Paper No. 12907 - Corrosion Service · 2021. 1. 12. · Calgary, AB, Canada T2P 5H1 C. Khattar, P. Eng. TransCanada Pipelines Limited 450 – 1st Street SW, Calgary, AB, Canada T2P

Findings from a Three-year AC Corrosion Field Study

S.M Segall, P. Eng. Corrosion Service Company Limited

9-280 Hillmount Rd. Markham, ON, Canada L6C 3A1

H. Bahgat, P. Eng. Corrosion Service Company Limited

9-280 Hillmount Rd. Markham, ON, Canada L6C 3A1

Simon Chen, P. Eng, PhD

TransCanada Pipelines Limited 450 – 1st Street SW,

Calgary, AB, Canada T2P 5H1

E. Gudino; P. Eng. TransCanada Pipelines Limited

450 – 1st Street SW, Calgary, AB, Canada T2P 5H1

C. Khattar, P. Eng.



C. Lidster



ABSTRACT

A field study was conducted to determine the influence of the AC current density and of the coating holiday size on the rate of AC corrosion.

This field study involved burying steel coupons of three different sizes (i.e., 1 cm2, 6 cm2, and 10 cm2), applying cathodic protection to an industry standard, and varying 60 Hz AC current densities (i.e., 20 A/m2, 50 A/m2, and 100 A/m2) for a three-year period. Four sets of 12 coupons each were installed for statistical relevance. Each set contained three coupons with no AC current applied (i.e., controls); one for each size. A special power supply cabinet was designed to provide uninterrupted DC and AC current, with each coupon being energized by a separate module.

At the end of the test, the coupons were retrieved, cleaned, photographed, and the corrosion rate measured.

This paper covers the results of the study, as well as the various findings from three years of data collection, including the effect of AC currents on the protection level of the coupons in clayish soils and changes in the spread impedance due to calcareous deposits.

Keywords: AC corrosion, AC current density, holiday size, DC current density, polarized OFF potential, AC coupons, AC/DC power supply cabinet.

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Paper No.

12907

©2019 by NACE International.Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing toNACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084.The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

INTRODUCTION

At the beginning of 2013, the AC corrosion was already recognized as a significant threat to pipeline integrity; however, no NACE standard criteria were available. Instead, Prinz general guidelines related to the AC current density (ACCD) were typically used. 1 These rules, based on laboratory tests, introduced a “grey area” for AC current densities between 20 A/m2 and 100 A/m2, where “AC corrosion was not predictable”.

This “grey area” was a concern because the cost of AC mitigation depends primarily on what the target current density is. Under some conditions, trying to mitigate the AC current density to the lower threshold of 20 A/m2 was prohibitively expensive. Some companies used 30 A/m2 as the lower threshold and some organizations used a target of 50 A/m2 based on the statement that “only current densities above 50 A/m2 are serious”.1

Furthermore, selecting the size of the holiday or the size of the coupon has a major impact on the calculated or measured AC current density. The typical size used in the industry was 1 cm2, but subsequent research indicated that maximum corrosion rates occur on a 6.45 cm2 holiday. 2

Subsequently, a field-based AC corrosion study was initiated as an R&D joint project to provide actual field data to document and reinforce future AC corrosion criteria. With the publication of the new European Standard and the progress in developing the new NACE Standard, special attention was given in monitoring both the AC and DC current densities applied to the coupons. 3,4

EXPERIMENTAL PROCEDURE

Forty-eight (48) steel coupons of three different sizes (i.e., 1 cm2, 6 cm2, and 10 cm2) were buried at 1.2 m depth in clay soil and cathodically protected to industry standard. Varying 60 Hz AC current densities (i.e., 20 A/m2, 50 A/m2, and 100A/m2) were applied for a period of three years. There were four sets of 12 coupons for statistical relevance. Each set contained three coupons with no AC current applied (i.e., controls), one for each size.

The coupons were installed on coupon holders to ensure uniform exposure to soil and to closely simulate the geometry of a coating holiday, as shown in Figure 1. The distance between the coupons was maximized to avoid equalization currents between coupons.

