Research addresses high-voltage interference, AC corrosion risk for cathodically protected pipelines

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Inductive interference due to fault currents in the high-voltage line. Provided the duration of fault current is smaller than 0.5 sec, the threshold is U_ac = 1,000 v.²

This article presents two examples of inductively interfered with pipelines and describes the measures that had been taken to reduce U_ac. Further presented are some results regarding AC corrosion followed by considerations for appropriate corrosion-protection measures.

The focus is on interference from operating currents in the high-voltage line. But one example regarding interference from fault currents will be presented to show that calculation methods are well suited to evaluate the expected U_ac, e.g., during the planning phase of a new pipeline.

Evaluation of U_ac

The evaluation of U_ac is generally based on the calculation of the induced longitudinal field strength (E) for a segment of the pipeline.¹ E is:

The value and frequency of the inducing current.

The mutual inductance, related to the unit length, between the conductor(s) of the high-voltage line and the pipeline, i.e., a function of their clearance and of the soil resistivity.

The reduction factor, allowing for the protective effect of adJacent earthed conductors.

For simplified conditions (e.g., parallel routing between high-voltage line and pipeline, uniform coating resistivity of the pipeline, constant soil resistivity, and both ends of the pipeline terminated by characteristic impedance), an analytical solution for U_ac as a function of pipeline length may be evaluated.

Generally, however, these calculations are carried out by use of computers that take into account the relevant parameters of the high-voltage line and the pipeline:¹

Drawings showing the routes of the high-voltage lines and the pipeline together with location of power stations, substations, transformer stations, overhead line towers, and pipeline stations.

• Rated voltage and type of system grounding of the high-voltage system.
• Inducing current of the high-voltage line; maximum working current.
• Form of towers and configuration of conductors.
• Diameter and coating resistivity (r_u) of the pipeline.
• Reduction factor of the high-voltage line and, if necessary, of other reducing conductors.

U_ac as a function of pipeline length (l) is then obtained from the vectorial sum of the induced voltage from each individual pipeline segment considered.

Coupon in above photo on the left from Location 4 (Example 2) appears after 2 years of installation: penetration rate 0.09 mm/year; pH on steel surface = 12; U_ac = 3.9 v (mean level over 1 week); U_on = –2.8 v-3.6 v (due to DC stray current); and U_off = –1.2 v (Fig. 5).

Excavated site in the above photo on the right shows corrosion on steel surface (Example 2): diameter 4 mm; depth 0.7 mm; penetration rate 0.3 mm/year; U_ac = 3.9 v; U_on = –2.2 v-–3.5 v (due to DC stray current); and U_off = –1.2 v (Fig. 6).

For practical reasons it is important to verify that U_{ac,calculated}≈ U_ac,measured, i.e., that a calculation of U_ac appropriately mirrors practical conditions. For this purpose Fig. 1b shows data from calculation (line) and measurement (dots) of U_ac from an inductively interfered pipeline section (Fig. 1a).

A test current (256 amp) had been injected into one conductor of the high-voltage line simulating a fault in the substation. With a slightly different frequency (f = 49.5 hz), it was possible to eliminate disturbing 50-hz interference. U_ac had been measured at each CP test post indicated with black dots in Fig. 1a. The results of both calculation and measurement as shown in Fig. 1b had been extrapolated to the real expected fault current, i.e., 12.2 kamp. (See also Reference 3.)

The correspondence of calculated and measured data is reasonable. In the following, this result will be used as a basis for the interpretation of calculated pipeline-to-ground potentials (U_ac).

First example

Fig. 2 shows the results of calculated U_ac for a pipeline (500 mm; 20 in.) constructed in 1999. The pipeline is inductively interfered by four 50-hz high-voltage lines (110 kv/220 kv); the level of operating currents is between 520 and 960 amp. The expected coating resistivity was set to 1 MΩ sq m which is typical for polyethylene-coated pipelines. Another existing section of the pipeline was coated with 100 kV sq m (Fig. 2).

Installing two ground electrodes U_ac is clearly below the safety threshold. In order not to interfere with the CP system, ground electrodes are electrically connected to the pipeline by antiparallel and unsymmetrical diodes.⁴ Cathodic protection was operated at about U_on = –2.7 v (with respect to a Cu/CuSO₄-electrode).

