Reference cathodic protection

The basic standard for cathodic protection was laid down for the first time in DIN 30676 to which all the application areas of the different branches of protection can be referred. In this the most important point is the technique for accurately measuring the object/soil potential [58]. The usual off-potential measurement method for underground installations has been slowly implemented and enforced in Europe since the 1960s [59]. 

The terms protection current and protection current densities refer to any values of total cathodic currents that meet the criterion in Eq. (2-40). However, in the field, and for designing cathodic protection stations, another term is of interest, the protection current requirement. This term is concerned with the lowest value of the protection current that fulfills the criteria in Eqs. (2-39) or (2-40). Since with an extended object having a surface S the polarization varies locally, only the current density for the region with the most positive potential has the value J. In other regions 17. 1 > 7. . For this reason, the protection current requirement 4 is given by ... 

Practical measurements providing data on corrosion risk or cathodic protection are predominantly electrical in nature. In principle they concern the determination of the three principal parameters of electrical technology voltage, current, and resistance. Also the measurement of the potential of metals in soil or in electrolytes is a high-resistance measurement of the voltage between the object and reference electrode and thus does not draw any current (see Table 3-1). 

In the cathodic protection of storage tanks, potentials should be measured in at least three places, i.e., at each end and at the top of the cover [16]. Widely different polarized areas arise due to the small distance which is normally the case between the impressed current anodes and the tank. Since such tanks are often buried under asphalt, it is recommended that permanent reference electrodes or fixed measuring points (plastic tubes under valve boxes) be installed. These should be located in areas not easily accessible to the cathodic protection current, for example between two tanks or between the tank wall and foundations. Since storage tanks usually have several anodes located near the tank, equalizing currents can flow between the differently loaded anodes on switching off the protection system and thus falsify the potential measurement. In such cases the anodes should be separated. 

Even with the superposition of the ac with a cathodic protection current, a large part of the anodic half wave persists for anodic corrosion. This process cannot be detected by the normal method (Section 3.3.2.1) of measuring the pipe/soil potential. The IR-free measurable voltage between an external probe and the reference electrode can be used as evidence of more positive potentials than the protection potential during the anodic phase. Investigations have shown, however, that the corrosion danger is considerably reduced, since only about 0.1 to 0.2% contributes to corrosion. 

Since cathodic protection of concrete structures in the United States has been very much advanced, protection criteria have been developed [46]. They correspond to the pragmatic criteria Nos. 3 and 4 in Table 3-3 (see Section 3.3.3.1). It is assumed that the protective effect is adequate if, upon switching off the protection current, the potential becomes more than 0.1 V more positive within 4 hours. The measurements are carried out in various parts of the protected object with built-in Ag-AgCl reference electrodes or with any electrodes on the external surface. 

Protection current devices with potential control are described in Section 8.6 (see Figs. 8.5 and 8.6) information on potentiostatic internal protection is given in Section 21.4.2.1. In these installations the reference electrode is sited in the most unfavorable location in the protected object. If the protection criterion according to Eq. (2-39) is reached there, it can be assumed that the remainder of the surface of the object to be protected is cathodically protected. 

As an example. Fig. 20-7 shows potential and protection currents of two parallel-connected 750-liter tanks as a function of service life. The protection equipment consists of a potential-controlled protection current rectifier, a 0.4-m long impressed current anode built into the manhole cover, and an Ag-AgCl electrode built into the same manhole [10,11]. A second reference electrode serves to control the tank potential this is attached separately to the opposite wall of the tank. During the whole of the control period, cathodic protection is ensured on the basis of the potential measurement. The sharp decrease in protection current in the first few months is due to the formation of calcareous deposits. 

Stainless steel pipes (buried in the ground) and the interiors of stainless steel heat exchangers have been successfully cathodically protected, but CP is rarely used for materials other than steel. The protection potential usually adopted for steel is —850 mV to the saturated calomel reference electrode. This varies with temperature and the presence of other aggressive species in the environment. 

In recent years it has been regarded as somewhat passe to refer to Sir Humphrey Davy in a text on cathodic protection. However, his role in the application of cathodic protection should not be ignored. In 1824 Davy presented a series of papers to the Royal Society in London in which he described how zinc and iron anodes could be used to prevent the corrosion of copper sheathing on the wooden hulls of British naval vessels. His paper shows a considerable intuitive awareness of what are now accepted as the principles of cathodic protection. Several practical tests were made on vessels in harbour and on sea-going ships, including the effect of various current densities on the level of protection of the copper. Davy also considered the use of an impressed current device based on a battery, but did not consider the method to be practicable. 

It is clear that to ensure adequate protection of a structure under cathodic protection it is necessary to measure its electrode potential. This can only be achieved by using a reference electrode placed in the same environment as the structure and measuring the e.m.f. of the cell so formed. Since the electrode potentials of different types of reference electrode vary, it is clear that the measured e.m.f. will also vary according to the particular reference electrode used. It follows that the potential measured must always be recorded with respect to the reference electrode deployed, which must always be stated. 

Finally, calomel electrodes (and more especially hydrogen electrodes) are not suitable for field measurements because they are not sufficiently robust. The calomel electrodes are however essential for calibrating the field reference electrodes. Saturated KCI calomel electrodes are the most suitable because there is then no doubt about the reference potential of the calibrating electrode. Lack of adequate calibration is a common cause of cathodic protection system mismanagement. 

