Detection of Flow-Accelerated Corrosion (FAC): Methods & Testing Services

Flow Accelerated Corrosion (FAC) — the enhanced dissolution of the protective iron oxide layer from carbon steel surfaces by flowing water or wet steam — has caused some of the most catastrophic and costly failures in power generation and process industry piping systems. The 1986 Surry Nuclear Power Plant condensate line rupture and the 2004 Mihama Unit 3 accident in Japan, both caused by FAC, killed workers and required extensive plant repair and downtime. In the power generation & infrastructure industry, systematic FAC detection, monitoring, and management programs are essential for plant safety and operational reliability.

What Is Flow Accelerated Corrosion?

FAC is a mass transfer-controlled corrosion mechanism where the protective magnetite (Fe₃O₄) film on the inner surface of carbon steel piping is continuously dissolved and carried away by the flowing water or wet steam, exposing fresh metal to further corrosion. This occurs in a continuous cycle:

1. The flowing fluid dissolves iron from the magnetite film (Fe₃O₄ → Fe²⁺ in solution)
2. Dissolved iron is transported away by fluid convection
3. The fresh metal surface oxidizes to reform magnetite
4. The flowing fluid again dissolves the oxide

When the rate of oxide dissolution exceeds the rate of oxide reformation, net material loss occurs — producing progressive wall thinning that can lead to rupture without external visual evidence of degradation.

Conditions That Promote FAC

FAC requires the simultaneous presence of several conditions — removing any one significantly reduces or eliminates FAC:

Single-phase liquid water FAC — occurs in condensate, feedwater, and extraction steam piping at temperatures of 100–250°C (optimal approximately 130°C), particularly in:

• Elbows, tees, reducers, and flow orifices where turbulence is highest
• Downstream of control valves, throttle valves, and orifice plates
• Extraction steam piping with entrained moisture

Two-phase (liquid-vapor) FAC — occurs in wet steam piping where condensate droplets impinge on metal surfaces, dramatically accelerating erosion-corrosion

Chemical Factors

• Low pH — acidic pH accelerates magnetite dissolution; feedwater pH below 8.5 dramatically increases FAC rates; target pH 9.0–9.6 with ammonia or morpholine treatment significantly reduces FAC
• Low dissolved oxygen — oxygen stabilizes the magnetite film; the near-zero dissolved oxygen in deaerated feedwater removes this protection
• Temperature — FAC peaks at approximately 130°C for single-phase water; the rate decreases at both higher and lower temperatures
• Water velocity — higher velocity increases turbulence and mass transfer coefficient, accelerating magnetite dissolution

Material Factors

• Carbon steel — most susceptible; chromium additions of 1–2% dramatically reduce FAC susceptibility
• Low-alloy steels (Cr-Mo steels) — 1% Cr reduces FAC rate by approximately 75%; 2.25% Cr provides near-complete immunity
• Stainless steels and nickel alloys — essentially immune to FAC

FAC Detection and Monitoring Methods

Ultrasonic Thickness Measurement (UT)

Ultrasonic testing is the primary tool for FAC wall thickness monitoring in operating power plants. Contact UT (using piezoelectric transducers) or immersion UT measures the remaining wall thickness at selected measurement points, then compares it to the original wall thickness to calculate the metal loss rate. Automated UT scanning systems map thickness distributions across pipe sections, generating color-coded thickness maps that reveal FAC attack patterns.

A-scan UT — single-point measurement; rapid for spot checks but limited in area coverage

B-scan (cross-sectional imaging) and C-scan (planar mapping) — provide area-averaged thickness data that reveals the characteristic “orange peel” surface texture of FAC-attacked pipe walls.

Radiographic Testing (RT)

Profile radiography reveals wall-thickness variations by projecting X-rays through the pipe wall — thinner walls transmit more radiation, appearing as lighter regions on film or digital detector images. RT is particularly effective for detecting localized FAC attack without requiring insulation removal.

Tangential radiography — X-ray beam tangent to the pipe wall clearly reveals the wall thickness profile and detects the scalloped inner surface characteristic of FAC.

Digital radiography (DR) and computed radiography (CR) — replace film with digital detectors; enable digital image analysis and measurement of wall thickness profiles

Pulsed Eddy Current (PEC) Testing

PEC testing detects wall thickness through insulation and coatings without contacting the pipe surface — particularly valuable for FAC monitoring of insulated piping,g where insulation removal is costly and time-consuming. A pulsed electromagnetic field penetrates insulation and metal to measure remaining wall thickness from the characteristic signal decay profile.

CHECWORKS and Predictive Modeling

CHECWORKS (Chemistry and Hydraulics Engineering CORROSION WORKS Study) is the industry-standard predictive software for FAC susceptibility assessment in nuclear power plant piping. The software calculates FAC wear rates for each piping segment based on:

• Fluid chemistry (pH, dissolved oxygen, temperature)
• Flow geometry (component type, velocity, quality)
• Material chromium content
• Operating history

CHECWORKS predictions prioritize inspection resources — focusing UT inspection on the highest-risk components first and allowing less-susceptible components to be inspected less frequently.

Electrochemical and Online Monitoring

Electrochemical probes installed in water systems measure corrosion potential and corrosion current in real time, providing a continuous indication of FAC activity. Changes in pH, dissolved iron content (measured by online iron analyzers), and electrochemical noise patterns signal changes in FAC activity before measurable wall loss occurs.

Regulatory and Industry Standards

• EPRI Report TR-106611 — FAC program guidelines for fossil fuel power plants
• NSAC-202L — FAC inspection program guidelines for nuclear power plants (EPRI)
• NRC GL 89-08 — NRC Generic Letter on erosion/corrosion program requirements for nuclear plants

VGB-S-011-T-00 — FAC guidelines for German power plants

Conclusion

Flow accelerated corrosion is a silent, progressive wall-thinning mechanism that can cause catastrophic pipe ruptures with no external warning — making systematic detection and monitoring programs non-negotiable for power generation and process piping systems. Combining ultrasonic thickness mapping, radiographic profiling, pulsed eddy current inspection through insulation, and CHECWORKS predictive modeling gives plant engineers the complete picture needed to prioritize inspections, schedule replacements, and implement chemistry controls before FAC progresses to failure.

Why Choose Infinita Lab for Flow Accelerated Corrosion Detection and Analysis?

Infinita Lab supports FAC management programs with materials characterization and analytical services — including carbon and chromium content analysis (ASTM E415, E1086) for FAC susceptibility assessment, corrosion product analysis (SEM/EDS, XRD for magnetite characterization), water chemistry analysis (pH, dissolved iron, dissolved oxygen by ICP-OES), and metallographic examination of FAC-damaged pipe specimens for wear rate verification — serving power generation & infrastructure clients with the analytical evidence needed for plant safety programs and regulatory compliance documentation. Contact Infinita Lab at infinitalab.com to discuss analytical support for your FAC monitoring program.

Frequently Asked Questions