Materials Performance

OCT 2017

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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33 NACE INTERNATIONAL: VOL. 56, NO. 10 MATERIALS PERFORMANCE OCTOBER 2017 wet and gel electrodes, a composi- t ion di fference ex ists ac ross t he membrane separating the element's electrolyte from the environment. Th is difference acts as a driving force for diffusion, which is outward for the active species and inward for contaminants. As diffusion changes the composition of the electrolyte, the reference potential changes. In a gel electrode, this change is perma- nent because the electrolyte cannot be renewed. Methods used to extend t he life of a permanent reference electrode, therefore, must focus on extending the time before the effects of diffusion reach the element-elec- trolyte interface where the reference potential is established. Three broad factors affect the ser- vice life of a permanent reference electrode: the environment in which it operates; the chemical make-up of the gel; and the physical design of the electrode itself. Environmental effects include both temperature and concentration difference across the membrane. Increasing eit her one will reduce service life by increasing diffusion rate. Permanent electrodes use a saturated salt solution as the basis for t he electrolyte gel. The chemical make-up of the gel can be modified to provide a reserve of salt to make up for that which is lost by diffusion. Both the amount and dis- tribution of this reserve affects elec- trode life. There are limits, however, to the amount of excess that can be added to the gel before its properties are affected. Two desig n details of an elec- trode have a significant effect on ser- vice life: membrane type and elec- trolyte path length. The nature of the membrane controls the quantity of salt that diffuses through it. Re- ducing this quantity can extend ser- vice life, but it also will increase in- ternal cell resistance. Clearly, there are limits to the extent to which this technique can be used for life exten- sion because a high internal resis- tance will cause measurement er- rors. Electrolyte path length is also importa nt because diff usion is a time-dependent phenomenon. In- creasing the internal electro- lyte path length, that is, the distance from the element to the membrane, increases the time for the effects of diffu- sion to reach the element-gel interface where the reference potential is established. This factor is particularly signifi- cant when using gels with low ionic mobility. When the electrolyte next to the ele- ment becomes diluted, the reference potential will shift, ending the useful life of the electrode. Another very important design criterion for a permanent reference electrode is that it be en- g i neered spec i f ica l ly for it s i n- tended service. There are a wide variety of applications for perma- nent reference electrodes. For each application, it is necessary to con- sider such factors as size limita- tions, ambient temperature range, operating pressure, and installation procedures. If these requirements are ig nored, ser vice life will be drastically shortened. 4 Conclusions • Accurate readings with portable references require that the ambi- ent temperature be recorded and a temperature correction be applied. • Light striking the element of a portable Cu/CuSO 4 reference through the clear-view window can induce significant photo- errors. This window should be covered with dark tape. • Portable references should be cleaned and the electrolyte re- placed on a regular basis for them to maintain accuracy. • Dry electrodes should be used only in electrolytes known to have a consistent composition and that contain nothing that can adversely react with the element metal. • Permanently installed references are usually not affected by photo- errors, and their ambient tempera- ture range is much narrower (not including concrete bridge deck applications). For this reason, they Factors Affecting the Accuracy of Reference Electrodes are more accurate than a portable reference. References 1. J. Morgan, Cathodic Protection, 2nd ed. (Houston, TX: NACE International, 1987). 2. D.G. Ives, G.J. Janz, Reference Electrodes, Theory and Practice (New York, NY: Academic Press, 1961). 3. M.H. Peterson, R.E. Groover, MP 11 (1972): p. 19. 4. F.J. Ansuini, J.R. Dimond, "Long-Term Stability Testing of Reference Electrodes for Reinforced Con- crete," CORROSION/94 paper no. 295 (Houston, TX: NACE, 1994). From the Author—Today's Practice Thirty years ago, reference electrodes were blamed when the potential measurements weren't what were expected. A common impres- sion was that reference cells were not reliable. In truth, they follow well-established laws of nature. So, if they seemed to be inaccurate, there was a good reason why. While many corrosion engi- neers knew how to make a reference electrode, few understood the factors affecting their accu- racy. Since our company's primary business is manufacturing reference electrodes, we be- lieved that it would be in our interest to deter- mine and quantify these factors. The results were presented as a refereed paper at a NACE conference, and a shortened version was pub- lished in MP to reach the full NACE member- ship. Since this article was published in 1994, the industry has become more technically sophisti- cated. Procedures have been developed for test- ing projected longevity of permanently installed electrodes and accuracy of portable electrodes. Manufacturers continue to improve the quality of reference electrodes while users are gathering additional useful data from electrical resistance probes, concentric coupons, and electrolyte re- sistivity sensors. All of this information provides a better understanding of what is actually happen- ing at the structure. Today, when measured po- tentials are not what is expected, don't just blame the reference electrode. The engineer or techni- cian should apply their knowledge and training to the dynamics at each site and determine what is actually happening. As an industry, we have ad- vanced due to this more analytical approach. —Frank J. Ansuini FIGURE 5 Effect of chloride contamination on the reference potential of Cu/CuSO 4 reference electrodes.

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