The three control coupons of each set that will receive only DC current were mounted on one holder, as shown in Figure 2, since the DC voltage differences were too small to produce equalization currents and subsequently the distances between coupons could be reduced to less than 75 cm.

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Figure 1: 1 cm2 coupon on holder.

Figure 2: Three-Coupon holder.

Monitoring boxes were provided (one for each set of coupons) to allow measuring OFF polarized potentials and AC currents, as shown in Figure 3.

Figure 3: Monitoring box.

Monitoring ports (i.e., vertical NPS2 PVC pipes filled with a bentonite/sand mixture to provide an electrolytic bridge) were provided to measure potentials, in order to minimize any IR drop from other CP sources. Both the AC and DC currents were cyclically interrupted during these measurements.

The power cabinet was designed to provide uninterrupted DC and 60 Hz AC current to 36 coupons and only DC current to 12 “control” coupons. Each coupon was energized by a separate specially designed module.

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A remote monitoring unit (i.e., RMU5) was installed at the electrical panel to immediately send notifications in case of AC power outage.

A partial view of the power supply cabinet is shown in Figure 4.

Figure 4: Power supply cabinet.

The coupons were initially energized in DC mode only and the currents were adjusted to maintain cathodic protection level within the industry recommended limits (i.e., OFF polarized potentials between -850 mVCSE and -1100 mVCSE). The upper limit (i.e., -1100 mVCSE) was imposed to avoid excessive rates of AC corrosion at highly electronegative potentials. With no AC voltage applied, very low DC current densities (i.e., less than 10 mA/m2) were required to polarize the coupons in clay.

Following the application of AC current in June 2015, the OFF potential of the coupons dropped to an average of -638 mVCSE, with two coupons displaying potentials more electropositive than -500 mVCSE, as shown in Figure 5 and Figure 6. The coupons are identified by letter C, followed by coupon size in cm2, followed by the AC current density target in A/m2, followed by the set number (from 1 to 4). For example, C-01-20-01 was a 1 cm2 coupon from the first set, exposed to an AC current density of 20 A/m2.

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Figure 5: Sets 01 and 02 coupon-to-soil OFF potential measurements.

Figure 6: Sets 03 and 04 coupon-to-soil OFF potential measurements.

The DC current density (DCCD) to restore protection had to be increased by more than 5000% to an average of 483.7 mA/m2. Although it was known that rectification of the AC current affects the protection level, the magnitude of this effect was unexpected.

Once the protection level was restored, the intent was to monitor the AC and DC current, as well as the OFF polarized potentials, with a minimum amount of adjustments.

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However, starting in 2016, significant seasonal variations in conjunction with the depolarization effects of the AC current resulted in severe drops in protection level at several coupons. Subsequently, it was decided to intervene and to increase the current in selected cases to maintain the AC corrosion as the determinant factor in coupons’ metal loss.

A second challenge was the increase in coupon impedance during the test, probably due to building of calcareous deposits. The highest impedance was recorded on coupon C-01-20-01 at 274 kΩ in June 2017, with large seasonal variations, as shown in Figure 7. For reference, the calculated resistance of a 1 cm2 coupon in 5000 Ω-cm clay soil is only 2.21 kΩ.

Figure 7: Coupon C-01-20-01 variation of coupon impedance.

Because of this increase, at several coupons the AC current required to provide the desired AC current density could not be reached, even at AC voltages well above 100 V, as shown in Figure 8.

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Figure 8: Variation of AC current for high impedance coupons

Since the important factor in assessing the risk of AC corrosion is the average AC current density, this challenge was not considered critical. Following initial adjustments, the actual AC current was recorded during the periodical surveys to allow calculating the AC current density.

The relevant information(1) regarding the variation of protection level and the average AC and DC current densities is summarized in Table 1.

