The Challenge of Internal Corrosion in Steel Pipelines
Internal corrosion is one of the leading causes of pipeline failure in oil and gas transmission, water distribution, and process piping systems. Unlike external corrosion, which is visible and relatively easy to address, internal corrosion develops out of sight and can cause catastrophic failures without warning. Effective monitoring and pigging programs are essential for pipeline integrity management. This guide covers all aspects of internal corrosion monitoring and pigging for 2026 pipeline operations.
Mechanisms of Internal Corrosion
CO₂ Corrosion (Sweet Corrosion)
- Most common form in oil and gas pipelines
- CO₂ dissolves in water to form carbonic acid (H₂CO₃)
- Causes localized pitting and mesa attack
- Rate depends on CO₂ partial pressure, temperature, water chemistry, and flow velocity
- de Waard-Milliams model is the standard prediction method
H₂S Corrosion (Sour Corrosion)
- Hydrogen sulfide forms iron sulfide (FeS) on the steel surface
- Can provide some protection (stable FeS film) or accelerate corrosion (unstable film)
- Causes sulfide stress cracking (SSC) and hydrogen-induced cracking (HIC)
- NACE MR0175/ISO 15156 defines material requirements for sour service
Oxygen Corrosion
- Even trace amounts of dissolved oxygen (>10 ppb) cause severe pitting
- Common in water injection systems where oxygen ingress occurs
- Much more aggressive than CO₂ corrosion on a per-ppm basis
Microbiologically Influenced Corrosion (MIC)
- Sulfate-reducing bacteria (SRB) produce H₂S as metabolic byproduct
- Acid-producing bacteria (APB) create localized acidic environments
- Common in stagnant or low-flow areas, water systems, and海底 pipelines
- Requires biocide treatment in addition to corrosion inhibitors
Erosion-Corrosion
- Combined mechanical erosion and electrochemical corrosion
- Caused by sand, sediment, or high-velocity flow removing protective films
- Occurs at bends, tees, restrictions, and downstream of injection points
Corrosion Monitoring Methods
1. Corrosion Coupons
- Small metal specimens inserted into the pipeline via coupon holders
- Retrieved periodically (typically 3-6 months) and analyzed
- Measure weight loss to calculate corrosion rate in mm/year or mpy
- Simple, reliable, and inexpensive
- Limitation: Provides historical data only (time-averaged rate)
2. Electrical Resistance (ER) Probes
- Measure metal loss in real-time by monitoring resistance change of a sensing element
- Provides continuous, real-time corrosion rate data
- Can trigger alarms when corrosion rate exceeds threshold
- Not affected by changes in water chemistry or conductivity
3. Linear Polarization Resistance (LPR) Probes
- Instantaneous corrosion rate measurement using electrochemical technique
- Responds rapidly to changes in corrosivity
- Requires conductive fluid (water phase present)
- Best for monitoring inhibitor effectiveness
4. Hydrogen Flux Probes
- Measure hydrogen permeation through the pipe wall
- Direct indicator of H₂S corrosion activity and SSC risk
- Used in sour service pipelines
5. Ultrasonic Wall Thickness Monitoring
- Permanently installed ultrasonic sensors measure wall thickness at critical locations
- Provides continuous or periodic wall thickness data
- Can detect localized corrosion (pitting) and general metal loss
- Advanced systems use phased array or guided wave technology
6. Corrosion Inhibitor Residual Monitoring
- Monitors the concentration of film-forming inhibitor in the fluid
- Ensures adequate inhibitor dosage is maintained
- Uses fluorescence, IR spectroscopy, or HPLC techniques
Pigging: The Backbone of Pipeline Integrity
What Is Pipeline Pigging?
A “pig” is a device inserted into a pipeline that travels through the pipeline (pushed by product flow or a dedicated launcher) to perform various functions: cleaning, inspection, batching, or sealing.
