Flow accelerated corrosion silently removes metal from the inside of carbon steel boiler tubes and piping systems, leaving behind a smooth, clean surface that looks completely healthy during a visual inspection. The dissolved iron is carried downstream by the flow, so there is no rust, no scale, and no visible indicator that wall thickness is disappearing at rates that can exceed ten mils per year in susceptible locations. By the time a leak occurs, the thinning has often progressed over tens of thousands of operating hours without any outward sign. The only way to manage FAC is to predict where it will occur, inspect those locations systematically, and track the thickness data over time to verify that your chemistry and flow conditions are actually keeping the corrosion rate under control. See how iFactory structures FAC prediction and inspection data by visiting iFactory support.
Boiler Chemistry · FAC Management
Flow Accelerated Corrosion Eats Pipe From the Inside Out
No visible scale, no rust, no warning. FAC dissolves carbon steel smoothly and silently. The only defense is knowing exactly where it strikes, predicting the rate, and inspecting before the wall is gone.
The Mechanism
What Actually Happens During Flow Accelerated Corrosion
FAC is not erosion. It is a electrochemical dissolution process where flowing water prevents the natural protective magnetite layer from forming or surviving on the carbon steel surface. The visualization below traces the mechanism step by step from clean metal to wall failure.
1
Exposed Metal Surface
In areas of high flow, turbulence, or flow disturbance, the protective magnetite layer either never forms or is continuously stripped away, leaving bare carbon steel in direct contact with the flowing water or steam-water mixture.
2
Iron Dissolves Into Flow
Ferrous iron ions dissolve from the bare steel surface into the water. The dissolution rate depends on water temperature, pH at the operating temperature, and the local flow velocity. Higher temperature and lower pH both accelerate the reaction significantly.
3
Flow Carries Ions Away
The flowing water continuously removes dissolved iron from the near-wall boundary layer. This mass transport is what distinguishes FAC from stagnant corrosion. Without flow, iron concentration builds up and the dissolution slows. With flow, the concentration stays low and the metal keeps dissolving.
4
Smooth Wall Thinning Progresses
The result is a smooth, polished-looking inner surface with uniform or locally concentrated wall loss. Because no scale or deposit forms, the surface looks healthy to anyone inspecting visually. Only ultrasonic thickness measurement reveals the actual remaining wall.
5
Wall Reaches Minimum Thickness
When the remaining wall can no longer contain the design pressure, the tube or pipe ruptures. The failure is typically a small pinhole leak rather than a catastrophic burst because the thinning is often localized, and the surrounding thicker material restrains the opening initially.
Susceptible Areas
Where FAC Strikes Inside a Boiler System
FAC does not attack uniformly. It concentrates at locations where flow changes direction, velocity increases, or geometry creates turbulence. The grid below maps the most common FAC attack zones across a typical boiler feedwater and steam system, ranked by historical failure frequency.
Critical Risk
Extraction Steam Lines
Two-phase flow with high velocity and flow regime changes cause aggressive thinning at bends, elbows, and downstream of orifices and valves
Critical Risk
Economizer Inlet Headers
Feedwater temperature entering the economizer falls in the peak FAC susceptibility window and header stub tubes create flow disturbances that accelerate local thinning
High Risk
Feedwater Piping Bends
Changes in flow direction at elbows create secondary flows and increased local velocity that prevent protective oxide formation on the outer radius of the bend
High Risk
Downcomer Tubes
High single-phase water velocity combined with any flow disturbance from inlet fittings or geometry changes creates localized thinning that is difficult to access for inspection
Moderate Risk
Economizer Tube Bends
Bend geometry in the economizer circuit creates flow acceleration and secondary flows, particularly at tight-radius bends where the outer wall sees increased velocity
Moderate Risk
Drum Internal Piping
Distribution pipes inside the steam drum experience complex flow patterns and temperature gradients that can create localized FAC, though access for inspection is extremely limited
Lower Risk
Straight Feedwater Runs
Long straight sections with undisturbed flow are the lowest risk areas because the boundary layer remains stable and protective oxide can form and survive under controlled chemistry conditions
Lower Risk
Main Steam Piping
Superheated steam carries no dissolved oxygen and operates above the FAC temperature susceptibility window, making main steam lines inherently resistant to the mechanism
Flow Regimes
Single-Phase vs Two-Phase FAC: Different Flow, Different Damage Pattern
FAC behaves differently depending on whether the fluid is all liquid or a mixture of liquid and steam. The temperature range, susceptible geometry, and failure morphology all shift between the two regimes. Misidentifying which regime is active at a failure location leads to incorrect corrective actions.