In order to indicate if any AC-corrosion risk exists at coating faults, coupons had been installed—and electrically connected to the pipeline—at five locations (Fig. 2), which had been selected according to the maximum level of U_ac and to the presence of low soil resistivity in swampy and humid areas. After an operating time of somewhat more than 2 years, the coupons were excavated at three locations. Table 1 gives the results.

The measured level of U_ac is less than the calculated one, indicating a lower level of actual operating currents in the high-voltage lines compared to the maximum one which had been used for the calculation.

The values for U_on and U_off indicate effective cathodic protection that is verified by pH>10 of any surface layers and/or corrosion product which had been found on the coupons.

The spread resistance varies among different test locations and also among the three coupons that were installed at one test site (see also Reference 6). It is interesting to note that spread resistance of coupons is relatively high at Location 4 and consequently J_ac is low. The relatively high penetration rate, which had been found on coupons Nos. 777 and 778 may be due to a temporarily low spread resistance causing higher AC current densities at constant U_ac.

From these results it has been concluded that an AC-corrosion risk exists for the new pipeline section and IFO measurements (IFO = intensive coating fault location4) had been carried out to identify small coating faults on the pipeline. Fig 3 shows an excavated coating fault where the steel surface is slightly corroded, which had been located at km 64.045 (Fig. 2). This result confirms the conclusion from coupon measurements.

Second example

Fig. 4 shows the results of calculated U_ac for a pipeline, DN 1,000, constructed in 1999. The pipeline is crossing or running parallel to eight 50-hz high-voltage lines (110 kv/380 kv), each contributing to U_ac which is calculated as the geometrical sum of all individual interference voltages. The expected coating resistivity was set to 1 MΩ sq m.

Installing four ground electrodes U_ac is clearly below the safety threshold. In order not to interfere with the CP system, ground electrodes are electrically connected to the pipeline by antiparallel and unsymmetrical diodes.⁴ Cathodic protection was operated at on-potentials varying between –1.6 v and –4.0 v (with respect to a Cu/CuSO₄-electrode) due to interference from DC-operated electrified railways.

In order to indicate if any AC-corrosion risk exists at coating faults, coupons had been installed—and electrically connected to the pipeline—at four locations (Fig. 4) that had been selected according to the maximum level of U_ac and to the presence of low soil resistivity in swampy and humid areas. After 2 years the coupons were excavated. Fig. 5 shows coupon No. 587 from Location 4 (Fig. 4). From these results, it had been concluded that an AC-corrosion risk exists for the new pipeline and IFO-measurements had been carried out to identify small coating faults.

Fig. 6 shows an excavated coating fault where the steel surface is slightly corroded, which had been located about 2 km apart from coupon No. 587. This result confirms the conclusion from coupon measurements.

Corrosion protection

The need for additional corrosion-protection measures is clearly indicated by both examples. As a guideline one may use criteria mentioned in Reference 7 to extract those parameters that have to be adjusted in order to achieve corrosion protection. The criteria are:

Criterion A:

J_ac > 30 amp/sq m → corrosion risk high

J_ac < 30 amp/sq m → corrosion risk low

Following this criterion, U_ac should be reduced, for example, by installing additional ground electrodes. The effectiveness of this measure has been verified by several investigations^{5 7 8} and will not be discussed here.

Criterion B:

U_IR-free < –0.85 v at any moment or U_IR-free < –0.950 MΩ in anaerobic soil containing sulfate-reducing bacteria → corrosion risk low

Criterion C:

J_ac/J_dc > 10 → corrosion risk high

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where:

U_IR-free = the IR-free potential of the coupon/coating fault.

In case of polyethylene-coated pipelines with generally small coating faults, U_IR-free.< U_off may be assumed, thus neglecting a possible difference, e.g., 50-100 MΩ, between both values.

Table 2 shows data that have been evaluated from the figures presented in Examples 1 and 2.

Following the results of Table 2 one may consider adjusting the level of the CP systems (of both pipelines) to a slightly more negative level in order to avoid AC corrosion. It has been demonstrated in References 5 and 11, however, that increasing pH of that medium, which is in contact with the steel surface and which generally accompanies an increasing CP level, may accelerate AC corrosion.

In order to clarify this contradiction, laboratory experiments have been carried out. The results are as follows.

Corrosion rate

Table 3 describes the conditions for the first series of laboratory investigations. Coupons with a bare steel surface of 0.5 sq cm had been exposed to U_ac = 0, 2, 6, and 10 v while two different CP potentials U_on = -1.5 v (Series A) and U_on = -3 v (Series B) had been applied. Soil had been taken from the right-of-way of a high-voltage interfered pipeline and the test period was 150 days.