Fig. 10.9 Diagram illustrating the source of the IR error in potential measurements on a cathodically protected structure. BA is the absolute electrode potential of the structure CD is the absolute electrode potential of the anode and CB is the field gradient in the environment due to cathodic protection current flux. A reference electrode placed at E will produce an IR error of EFin the potential measurement of the structure potential. If placed at G the error will be reduced to GH. At B there would be no error, but the point is too close to the structure to permit insertion of a reference electrode. If the current is interrupted the field immediately becomes as shown by the dotted line, and no IR is included...

An increase in carbonate-ion concentration moves the equilibrium in favour of calcium carbonate deposition. Thus one secondary effect of cathodic protection in seawater is the production of OH , which favours the production of CO, , which in turn promotes the deposition of CaCOj. Cathodically protected surfaces in seawater will often develop an aragonite (CaCOj) film. This film is commonly referred to as a calcareous deposit. 

Table 10.23 must be taken only as a guide and interpreted in the best manner available, preferably using experience in that particular environment or operational requirement. Table 10.23 should be consulted in conjunction with the text and references, specifically those covering the whole range of cathodic protection anodes Consideration must be given to... 

Manually Controlled System A manually controlled system comprises one or more transformer-rectifiers each with its associated control panels which supply the d.c. to the various anodes installed in the water box spaces. Each transformer-rectifier is provided with its own control panel where each anode is provided with a fuse, shunt and variable resistor. These enable the current to each anode to be adjusted as required. Reference cells should be provided in order to monitor the cathodic protection system. In the case of a major power station, one transformer-rectifier and associated control panel should be provided for separate protection of screens, circulating water pumps and for each main condenser and associated equipment. 

Stray currents are produced in the electrolyte during the operation of cathodic-protection systems and part of the protection current may traverse nearby immersed structures which are not being cathodically protected. The resultant corrosion produced on the unprotected structure is referred to as corrosion interaction or corrosion interference. 

Reference electrodes The generally accepted criterion for the effectiveness of a cathodic-protection system is the structure/electrolyte potential (Section 10.1). In order to determine this potential it is necessary to make a contact on the structure itself and a contact with the electrolyte (soil or water). The problem of connection to the structure normally presents no difficulties, but contact with the electrolyte must be made with a reference electrode. (If for example an ordinary steel prol e were used as a reference electrode, then inaccuracies would result for two main reasons first, electrochemical action between the probe and the soil, and second, polarisatibn of the probe owing to current flow through the measuring circuit.)... 

Reference Book On Instruments for Electrolysis Corrosion and Cathodic Protection Testing, American Gas Association, New York (1951)... 

It is apparent that since the electrode potential of a metal/solution interface can only be evaluated from the e.m.f. of a cell, the reference electrode used for that purpose must be specified precisely, e.g. the criterion for the cathodic protection of steel is —0-85 V (vs. Cu/CuSOg, sat.), but this can be expressed as a potential with respect to the standard hydrogen electrode (S.H.E.), i.e. -0-55 V (vs. S.H.E.) or with respect to any other reference electrode. Potentials of reference electrodes are given in Table 21.7. 

Remote Potentials in dealing with a pipeline cathodic-protection system, it is often advantageous to refer all measurements to a half-cell located at a distance from the pipe. Such measurements are referred to as remote potentials. 

Cathodic protection involves connecting a metal to be protected to another metal that is more easily oxidized. The more easily oxidized metal serves as the anode and the metal to be protected is the cathode in an electrochemical cell. The metal that is oxidized is called the sacrificial anode because it is sacrificed to protect another metal. Metals such as zinc and magnesium are often used to protect iron. Cathodic protection is demonstrated in this activity by using two steel nails. The nails are placed on a shallow dish. Using a white or light-colored dish displays the oxidation better [iron (III) oxide, Fe Oj, referred to commonly as rust is more visible on a light-... 

Monitoring of the electrochemical potential of steel reinforcement in concrete is a well established technique for assessing the severity of corrosion and for controlling cathodic protection systems. A reference electrode is the electrochemical device used for measuring these potentials. The reference electrode is either placed on the concrete surface during the measurements or permanently embedded in the concrete in close proximity to the steel. The latter technique enables remote long-term monitoring. 

Most of the reference electrodes embedded in concrete are used for control of cathodic protection (CP) systems. Potential stability is then less important, compared to corrosion state monitoring. Control of CP systems requires only short-term stability, maximum 24 hours. Corrosion rate measurement, like linear polarisation resistance (LPR) measurements, also requires short-term reference electrode stability. However, regardless of application, a reference electrode which is to be permanently embedded in the test solution, e.g. concrete, must have a long life when exposed to this environment. 

Obviously, very stable and long life embeddable reference electrodes are a must for this type of application (long-term corrosion monitoring). However, for cathodic protection control applications the stability requirement is not that important. It is surely desirable for the reference electrode to have a long embedded life . More reports on held experiences with embeddable reference electrodes for concrete would be highly appreciated. 

K. N. Gurusamy and M. P. Geoghegan, The long term performance of embedded reference electrodes for cathodic protection and insitu monitoring of steel in concrete , in Corrosion of Reinforcement in Concrete, edited by C. L. Page, K. W. J. Treadaway and P. B. Bamforth, Elsevier Applied Science, London, UK, 1990, pp 333-347. 

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