Characteristic
Single-Phase FAC
Two-Phase FAC
Fluid State
Subcooled liquid water only
Mixture of water and steam
Temperature Range
250°F to 450°F peak susceptibility
350°F to 550°F peak susceptibility
Peak Susceptibility
Around 280°F to 300°F for most plants
Around 400°F to 450°F at high quality
Typical Locations
Feedwater piping, economizer inlets, downcomers
Extraction steam lines, heater drains, HRSG circuits
Geometry Factor
Bends, elbows, tees, flow restrictions
Downstream of orifices, valves, bends, at flow regime transitions
Thinning Pattern
Usually on outer radius of bends or downstream of disturbances
Can be circumferential or highly localized at flow impingement points
Failure Appearance
Smooth, polished inner surface, often with a characteristic tiger-stripe pattern at bends
Smooth surface with possible wavy morphology where liquid droplets impinge at high velocity
Driving Factors
Four Variables That Control Whether FAC Happens and How Fast
FAC rate is not fixed. It responds in real time to changes in water chemistry, temperature, material composition, and flow conditions. Understanding how each variable shifts the corrosion rate is the basis for both prediction and prevention. The cards below show the direction and magnitude of each effect.
Water Chemistry pH
Higher pH reduces FAC rate dramatically
FAC rate drops by roughly a factor of ten for each unit increase in pH within the operating range. Raising feedwater pH from 8.8 to 9.2 or 9.6 through ammonia or amine treatment is the single most effective control lever available. Below pH 8.5, FAC rates in susceptible geometries can become extremely aggressive and difficult to manage through inspection alone.
Temperature
Bell-shaped curve with a sharp peak
FAC rate follows a characteristic bell curve versus temperature. Below 200°F the dissolution reaction is too slow for significant thinning. The rate peaks between 250°F and 450°F depending on pH and material. Above the peak, a more stable oxide forms and the rate drops again. Operating your system at or near the peak temperature window without compensating chemistry is the most common root cause of aggressive FAC.
Chromium Content
Even 0.1% chromium virtually eliminates FAC
Carbon steel with less than 0.05% chromium is highly susceptible to FAC. Low-alloy steels containing 0.5% to 1.0% chromium form a chromium-rich oxide layer that is resistant to dissolution under the same flow conditions that aggressively attack plain carbon steel. Material substitution is often the most reliable fix for locations where chemistry control alone cannot reduce the rate to acceptable levels.
Flow Velocity and Geometry
Higher velocity and turbulence increase FAC rate
FAC requires flow to transport dissolved iron away from the surface. Higher velocity increases mass transport and accelerates thinning. Geometry effects are often more important than bulk velocity because elbows, tees, reducers, and restrictions create local turbulence, flow separation, and secondary flows that dramatically increase the local mass transfer coefficient even when the bulk pipe velocity is moderate.
Predictive Modeling
From Qualitative Guessing to Quantitative FAC Prediction
For decades, plants managed FAC by inspecting locations that had failed before and hoping they caught the next one in time. Predictive modeling changed this by calculating expected thinning rates for every component in the system based on its geometry, material, temperature, and chemistry conditions. The result is a risk-ranked component list that tells you where to inspect before a failure occurs.
01
Build System Model
Map the entire piping and tube circuit including pipe sizes, materials, flow directions, elbows, tees, valves, orifices, and temperature at each node. This geometry model is the foundation that determines where local flow conditions differ from bulk conditions.
02
Input Chemistry Data
Feed actual operating pH, conductivity, oxygen, and treatment chemical data into the model. The prediction accuracy depends directly on how well the chemistry inputs represent the actual water conditions at each temperature point in the system over the operating history.
03
Calculate Thinning Rates
The model applies empirical FAC correlations to each component based on its local temperature, geometry factor, material chromium content, and chemistry conditions to produce a predicted thinning rate in mils per thousand hours for every modeled location.
04
Rank by Risk
Components are ranked by predicted thinning rate combined with remaining wall margin to produce a risk priority list. The highest-risk locations are the ones where predicted thinning will consume the available wall thickness in the shortest remaining operating time.
05
Generate Inspection Plan
The risk ranking is converted into an inspection plan that specifies which locations to measure, what measurement technique to use, and how often to reinspect based on the predicted time to reach minimum wall thickness at the calculated thinning rate.
06
Validate and Calibrate
Actual thickness measurements from inspections are fed back into the model to compare predicted versus measured thinning rates. Where discrepancies exist, the model inputs are adjusted to improve future prediction accuracy for those circuit conditions.
Inspection Execution
Inspection Methods for FAC-Affected Components
FAC produces smooth wall loss with no external signs, so inspection relies entirely on thickness measurement technology. The method chosen determines how much of each component you can assess, how accurate the number is, and how long the inspection takes relative to your available outage window.
Manual UT Point Readings
Technician places a transducer at discrete points around the circumference and along the length of each component. Each reading takes 15 to 30 seconds. Good for targeted inspection of known high-risk locations but cannot provide full coverage of long pipe runs or complex geometries in a reasonable time frame.
Best for: Spot checks on risk-ranked locations
Automated UT Scanning
A motorized scanner moves the transducer continuously around the pipe circumference and along the axis, producing a dense thickness map with thousands of data points per foot of pipe. Requires clean surface access but provides the coverage needed to find localized thinning that point readings would miss between measurement locations.
