Erosion Corrosion and Flow-Accelerated Corrosion is the combined mechanical and electrochemical attack that occurs when high-velocity or turbulent fluid strips the protective oxide or passive film off a metal surface faster than the film can reform, exposing fresh metal that then corrodes again in a repeating loop. The result is directional metal loss that follows the flow path, not the random pitting seen in stagnant conditions. Flow-accelerated corrosion (FAC) is a specific, high-consequence form of this mechanism that thins carbon-steel piping in wet-steam and feedwater systems.
Most engineering metals survive in aggressive fluids only because a thin, adherent film separates the metal from the electrolyte. Carbon steel relies on magnetite (Fe3O4), copper alloys on a cuprous oxide layer, and stainless steels on a chromium-rich passive film. Erosion corrosion begins when wall shear stress, turbulence, impinging droplets, or entrained solids mechanically remove that film locally. The bare metal corrodes rapidly, a new film starts to grow, and the flow removes it again. Because dissolution and mechanical removal reinforce each other, the total metal loss is far greater than either process acting alone. The rate is governed largely by mass transfer at the wall, so anything that raises turbulence or local velocity raises the rate.
Erosion corrosion has a distinctive, directional morphology that separates it from pitting corrosion and other localized forms. Typical signatures include:
Erosion corrosion concentrates wherever the flow accelerates, changes direction, or becomes turbulent. The table below maps the common sites to their dominant cause and the practical mitigation.
| Site | Dominant cause | Mitigation |
|---|---|---|
| Elbows, bends, tees | Turbulence and impingement at direction change | Long-radius bends, thicker wall, FAC-resistant alloy |
| Pump impellers and casings | High velocity plus cavitation collapse | Raise NPSH, harder alloys, correct sizing |
| Valve seats and throttling trim | Flashing and high local velocity across the restriction | Stellite or hardened trim, multi-stage let-down |
| Condenser and heat-exchanger tube inlets | Inlet turbulence and entrained solids | Inlet ferrules or inserts, velocity limits |
| Feedwater and wet-steam piping | Flow-accelerated corrosion of carbon steel | Chromium alloy, pH and oxygen control |
| Orifices and reducers | Jetting downstream of the restriction | Streamlined geometry, wear-resistant liners |
Inlet-end erosion corrosion is a frequent driver of tube failure and lost thermal duty, so tube inlets deserve dedicated attention during condition monitoring.
FAC is the dissolution of the protective magnetite layer on carbon and low-alloy steel in flowing water or wet steam. It has caused catastrophic pipe ruptures, including the 1986 Surry and 2004 Mihama condensate and feedwater line failures, because thinning can be internal and invisible until the wall fails. FAC is most aggressive within a specific window of conditions rather than at simply the highest temperature. Single-phase FAC peaks roughly in the 130 to 150 C range, while two-phase (wet-steam) FAC peaks nearer 180 C. The feedwater chemistry set in the deaerator strongly influences the rate, which is why the deaerator and boiler feedwater chemistry must be controlled deliberately.
Four levers dominate the FAC rate, and each has a clear engineering response.
| Variable | Effect on FAC | Control target |
|---|---|---|
| Chromium content | Even about 0.1% Cr sharply lowers the rate; 1.25Cr-0.5Mo and 2.25Cr-1Mo are highly resistant | Upgrade susceptible components to low-alloy steel |
| Feedwater pH | Higher pH stabilizes magnetite | pH 9.2 to 9.8 with all-volatile treatment |
| Dissolved oxygen | A trace of oxygen forms a more protective oxide | Oxygenated treatment on high-purity water only |
| Flow velocity and geometry | Turbulence raises mass transfer | Velocity limits, long-radius fittings |
When replacing suspect spools, confirm the delivered material actually contains the specified chromium; a nominally identical carbon-steel fitting will thin again. Field alloy verification with positive material identification prevents that mix-up.
For single-phase erosion corrosion, the most direct defense is keeping fluid velocity below the threshold at which the protective film cannot survive. These approximate design velocities for flowing seawater illustrate how alloy choice buys headroom.
| Material | Approx. max continuous seawater velocity (m/s) |
|---|---|
| Copper | 1.0 |
| Admiralty brass | 1.5 |
| 90-10 copper-nickel | 3.5 |
| 70-30 copper-nickel | 4.5 |
| 316 stainless steel and titanium | Very high (film stable) |
A durable program combines geometry, chemistry, and materials with disciplined wall-thickness surveillance. Use long-radius bends and streamlined reducers, hold velocities within alloy limits, control feedwater pH and oxygen, and upgrade high-susceptibility locations to chromium-bearing steel. Then inspect on a predictive schedule: grid ultrasonic thickness readings at bends, tees, and downstream of orifices, trended over time to project remaining life before a minimum wall is reached. Managing those inspection routes, thickness histories, and alloy records inside a CMMS keeps thinning trends visible instead of buried in spreadsheets. Platforms such as Fabrico let teams schedule thickness surveys and track component condition against remaining-life limits. Book a Fabrico demo to see how that fits an existing inspection program.
Both involve mechanical film removal, but cavitation damage comes from vapor bubbles collapsing against the surface and produces a rough, spongy, cratered appearance. Erosion corrosion produces smooth, directional grooves and undercut pits that follow the flow path.
FAC works by dissolving magnetite. Chromium in the steel forms a mixed oxide that is far less soluble in high-temperature water, so even small chromium additions, and certainly stainless grades, resist the mechanism.
Raising pH within the 9.2 to 9.8 range stabilizes the protective oxide and lowers the rate, but the benefit depends on the amine used and its distribution between water and steam phases. Chemistry should be validated for each specific cycle.
Trended ultrasonic wall-thickness measurement at known-susceptible locations. A single reading only shows current wall; the projected remaining life comes from comparing successive surveys over time.
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