Table 1 July 2015 to July 2018 Coupons Monitoring Summary

Coupon Days AC current

density below target

Average ACCD (A/m2)

Days not Fully Protected

Days DCCD greater than 1 A/m2

C-01-20-01 584 53% 12.3 26 2% 0 0%

C-01-50-01 540 49% 39.7 71 6% 402 37%

C-01-100-01 23 2% 103.9 30 3% 643 59%

C-01-20-02 164 15% 18.1 54 5% 0 0%

C-01-50-02 324 30% 43.7 73 7% 0 0%

C-01-100-02 264 24% 92.3 6 1% 0 0%

C-01-20-03 454 41% 23.7 43 4% 0 0%

C-01-50-03 852 78% 34.6 17 2% 0 0%

C-01-100-03 877 80% 65.7 27 2% 0 0%

C-01-20-04 118 11% 19.2 82 8% 0 0%

C-01-50-04 536 49% 44.1 129 12% 27 3%

C-01-100-04 93 8% 96.6 84 8% 681 62%

C-10-20-01 17 2% 20.1 6 1% 0 0%

C-10-50-01 91 8% 61.5 94 9% 590 54%

(1) The relevance was established with respect to the proposed AC corrosion criteria during the development of NACE standard SP21424-2018.

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Table 1 July 2015 to July 2018 Coupons Monitoring Summary

Coupon Days AC current

density below target

Average ACCD (A/m2)

Days not Fully Protected

Days DCCD greater than 1 A/m2

C-10-100-01 241 22% 96.9 26 2% 0 0%

C-10-20-02 429 39% 16.7 67 6% 476 43%

C-10-50-02 655 60% 25.9 199 18% 134 12%

C-10-100-02 58 5% 99.9 27 3% 0 0%

C-10-20-03 528 48% 13.4 186 17% 0 0%

C-10-50-03 409 37% 39.2 86 8% 473 43%

C-10-100-03 63 6% 101.9 60 5% 676 62%

C-10-20-04 0 0% 20.2 43 4% 0 0%

C-10-50-04 123 11% 47.4 74 7% 579 53%

C-10-100-04 724 66% 60.0 255 23% 542 49%

C-06-20-01 162 15% 18.9 30 3% 0 0%

C-06-50-01 21 2% 52.3 27 3% 0 0%

C-06-100-01 41 4% 103.2 6 1% 0 0%

C-06-20-02 146 13% 18.5 73 7% 0 0%

C-06-50-02 252 23% 42.3 45 4% 519 47%

C-06-100-02 20 2% 105.0 52 5% 696 63%

C-06-20-03 170 15% 18.4 86 8% 0 0%

C-06-50-03 204 19% 43.4 58 5% 0 0%

C-06-100-03 680 62% 71.1 49 4% 671 61%

C-06-20-04 191 17% 18.1 80 7% 21 2%

C-06-50-04 186 17% 45.8 105 10% 556 51%

C-06-100-04 17 2% 104.2 142 13% 711 65%

C-01-00-04 0 0% 0.0 0 0% 310 28%

C-01-00-03 0 0% 0.0 17 2% 0 0%

C-01-00-01 0 0% 0.0 239 22% 0 0%

C-10-00-04 0 0% 0.0 17 2% 0 0%

C-10-00-02 0 0% 0.0 21 2% 0 0%

C-10-00-03 0 0% 0.0 0 0% 0 0%

C-10-00-01 0 0% 0.0 36 3% 0 0%

C-06-00-04 0 0% 0.0 44 4% 0 0%

C-06-00-02 0 0% 0.0 65 6% 0 0%

C-06-00-03 0 0% 0.0 33 3% 0 0%

C-06-00-01 0 0% 0.0 963 88% 371 34%

Other field related challenges, including highly electronegative OFF potentials and effects of exposure to high AC voltage on data loggers, would be discussed in future papers.

RESULTS

The field test results in terms of weight loss, average corrosion rate, and maximum pitting depth are summarized in Table 2.

