Potential survey

The cathodic protection of pipelines is best monitored by an intensive measurement technique according to Section 3.7, by an off potential survey eveiy 3 years and by remote monitoring of pipe/soil potentials. After installation of parallel pipelines, it can be ascertained by intensive measurements whether new damage of the pipe coating has occurred. These measurements provide evidence of possible external actions that can cause mechanical damage. 

The principle reference electrodes used in the onshore and offshore industries have remained unchanged. The CU/CUSO4 cell is used almost exclusively onshore in the form based on the plastic tube and porous wooden plug illustrated in Fig. 10.40, although certain potential survey instruments have... 

For potential surveys on offshore platforms it is necessary to locate numerous reference electrodes at all levels on the structure. The hard-wire connections from these electrodes together with, for example, similar connections from specially monitored sacrificial anodes are best terminated and displayed at the surface on mimic display monitoring panels. 

Rog, J., Computerised Close Order Potential Surveys for Land Based Applications. Paper No. HC-42, Harco Corp. (1980)... 

Failure to realize human potential (surveys of employee satis clion)... 

Energoinvest Sarajevo, Potential survey procedure, 19051-S-395-10-MC-0028-00, Sarajevo, 2006. 

To confirm the cause of abnormal pipe-to-soil potential readings, the effect of interrupting the suspected current source on potential survey along the protected pipeline or structure should be assessed. If there is no effect on the potential readings of the interfered structure, the search for the current source must continue until the actual interfering current source is located. 

The effectiveness of cathodic protection can be accomphshed by monitoring the pipeline potential using the close interval potential survey (CIPS) method or by using direct current voltage gradient (DCVG) or IR coupon techniques. Also, physical and electrochemical methods can be used to estimate the corrosion rates of a cathodicaUy protected system. 

Measuring the pipehne potential with reference to a nonpolarized reference electrode (e.g., CU/CUSO4) is one of the most commordy used methods for potential survey (PS) of the pipeline. This method is based on the potential criteria Usted in the previous section. Presently, battery operated, inexpensive, light (50 MO or higher) internal resistance voltmeters with a resolution of 1 mV are widely available. Microprocessor digital recorders caUed data loggers are more frequendy used for monitoring pipehne potential. 

For the cathodic protection of a steel pipeline, the following information has been obtained from the potential survey shown in Table El5.1. 

Table E15.1 Potential Survey Data for the Cathodic Protection of a Steel Pipeline Parameter Drain Point 1...

For carbonated concrete one method is first to do a potential survey with minimum wetting (if stable potentials can be established). Then wet the surface thoroughly and leave it for at least two hours or until potentials are stable. Resurvey once potentials have stabilized and then look for the most anodic (negative potential) areas. These are most probably active if a potential difference of 100 mV or more exists over a space of 1.0 m or less. A physical investigation is essential to see if there is reasonable correlation between corrosion and anodic areas. 

Figure 4.7 Potential survey as per delamination and visual survey Figure 4.3. Courtesy of the Highway Agency.

Figure 4.8 Reference Electrode Potential survey of a Bridge Pier, also recording cover meter chloride concentrations at three different depths. Courtesy Highways Agency AmeyMouchel.

Detailed methodologies for undertaking reference electrode potential surveys and their interpretation can be found in Concrete Society (2004), Chess and Gronvold (1996) and ASTM C876-91 (1999). 

As the technique is slower than taking reference electrode measurements it is important to take measurements at the most significant locations on the structure, for example, by following up a potential survey with strategic corrosion rate measurements. Rate measurements should be taken at the positions of the highest and lowest potentials and at the steepest potential gradients. 

Epoxy coated rebars present particular problems to determining the corrosion condition of the Steel. In the first place the bars are electrically isolated from the concrete except at areas of damage. The size and locations of the areas of damage are obviously unknown. Attempts to carry out reference electrode potential surveys and linear polarization measurements have therefore been unable to come up with definitive criteria for corroding and non-corroding areas. The other problem is that the bars are isolated from each other, therefore a connection must be made to each bar measured to be sure that there is electrical contact. 

One of the more confusing facts of cathodic protection is that when we carry out a reference electrode potential survey of a reinforced concrete structure (Section 4.8) the most negative areas are those that are at highest risk of corrosion while the areas with a positive potential are at the lowest risk of corrosion, that is, cathodic (Section 4.8.3). However, to achieve cathodic protection, we must depress the potential of the steel. The reasons for this are explained later. 

One of the more confusing facts of cathodic protection is that when we carry out a half cell potential survey of a reinforced concrete structure, the mo.st negative areas are those that are corroding most the... 

New galvanized steel footings, when buried, will exhibit a potential between -0.85 and -1.1 V with reference to CU/CUSO4. After the galvanizing is lost, the newly exposed bare steel surface will have potential between -0.5 and -0.7 V. Corroded steel with a heavy scale will have a potential between -0.3 and -0.5 V. In addition to a potential survey, an electrochemical polarization technique can be used to determine the rate of galvanic corrosion [3]. The device operates by passing a small current between the tower footing... 

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