Fundamentals and Practice of Electrical Measurements
3.3 Potential Measurement
3.3.3 Application of Protection Criteria
In contrast to direct voltage, alternating voltage can be measured using a ground- ing rod as reference electrode. The grounding resistance of the rod is considerably lower than that of the reference electrodes in Table 3-1 but can still be too high for measurements with soft iron, moving iron or electrodynamic measuring instru- ments. It is therefore also recommended that amplified voltmeters or amplified recorders be used which have high internal resistances, great accuracy and a linear scale. Attention to frequency and shape of curve is important in the technique of ac measurements. In general, measuring instruments are gauged for effective values for 50 Hz and sine curves. They can therefore give false readings for diverse fre- quencies and waveforms (phase control). Measurement errors due to different wave- forms can be recognized by the fact that they give different results in different measurement regions.
3.3.3.1 Pragmatic Protection Criteria for Nonalloyed Ferrous Materials In this section the pragmatic protection criteria of NACE [23] given in Table 3-3 are commented on in light of present knowledge.
Criterion No. 2: Uon-UR<-0.3V (3-31) This criterion is derived from the fact that the free corrosion potential in soil is generally f/Cu-cuso4 = -0-55 V. Ohmic voltage drop and protective surface films are not taken into consideration. According to the information in Chapter 4, a maxi- mum corrosion rate for uniform corrosion in soil of 0.1 mm a"1 can be assumed.
This corresponds to a current density of 0.1 A nr2. In Fig. 2-9, the corrosion cur- rent density for steel without surface film changes by a factor of 10 with a reduc- tion in potential of about 70 mV. To reduce it to 1 /um ar\ 0.14 V would be necessary.
The same level would be available for an ohmic voltage drop. With surfaces cov- ered with films, corrosion at the rest potential and the potential dependence of corrosion in comparison with UR act contrary to each other so that qualitatively the situation remains the same. More relevant is
Criterion No. 3: UoS- UR < -0.1 V (3-32) Here UR is measured after switching off of the protection current and after step polarization. The potential difference corresponds to an 7/?-free potential decay.
From the slope in Fig. 2-9, a reduction in the corrosion rate of 100 to 4 fj.m a"1 results.
Criterion No. 4: U < potential at the bend in the U (log I) curve (3-33) This criterion is understood from the shape of the I(U) curves described in Fig. 2-4 and Eq. (2-35) assuming two cathodic partial reactions according to Eqs. (2-17) and (2-19) [27]. For oxygen corrosion, J0 > Gc so that in the relevant potential range for this reaction there is a limiting current, which also corresponds to the corrosion rate at the rest potential and to the protection currents. For H2 evolution, J0< Gc. This reaction only occurs at more negative potentials than the protection potential and follows a Tafel slope, which on a logarithmic plot of the I(U) curve shows a marked deviation at the transition from O2 diffusion current to H2 evolu- tion [see Fig. 2-2la and the explanation for Uh in Section 24.4 relating to Eq. (24-68b)]. Polarization in this region of the curve shows that the protection current is greater than the O2 diffusion current and thus according to Eq. (2-40), cathodic protection is occurring.
Criterion No. 5: U£<0 (3-34)
Table 3-3 Practical criteria for cathodic protection of plain carbon and low- alloy steels in soil
No. Basis Explanation Application 1 U < Us IR voltage drop "should General
be considered"
2 At/ > 300 mV 300-mV negative change Pipeline without from rest potential on connections to foreign switching on protection cathodic structures current
3 A f / > 1 0 0 m V Measurement of Uncoated pipelines depolarization on
switching off protection current
4 U = /(log /) The protection current is Well casings determined from the bend
in the current density vs.
potential curve by a drainage test
5 A U < 0 at All voltage drops Pipelines with grounds the pipe perpendicular to the
pipeline must be negative to the pipeline
6 U < U - 0.3 V Cancelling the cell Mixed installationon i- ° (near foreign voltage with foreign cathodic cathode) structures (steel-
reinforced concrete structures)
7 UM < Us The potential test probe If the off potential (provided with a near measurement is not reference electrode) is possible, contact with connnected to the foreign cathodic pipeline, and the structures (steel- connection is interrupted reinforced concrete for measurement structures)
This criterion indicates that cathodic current is entering the pipeline and that there is no more cell activity present (see Figs. 3-24 and 3-25 as well as the explanation of Fig. 2-7). The criterion for cathodic protection of pipelines with connected gal- vanized grounds against high-voltage interference is also comparable with this criterion [22]. In the connection between ground and pipeline, the current must flow to the pipeline.
In DIN 30676 [22] there is a further criterion (No. 6: Uon <US- 0.3 V) for protection of intermeshed objects with foreign cathodic structures. The on poten- tial should be measured in the vicinity of the foreign cathodic structure. Here it is assumed that in spite of a high IR term, the potential of the foreign cathodic struc- ture is so negative that cell formation is not expected to occur and the object also experiences cathodic protection. Verification is only possible with Criterion No. 7, i.e., with potential test probes.
3.3.3.2 Potential Measurement with Potential Test Probes
This measuring technique is applied when there are relatively high IR values due to cell currents with intermeshed objects or contact with steel-concrete struc- tures, as well as the influence of stray currents, which cannot be switched off. The principle corresponds to the information in Fig. 2-3 on the use of measuring probes.
Test coupons of steel of a specified size are buried near the pipeline and con- nected by cable at the test point with the cathodically protected pipeline. They simulate artificial defects in the coating. The protection current taken from the test coupon can be measured via the cable connection and the true potential deter- mined from a reference electrode in front of the test coupon by momentarily inter- rupting the cable connection [28]. Ohmic potential drops between the reference electrode and the test coupon are obtained from a measuring test probe that has a built-in reference electrode on the back of it to measure the 7/?-free potential directly [see Eq. (2-34) with s —» 0] without having to switch off the protection current or interrupt the cable connection to the pipeline (see Fig. 3-11). The pipe/
soil potential at the plate is measured by means of a permanent reference electrode that is installed in a plastic tube behind the plate filled with electrolyte. Contact is made via a diaphragm in the plate [24]. The steel plate and the diaphragm that is led through the steel plate must be carefully insulated since otherwise the potential of the test coupon unaffected by the protection current inside the plastic tube will be measured instead of that of the external steel probe. A comparison of values from the potential test probe with the external reference electrode, test probes with built-in reference electrodes and the off potentials of the pipeline shows discrepan- cies of less than 20 mV. Since the potential becomes more negative with defects of decreasing size [see Eq. (3-21)], all coating defects in areas of similar resistivity that are smaller than the test probe must show more negative potentials (Crite-
rion 7). Potential test probes are only efficient if they are located in the same soil as the pipeline.