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Table 2 Weight Loss, Average Corrosion Rate, and Maximum Pitting Depth versus AC Current Density

for Various Size Coupons

Coupon Weight Loss (g)

Maximum Pitting Depth(2)

(mm)

Pitting Factor

Average Corrosion

Rate(3) (mm/y)

Average ACCD (A/m2)

Time not Fully

Protected

Time with

DCCD Greater than 1 A/m2

C-01-20-01 0.0117 0.1980 13.302 0.005 12 2% 0%

C-01-50-01 0.0433 0.2080 3.776 0.019 40 6% 37%

C-01-100-01 0.0273 0.2620 7.543 0.012 104 3% 59%

C-01-20-02 0.0118 0.0850 5.662 0.005 18 5% 0%

C-01-50-02 0.0172 0.0710 3.245 0.007 44 7% 0%

C-01-100-02 0.0206 0.1080 4.121 0.009 92 1% 0%

C-01-20-03 0.0143 0.0910 5.002 0.006 24 4% 0%

C-01-50-03 0.0222 0.1490 5.275 0.010 35 2% 0%

C-01-100-03 0.0372 0.2030 4.289 0.016 66 2% 0%

C-01-20-04 0.0162 0.0390 1.892 0.007 19 8% 0%

C-01-50-04 0.0136 0.0890 5.144 0.006 44 12% 3%

C-01-100-04 0.0185 0.0590 2.507 0.008 97 8% 62%

C-10-20-01 0.0809 0.2530 24.581 0.003 20 1% 0%

C-10-50-01 0.2170 0.1310 4.745 0.009 62 9% 54%

C-10-100-01 0.1841 0.3910 16.693 0.008 97 2% 0%

C-10-20-02 0.1333 0.1780 10.496 0.006 17 6% 43%

C-10-50-02 0.3328 0.1050 2.480 0.014 26 18% 12%

C-10-100-02 0.1795 0.2190 9.590 0.008 100 3% 0%

C-10-20-03 0.2305 0.1560 5.320 0.010 13 17% 0%

C-10-50-03 0.3855 0.1950 3.976 0.017 39 8% 43%

C-10-100-03 0.1649 0.2650 12.631 0.007 102 5% 62%

C-10-20-04 0.0480 0.0280 4.585 0.002 20 4% 0%

C-10-50-04 0.2092 0.1400 5.260 0.009 47 7% 53%

C-10-100-04 0.1764 0.2720 12.120 0.008 60 23% 49%

C-06-20-01 0.1009 0.2110 9.862 0.007 19 3% 0%

C-06-50-01 0.1427 0.2940 9.716 0.010 52 3% 0%

C-06-100-01 0.1883 0.4390 10.995 0.013 103 1% 0%

C-06-20-02 0.0472 0.0830 8.293 0.003 19 7% 0%

C-06-50-02 0.1575 0.3060 9.163 0.011 42 4% 47%

C-06-100-02 0.2577 0.2160 3.953 0.018 105 5% 63%

C-06-20-03 0.0267 0.0770 13.600 0.002 18 8% 0%

C-06-50-03 0.1160 0.2510 10.204 0.008 43 5% 0%

C-06-100-03 0.0840 0.4660 26.163 0.006 71 4% 61%

C-06-20-04 0.0551 0.1830 15.663 0.004 18 7% 2%

C-06-50-04 0.0910 0.4070 21.092 0.007 46 10% 51%

C-06-100-04 0.0821 0.1880 10.799 0.006 104 13% 65%

C-01-00-04 0.0084 0.0230 2.152 0.004 0 0% 28%

C-01-00-03 0.0110 0.0000 0.000 0.005 0 2% 0%

C-01-00-01 0.0130 0.0240 1.451 0.006 0 22% 0%

(2) Measured using optical microscopy, as per ASTM G46 standard.5

(3) Calculated in accordance with ASTM G1 standard.6

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Table 2 Weight Loss, Average Corrosion Rate, and Maximum Pitting Depth versus AC Current Density

for Various Size Coupons

Coupon Weight Loss (g)

Maximum Pitting Depth(2)

(mm)

Pitting Factor

Average Corrosion

Rate(3) (mm/y)

Average ACCD (A/m2)

