Permanent corrosion monitoring systems

A number of the techniques described in Chapter 4 are suitable for embedding in concrete for permanent monitoring. The main requirements are that they are durable when cast in concrete and exposed to the environment, the readings are meaningful and that readings do not drift with time as recalibration is difficult or impossible. 

The simplest probe is the reference electrode. These are designed for exposure in concrete for installation with cathodic protection systems (Section 7.3). 

However, it has been shown that once embedded in concrete they cannot be recalibrated if they drift (Ansuini and Dimond, 1994) and a very large number are required if a useful potential map (Section 4.8.4) is to be produced. Reference electrodes are incorporated into LPR probes (Section 5.2.2) but are rarely used on their own. 

As stated in Section 4.12, corrosion rates can be measured by linear polarization or by galvanic or macrocell techniques. Both have their merits and limitations as discussed in the Sections 5.2.2 and 5.2.3. 

Reference electrode (with cap on) and notice to remove before pouring concrete 

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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. 

In the discussion of cathodic protection monitoring, two important distinct areas can be identified. The first domain lies in monitoring the condition and performance of the CP system hardware. Monitoring of rectifier output, pipe-to-soil potential and current measurements at buried sacrificial anodes, inspection of bonds, fuses, insulators, test posts, and permanent reference electrodes are relevant to this area. The second domain concerns the condition of the pipeline (or buried structure) itself and largely deals with surveys along the length of the pipeline to assess its condition and to identify high corrosion-risk areas. 

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