Types of Pigs
Utility Pigs (Cleaning)
- Foam pigs: Open-cell polyurethane, used for batching, dewatering, light cleaning
- Scraper pigs: With metal blades or wire brushes for removing hard deposits (wax, scale, mill scale)
- Cup pigs: With cup-shaped discs for sealing and pushing debris
- Sphere pigs: For liquid displacement and light cleaning in multiphase lines
Intelligent Pigs (Inspection Tools / ILI Tools)
- MFL (Magnetic Flux Leakage): Most common for metal loss detection. Uses magnets to saturate the pipe wall; sensors detect flux leakage at corrosion/defect locations. Accuracy: ±10% wall thickness.
- UT (Ultrasonic Testing): Uses ultrasonic pulses to measure wall thickness. Higher accuracy than MFL (±0.1mm). Requires liquid couplant (not suitable for gas pipelines without liquid fill).
- EMAT (Electromagnetic Acoustic Transducer): UT without couplant — suitable for gas pipelines. Slightly lower accuracy than conventional UT.
- Geometry pigs: Measure internal profile to detect dents, ovality, and buckling using mechanical arms or laser scanning.
- Crack detection pigs: Use EMAT or high-resolution UT to detect and size cracks (stress corrosion cracking, fatigue cracks).
Pig Launcher and Receiver Design
- Launcher: Upstream facility with quick-opening closure, pig insertion mechanism, and flow valves
- Receiver: Downstream facility with indicator (signaling pig arrival), closure, and debris trap
- Standard per ASME B31.4 (liquids) and B31.8 (gas)
- Must accommodate the largest pig diameter and longest pig length to be run
Pigging Frequency
| Pig Type | Typical Frequency | Purpose |
|---|---|---|
| Foam pig (batching) | Weekly to monthly | Separate products, remove liquids |
| Scraper pig (cleaning) | Monthly to quarterly | Remove wax, scale, debris |
| Intelligent pig (ILI) | Every 3-5 years | Wall thickness, corrosion mapping |
| Geometry pig | After construction, post-event | Detect dents, deformation |
| Crack detection pig | When risk indicates | Detect SCC, fatigue cracks |
Corrosion Management Strategy
Integrated Approach
- Baseline assessment: Initial intelligent pig run after commissioning to establish baseline wall thickness
- Continuous monitoring: ER probes, LPR probes, and coupons at strategic locations
- Chemical treatment: Corrosion inhibitor injection, biocide treatment, oxygen scavenging
- Regular pigging: Cleaning pigs on schedule to remove deposits that promote under-deposit corrosion
- Periodic ILI: Intelligent pig runs every 3-5 years to map corrosion progression
- Remaining life assessment: Calculate remaining service life based on corrosion rate and minimum wall thickness
- Repair and replacement: Schedule repairs for areas approaching minimum allowable wall thickness
2026 Technology Advances
- AI-driven corrosion prediction: Machine learning models combining monitoring data, fluid composition, flow regime, and historical inspection data to predict corrosion rates and optimize inhibitor dosing
- Autonomous inspection vehicles (AIVs): Free-swimming robots for subsea pipeline inspection without tether
- Fiber optic sensing: Distributed acoustic sensing (DAS) and distributed temperature sensing (DTS) for real-time pipeline monitoring
- Digital twins: Virtual pipeline models updated with real-time data for predictive maintenance
CoreMetal Steel: Pipeline-Grade Steel Products
CoreMetal Steel supplies API 5L pipeline steel in Grades B through X120, seamless and welded. Our pipe is manufactured for pigging compatibility with proper roundness, internal surface quality, and dimensional tolerances. Available with internal coating and lining per project specifications.
Contact: Tracy | tracy@coremetalsteel.com | +86 18291910632
Conclusion
Internal corrosion monitoring and pigging are essential components of pipeline integrity management. Combining continuous monitoring (coupons, ER probes, LPR), regular cleaning pigging, and periodic intelligent pig inspection provides comprehensive coverage for detecting and managing internal corrosion. Visit CoreMetal Steel Blog for more pipeline engineering resources.