Time not Fully

Protected

Time with

DCCD Greater than 1 A/m2

C-10-00-04 0.0383 0.0160 3.284 0.002 0 2% 0%

C-10-00-02 0.0475 0.0610 10.094 0.002 0 2% 0%

C-10-00-03 0.0380 0.0270 5.585 0.002 0 0% 0%

C-10-00-01 0.0531 0.1070 15.838 0.002 0 3% 0%

C-06-00-04 0.0250 0.0390 7.357 0.002 0 4% 0%

C-06-00-02 0.0433 0.0070 0.762 0.003 0 6% 0%

C-06-00-03 0.0264 0.0200 3.573 0.002 0 3% 0%

C-06-00-01 0.1231 0.2750 10.535 0.009 0 88% 34%

The average rate of corrosion for the cathodically protected coupons exposed to AC interference varies between 0.002 mm/y and 0.019 mm/y, while the maximum pitting depth varies between 0.028 mm (0.006 mm/y) at coupon C10-20-04 and 0.466 mm (0.157 mm/y) at coupon C-06-100-03 – see Figures 8 and 9, respectively. For reference, the average rate of corrosion for the disconnected coupon C-01-00-02, which was left to freely corrode in soil, with no AC current applied, was 0.052 mm/y – see Figure 10.

Figure 8: C-10-20-04.

Figure 9: C-06-100-03.

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Figure 10: C-01-00-02.

The pitting factor varied between 2.48 and 26.123. These values are similar to the values indicated by Romanoff7 for soil corrosion (i.e., from 1 to 25.5).

All the average corrosion rates are less than 0.025 mm/y indicating no risk of AC corrosion, although significant pitting was recorded on the coupons operating at high AC current density and DC current density exceeding 1 A/m2 for long periods of time.

The highest pit depth was recorded at coupon C-06-100-03 (0.466 mm), at an average current density of 71.1 A/m2, pitting factor 26.163, with the DC current density exceeding 1 A/m2 for 61% of the test duration. Deep pitting was also recorded on coupon C-06-100-01 (0.439 mm) at an average current density of 103.2 A/m2, pitting factor 10.995 and C-06-50-04 (0.407 mm) at an average current density of 45.8 A/m2, pitting factor 21.092 – see Figures 11 and 12, respectively. The DC current density for coupon C-06-50-04 exceeded 1 A/m2 for 51% of the test duration.

Figure 11: C-06-100-01.

Figure 12: C-06-50-04.

Based on the results of this three-years field study, low average rates of corrosion, as measured by weight loss or ER probes, do not exclude the high rates of pitting. Paragraph 8.3.1 of NACE standard SP21424-2018, also states the primary disadvantage of the ER probes is that the accuracy is compromised if localized corrosion occurs.4 Subsequently we recommend assessing the risk of AC

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corrosion on both coupons and ER probes data and not to rely on the ER probe only. Alternatively, perforation probes may be used for early detection of deep pitting, as per BS EN ISO 18086.3

The majority of the coupons operated at DC current densities less than 1 A/m2, as the initial objective was to maintain the coupons cathodically protected. Subsequently, the test data provided only limited information regarding conformance to NACE standard SP21424-2018 criteria.4

A preliminary analysis based on the three-years AC average current density and the total period of time the 1 A/m2 DC current density was exceeded did not provide relevant results. The exposure to DC current densities higher than 1 A/m2 varied between no exposure at 28 coupons and 710.5 days (65%) for coupon C-06-100-04, which displayed an average corrosion rate of only 0.006 mm/y, with a maximum pit depth of 0.272 mm (0.063 mm/y) at an average current density of 104.2 A/m2 – see Figure 13. Long exposure (i.e., over 50% of the test duration) to DC current densities higher than 1 A/m2 was also recorded at other eight coupons, with the highest average corrosion rate of 0.019 mm/y at coupon C-01-50-01 – see Figure 14. Only six of these coupons displayed pitting rates higher than 0.052 mm/y average corrosion rate for free corrosion in soil, as measured on coupon C-01-00-02.

Figure 13: C-06-100-04.

Figure 14: C-01-50-01.

