AC Corrosion Study Challenges ISO Threshold Assumptions
Share
New Research Advances Understanding of AC Corrosion on Coated Pipelines in Sabkha Soils
By Craig Botha
Presented in collaboration with Kuwait Oil Company (KOC)
Introduction
In a ground-breaking field study conducted in the sabkha soils of Southern Kuwait, Craig Botha, founder of RASC and CEO of Re:Ignite, in partnership with the Kuwait Oil Company, has advanced industry understanding of alternating current (AC) corrosion under cathodic protection conditions. The research uniquely evaluated the real-time performance of 3LPE and HBPU coating systems under high AC and DC current densities—conditions that routinely exceed ISO18086 acceptable interference thresholds.
Context: AC Interference and Cathodic Protection
When high-voltage AC power lines run parallel to cathodically protected pipelines, induced AC voltages can accelerate localized corrosion, particularly at coating defects. While standards like ISO18086, NACE SP0169, and ISO22426 set thresholds for AC voltage, current density, and DC/AC ratios, these are not always achievable in low-resistivity environments.
The key challenge: corrosion behaviour in these extreme conditions remains poorly understood, particularly for small defects where temperature effects, oxide formation, and electrochemical kinetics may diverge significantly from controlled laboratory conditions.
Research Objective
This research aimed to:
- Evaluate AC corrosion at real-world coating defects on 3LPE and HBPU coatings.
- Simulate field conditions with excessive AC interference.
- Quantify corrosion depths, product formation, and electrochemical characteristics over seven months.
- Assess whether corrosion self-limits under long-term exposure.
Methodology
A 40" pipeline spool, half coated with 3LPE and half with High Build Polyurethane (HBPU), was buried in sabkha soil and connected to a live pipeline system near CPTR 2-410. Artificial defects (8–19.5 mm in diameter) were introduced and monitored for voltage, current density (AC and DC), and corrosion product formation over 7 months (Sep 2023 – Apr 2024).
Measured conditions:
- Initial AC voltage: 3.95 V
- AC current density: up to 1,348 A/m²
- Soil resistivity: 0.5–0.83 Ω·m
- Final corrosion depths: up to 2.4 mm
Key Findings
1. Corrosion Depths and Coating Behaviour
- Maximum corrosion depth on 3LPE: 2.02 mm
- Maximum corrosion depth on HBPU: 2.4 mm
- Coating disbondment was more extensive in HBPU, with larger corrosion product zones.
2. Corrosion Product Composition
XRD analysis revealed a critical finding: the predominance of goethite (α-FeOOH) at defect sites on the pipe spool, contrasting with magnetite on ER probes. Goethite’s high resistivity (10⁴–10⁸ Ω·m) suggests a self-limiting corrosion mechanism due to:
- Physical barrier formation
- Surface passivation
- Decreased electrochemical activity
3. Temperature Effects from Joule Heating
Using the Joule effect, the study modelled temperature increases at the defect/electrolyte interface. For a 1 cm² defect, the localized temperature increase exceeded 15°C/min—significantly altering electrochemical kinetics.
- Higher temperatures favour goethite formation over magnetite.
- Lower solubility of Fe³⁺ at elevated temperatures promotes precipitation.
- Thermodynamic modelling (ΔG) confirms goethite’s preferential formation in heated environments.
4. Current Density Decline Over Time
Despite initial AC current densities far exceeding the ISO18086 30 A/m² threshold, current densities dropped by over 50%, correlating with:
- Spool impedance increase of 52.6%
- Formation of high-resistance oxide layers
- Reduction in corrosion rates despite ongoing AC exposure
Standards Contextualization
This study calls into question the universal applicability of:
- ISO18086 AC and DC thresholds
- ISO22426 assumptions on defect geometry
- The validity of using flat ER probes as corrosion predictors under high i<sub>ac</sub> environments
Craig Botha’s work highlights the need to factor in defect geometry, coating thickness, and temperature effects when modelling corrosion rates in field conditions.
Implications for Pipeline Integrity Management
This research provides strong evidence that:
- Small coating defects (≤1 cm²) may experience rapid initial corrosion but then stabilize due to goethite formation.
- AC corrosion in sabkha soils may self-limit under specific conditions, challenging worst-case linear extrapolations.
- Current industry standards may benefit from refinement to incorporate thermal effects and oxide film resistivity.
Furthermore, reliance on ER probes without considering site-specific temperature and geometry conditions can yield misleading corrosion rates.
Conclusions and Future Research
The presence of goethite as the dominant corrosion product under high i<sub>ac</sub> suggests that thermal and electrochemical kinetics play a larger role than previously understood. Despite the exceedance of standard limits, corrosion rates did not scale linearly over time—indicating the potential for long-term stability under specific field conditions.
Next steps include:
- Investigating varied defect geometries and coating systems
- Developing machine learning models to predict field-specific AC corrosion risks
- Integrating real-time thermal and electrochemical monitoring in CP design
Final Thoughts
Craig Botha’s collaborative field investigation provides essential new data for understanding AC-induced corrosion under high-risk conditions. By moving beyond the lab and into complex real-world environments, this work opens the door to more data-driven, site-specific, and thermodynamically aware approaches to corrosion control.