in Figure 1, uses a filling solution of 4 M KCl, which is just under saturation. Its reference poten- tial is about 7 mV positive to the sat- urated electrode. When reporting corrosion poten- tials, it is important to describe the type of reference electrode used to measure the potentials. While the potent ial of a st r uct ure may not change (Figure 2), the number used to describe that potential will de- pend on the type of electrode used to measure it. The commonly used criterion of –0.85 V for cathodically protected steel only refers to mea- s u r e me nt s m ade w it h c opp er/ copper sulfate electrodes. External Influences Temperature, light, electrolyte concentration, and contamination all affect the potential of a reference electrode. These effects are most likely to affect portable references, which can operate in a variety of env i ron ment s. Per ma nent elec- trodes most often operate where there is no light and usually in a very narrow temperature range. If they have been properly designed and installed, permanent electrodes are less likely to be influenced by changes in electrolyte concentra- tion or contaminants. These points should be considered when a por- table reference is used to calibrate an installed permanent reference: the observed variations may be due to a shift in the potential of the por- table reference. Temperature Effects Temperature has both direct and indirect effects on reference poten- t ial. The direct effect is a linear variation of the reference potential with temperature. The indirect ef- fect is that as temperature increases, the quantity of salt that can be dis- solved in a saturated solution will also increase. This increase in salt concentration will affect potential. Only the linear variation will be seen in a nonsat urated electrode such as a 4 M or seawater Ag/AgCl electrode. Figure 3 shows the effect of temperature on the potential of electrodes in a fully saturated solu- tion. These data include both the direct and indirect effects. The tem- perature coefficient for a saturated Cu/CuSO 4 electrode is about 0.5 mV/°F (0.9 mV/°C). Saturated Ag/ AgCl electrodes have a much lower t e mp e r at u r e c o e f f ic i e nt : – 0.0 7 mV/°F (–0.13 mV/°C). These temperature coefficients are large enough to produce a sig- nificant error in potential measure- ments if they are left uncompen- sated. For example, the potential of a pipeline may be –865 mV when FIGURE 1 Relative potentials of common reference elec- trodes vs the saturated hydrogen electrode. FIGURE 2 Potential of cathodically protected steel as m e a s u r e d b y s e v e r a l c o m m o n r e f e r e n c e electrodes. mea su red w it h a por t able Cu/ CuSO 4 electrode on a 90°F (26°C) day. If the same measurement were made on a 40°F (5°C) day, the tem- perature effect will cause a 25-mV shift in the reference potential so the pipeline would now measure –840 mV. The drift occasionally re- ported to occur on buried perma- nent electrodes is also likely due to temperat ure-caused dr if t of t he portable electrode being used for calibration. When using a portable reference in the field, it is a good practice to record the ambient tem- perature and, if necessary, correct the readings. Light Effects Copper salts and silver salts are both photosensitive, so it should not be surprising that light has an Factors Affecting the Accuracy of Reference Electrodes