A more detailed analysis will be performed by selecting the specific periods when the DC current exceeded 1 A/m2 and calculating the average AC current density for this period. However, with measurements every two or three weeks, even these more detailed calculations are not expected to provide the resolution of the short time recordings. A future field-based testing designed specifically to validate NACE standard SP21424-2018 criteria would have to intentionally operate coupons at DC current densities below and above 1 A/m2 at AC current densities below and above 30 A/m2. The existing equipment may be used, with the added capability for short time (24 to 144 hours) recording.

Based on the results of the field study, the maximum pitting depths, as averaged on the four coupons sets, were recorded on the 6 cm2 coupons, as shown in Figure 15, confirming the findings of Goidanich.2

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Figure 15: Maximum Pitting Depth for Various Coupon Sizes

For AC current densities exceeding 30 A/m2, the maximum depth of pitting on the 6 cm2 coupons, is more than double the depth measured on 1 cm2 coupons.

CONCLUSIONS

Based on the results of this three-year field study, low average rates of corrosion, as measured by weight loss or ER probes, do not exclude high rates of pitting, confirming the cautionary notes of paragraph 8.3.1 of NACE standard SP21424-2018.4 The average corrosion rates were less than 0.025 mm/y, indicating no risk of AC corrosion, although significant pitting (i.e., up to 0.157 mm/y) was recorded on the coupons operating at high AC current density and DC current density exceeding 1 A/m2 for long periods of time. The pitting factor varied between 2.48 and 26.123. Subsequently we recommend assessing the risk of AC corrosion on both coupons and ER probes data and not to rely on the ER probe only. Alternatively, perforation probes may be used for early detection of deep pitting, as per BS EN ISO 18086.3

Despite regularly monitoring both AC and DC current densities, the test data did not provide sufficient information to allow validation of the new NACE standard SP21424-2018 criteria.4 A future field-based testing designed specifically to validate these criteria would have to intentionally operate coupons at DC current densities below and above 1 A/m2 at AC current densities below and above 30 A/m2. The existing equipment may be used, with the added capability for short time (24 to 144 hours) recording.

Based on the results of the field study, for AC current densities exceeding 30 A/m2, the maximum depth of pitting on the 6 cm2 coupons, is more than double the depth measured on 1 cm2 coupons.

Very low current densities (i.e., less than 10 mA/m2) were required to fully protect the coupons in clay. Following the application of AC current, the coupons depolarized to an average of -638 mVCSE. Subsequently, the DC current density had to be increased by more than 5000% to an average of 483.7 mA/m2 to restore protection, validating paragraph 4.2 of NACE Standard SP21424-2018.4

0.103

0.129

0.1580.139

0.3150.327

0.1540.143

0.287

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

20 50 100

Max

imu

m P

itti

ng

Dep

th (

mm

)

AC Current Density Target (A/m2)

1 cm² 6 cm² 10 cm²

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REFERENCES

1. W. Prinz, “AC Induced Corrosion on Cathodically Protected Pipelines,” UK Corrosion 92, vol.1 (1992).

2. S. Goidanich, et al., “Influence of AC on Carbon Steel Corrosion in Simulated Soil Conditions,” 16th ICC, paper 04-03, (Beijing, China: ICC, 2005).

3. BS EN ISO 18086 (2017), “Corrosion of metals and alloys. Determination of AC corrosion. Protection Criteria” (London, UK: BSI).

4. NACE SP21424 (2018), “Alternating Current Corrosion on Cathodically Protected Pipelines: Risk Assessment, Mitigation, and Monitoring” (Houston, TX: NACE).

5. ASTM G46 (2013), “Standard Guide for Examination and Evaluation of Pitting Corrosion”, (West Conshohocken, PA: ASTM).

6. ASTM G1 (2017), “Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens” (West Conshohocken, PA: ASTM).

7. M. Romanoff, Underground Corrosion. (Houston, TX: NACE, 1989).

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Documents

Paper No. 12907 - Corrosion Service · 2021. 1. 12. · Calgary, AB, Canada T2P 5H1 C. Khattar, P. Eng. TransCanada Pipelines Limited 450 – 1st Street SW, Calgary, AB, Canada T2P