Best for: Full-coverage surveys of critical piping
Inline Inspection (Pigs)
For large-diameter piping that is piggable, an instrumented pig can measure wall thickness from the inside while the system is out of service. Provides 100% coverage of the pipeline interior but requires pig launchers, receivers, and a pipeline geometry that accommodates the tool without damage or stuck-pig risk.
Best for: Long straight feedwater runs
Permanent Monitoring Probes
Fixed UT transducers installed on high-risk components provide continuous or periodic thickness readings without requiring scaffold access or surface preparation at each inspection interval. The trade-off is that each probe monitors only a small area, so multiple probes are needed to cover a single bend or tee comprehensively.
Best for: Highest-risk locations between outages
Stop Inspecting Where FAC Already Failed. Start Inspecting Where It Will Fail Next.
iFactory integrates FAC model predictions, inspection thickness data, chemistry records, and replacement history into a single system that ranks every component by actual risk and adjusts the plan as conditions change.
Common Questions
Flow Accelerated Corrosion — Frequently Asked
Can FAC occur in stainless steel piping?
Standard austenitic stainless steels like 304 and 316 contain enough chromium to form a stable, adherent chromium oxide layer that is highly resistant to FAC under normal boiler water chemistry conditions. FAC is predominantly a carbon steel and low-alloy steel problem. However, if the stainless steel surface is damaged, contaminated, or operates in an environment that breaks down the chromium oxide layer, some localized corrosion can occur, though it would not typically be classified as classical FAC. The practical answer is that material substitution to stainless steel or chromium-containing low-alloy steel is one of the most reliable ways to eliminate FAC at a specific location. Book a demo to see how iFactory tracks material upgrades against FAC risk reduction.
How does changing fuel source affect FAC risk?
Fuel changes affect FAC indirectly through their impact on water chemistry and system temperatures rather than through any direct interaction with the fuel itself. A fuel switch that changes boiler heat absorption patterns can shift temperatures in feedwater and economizer circuits, potentially moving components into or out of the peak FAC susceptibility window. Fuel changes that affect condenser leakage rates or introduce different contaminants into the condensate can shift water chemistry parameters, particularly pH and conductivity, which directly change the FAC rate in all susceptible locations. Any fuel change should trigger a review of temperatures and chemistry at known FAC-susceptible locations to determine whether the prediction model inputs need updating. Contact support for guidance on updating FAC models after operational changes.
What is the minimum chromium content needed to prevent FAC?
Research and operating experience consistently show that carbon steels with chromium content below approximately 0.05% are highly susceptible to FAC, while steels with 0.1% chromium or more show dramatically reduced thinning rates under identical flow and chemistry conditions. Low-alloy steels like T2 and T11 containing 0.5% to 1.25% chromium are effectively immune to FAC in most boiler water environments. The practical implication is that even small variations in chromium content between different heats of nominally identical carbon steel pipe can produce noticeably different FAC rates in the same circuit. When replacing FAC-damaged components, verifying the actual chromium content of the replacement material through a material test report is worth the effort. Book a demo to explore how iFactory tracks material chemistry against FAC susceptibility.
How often should FAC-susceptible locations be inspected?
Inspection interval should be based on the predicted time to reach minimum wall thickness at the calculated thinning rate, with a safety margin that accounts for measurement uncertainty and the possibility that actual conditions are worse than modeled. A common approach is to set the inspection interval so that the next inspection occurs when the predicted remaining wall is still 20% to 30% above the minimum required thickness. For high-risk locations with aggressive predicted rates, this may mean inspecting every outage. For lower-risk locations, the interval may extend to two or three outages. The key is that the interval is calculated from data, not chosen arbitrarily, and that it is recalculated whenever chemistry, temperature, or flow conditions change. Contact support to discuss setting up risk-based inspection intervals for your FAC program.
Can FAC be completely eliminated or only managed?
FAC can be effectively eliminated at specific locations through material substitution to chromium-containing alloys, which removes the susceptibility entirely regardless of flow conditions or chemistry variations. For the broader system where carbon steel remains in service, FAC can be reduced to very low rates through optimized chemistry control, particularly maintaining pH at the upper end of the recommended range using appropriate amine blends. However, complete elimination through chemistry alone is difficult because transient conditions like startups, shutdowns, chemistry upsets, and condenser leaks can create local environments where FAC rates spike even in a well-controlled system. The most robust approach combines material upgrades at the highest-risk locations with optimized chemistry for the remaining carbon steel system and systematic inspection to verify that actual thinning rates match predictions. Book a demo to see how iFactory manages this combined approach across your entire system.
The Pipe Looks Fine on the Outside. The Inside Tells a Different Story.
FAC prediction modeling, risk-ranked inspection planning, thickness trending, and material upgrade tracking, built for plants that refuse to wait for the next pinhole leak to find out where their thinnest pipe is.