Fig. 7 shows the electric circuit.

Fig. 8 shows plots of J_dc and J_ac vs. time as measured for coupons. The sharp increase of current densities after 35 days (Fig. 7a) and 21 days (Fig. 7b) results from adjusting the "groundwater level" in the container to where coupons were installed.

The shapes of the curves in Figs. 7a and 7b do not significantly differ from each other, except regarding J_dc, which is higher for Coupons 9B and 10B (compared to Coupons 11A and 12A) due to U_on = –3 v.

From the results of these measurements, average values (J_dc,av and J_ac,av) have been evaluated, neglecting those data which had been obtained before adding "groundwater" to the container. Table 4 presents the results of all measurements of Series 1.

In Table 4 w_l,max is the penetration rate as calculated from the deepest corrosion pit. Average values for U_on and U_off had been calculated as described before for J_ac. With reasonable accuracy Equation 1 may be verified.

Fig. 9 compares the results from the measurements of corrosion pit depth for coupons of Group A (U_on = –1.5 v) with Group B (U_on = –3 v). It should be noted that maximum corrosion pit depth is lower for coupons operated at U_on = –1.5 v. Table 5 shows the test conditions for Series 2. For these investigations, coupons were initially (i.e. t = 0) polarized (galvanostatically) with J_dc between 0.3 and 100 amp/sq m followed (after 24 hr) by superposing J_ac = 70 amp/sq m after 24 hr. From now on, U_on and U_ac were kept constant (see Table 5).

It should be noted that U_ac, needed to achieve J_ac =70 amp/sq m, increases with decreasing J_dc, indicating an increasing spread resistance of coupons.

Fig. 10a and 10b show J_dc and J_ac vs. time for one coupon out of each group and may be considered representative. Generally, J_ac initially increases; after 50 days, however, there is a tendency to decreasing current densities.

Comparing the curves should note the different levels of U_ac and U_on. After the test period of 100 days, coupons were removed and pH was measured on the steel surface followed by measuring the maximum corrosion pit depth from which the penetration rate (wl,max) was calculated.

Table 6 shows the results from Series 2. It should be noted that Equation 1 is fulfilled within reasonable accuracy. Penetration rates are relatively low, even in case of relatively high J_ac (Coupons 14 and 16). Fig. 11 shows penetration rates for all coupons (J_ac,av > 30 amp/sq m) of Series 1 and 2. In order to be able to compare the data, penetration rates refer to J_ac = 100 amp/sq m (which shall not imply a linear relation between w_l,max and J_ac).

Fig. 11 shows only one coupon from Series 1, which was operated at U_ac = 2 v; corrosion rate is low, however. Enhanced corrosion rates are found for 1 < U_ac/(U_off–U_on) < 30 auf. The soil used as the medium in Series 1 may favor the corrosion rate compared to the "sand + artificial soil" solution, that was used in Series 2.

Results

The results from corrosion-rate measurements on coupons that had been exposed to constant AC voltage U_ac at different cathodic protection on-potentials U_on point out that the level of U_on is an important factor that influences the corrosion rate.

It is assumed that alkalinity, which increases if U_on is adjusted to more negative levels, plays an important role.^{5 11}

In contradiction to Criterion c, however, from which a continuously decreasing corrosion rate may be concluded if CP current density is increased whereby U_on will be more negative, the results presented here suggest:

Relatively low corrosion rate if U_ac/(U_off–U_on) > 30. These conditions qualitatively reflect a "weak" adjustment of the CP system and care has to be taken to fulfill the requirements of the relevant CP standard (Reference 2, for example). It should be noted that the "weak" adjustment of U_on will most probably provide insufficient protection against AC corrosion, but corrosion protection shall be achieved by reducing U_ac.

Data suggest, however, that a careful adjustment of U_on is helpful for achieving low AC corrosion rates. This conclusion is supported by results from measurements using electric-resistance probes described in Reference 12.

Relatively low corrosion rate if U_ac/(U_off–U_on) < 1 which reflects conditions in which U_on nearly or fully compensates the positive (anodic) part of → This technique has been described elsewhere.¹³

The author
Hanns-Georg Schöneich has worked for 18 years at Ruhrgas AG, Essen, Germany, in the field of corrosion protection, coatings, high-voltage interference of pipelines. He holds a master's in physics and a PhD in physical chemistry from the University of Münster.