Guide
Which materials and design changes stop erosion corrosion in machined pump, valve and fitting parts?
Erosion corrosion is accelerated metal loss caused by a corrosive fluid moving fast or turbulently over a metal surface: the flow strips or thins the protective oxide or corrosion-product film, the bare metal corrodes, the film tries to reform and is stripped again. It shows up as fast, localized wall loss at pump casings and impellers, valve seats and bodies, elbows, tees, reducers and the first few diameters downstream of anything that disturbs the flow.
By Saola Manufacturing engineering team · Published · Updated
This guide is for the engineer or buyer holding a pump, valve or fitting that has thinned or leaked far sooner than its corrosion allowance predicted. It covers how to recognize erosion corrosion, how to confirm it on a returned part, which alloys tolerate higher velocity, which design and machining details create the local turbulence that starts it, and what to put on the drawing and RFQ so the replacement lasts. The short version: velocity and turbulence decide where it happens, the alloy decides how much velocity the surface can take, and many of the cheapest fixes are geometry details a machinist can hold if the drawing asks for them.
On this page
How do you recognize erosion corrosion and tell it apart from cavitation or uniform corrosion?
Erosion corrosion leaves directional, localized wall loss where flow is fastest or most turbulent: smooth grooves, gullies, waves or horseshoe-shaped pits lined up with the flow, often on clean, bright metal with little corrosion product left on it. Uniform corrosion thins the whole wetted surface evenly, and cavitation leaves a rough, spongy, closely pitted patch in a low-pressure zone.
Location is the strongest clue. Look at the outside radius of elbows, the inlet end of tubes, the bore just downstream of a weld bead, gasket, orifice, partly open valve or step in diameter, the lip of a valve seat, the cutwater and volute of a pump casing, and the leading edges of impeller vanes. AMPP's guidance singles out heat exchanger tubes, tank inlets, welded and flanged joints, and impingement plates and baffles, and points to workmanship: a burr on a cut tube end is enough to disturb the flow locally (AMPP, Erosion).
Carbon steel in hot water or wet steam has a related form called flow-accelerated corrosion, where the surface dissolves rather than being mechanically worn. After the 2004 Mihama Unit 3 condensate pipe rupture, the NRC reported that the carbon steel pipe failed downstream of a flow-measuring orifice, that the wall had thinned from a nominal 10 mm to 0.4 mm at its thinnest point, and that the inside showed a fish-scale pattern characteristic of flow-accelerated corrosion (NRC Information Notice 2006-08). If the part is a cast or wrought copper alloy and the attack is spongy and copper-colored under a white or green crust, check for dezincification or dealloying as well, because a material change, not a geometry change, fixes that.
What causes erosion corrosion in pumps, valves and fittings?
Local wall shear stress that exceeds what the protective film can withstand. The line-average velocity is often within limits; the damage starts where geometry, solids or bubbles raise the local velocity and turbulence far above the line average.
The usual causes on industrial pump, valve and fitting parts are:
- Local turbulence from geometry. Short-radius elbows, sharp port intersections, steps between a fitting bore and its mating pipe, misaligned flanges, protruding gaskets, weld reinforcement and burrs. Tuthill, citing work by Sato and Nagata, reports that water passing a partial obstruction in a condenser tube can reach 8 m/s while the overall velocity stays near 2 m/s, which explains failures just downstream of lodged debris (Tuthill, Guidelines for the use of copper alloys in seawater).
- Average velocity above the alloy's limit. Each alloy has a velocity at which shear strips its film. For copper alloys this limit is well documented, and the same paper shows the shear stress at a condenser tube inlet is about double that further down the tube, which is why inlet-end attack is so common.
- Throttling and pressure drop. A valve run partly open, an undersized control valve or an orifice plate turns pressure into a high-velocity jet. Where the local pressure falls below the vapor pressure, the jet also cavitates.
- Entrained solids, bubbles or droplets. Sand, scale, catalyst fines and slurry particles abrade the film mechanically; droplets do the same in wet steam and compressed air. AMPP lists filtration, settling and water traps as remedies.
- Water chemistry and temperature. For wall thinning of carbon steel piping in single-phase water, the NRC calls erosion/corrosion a complex process that depends on alloy content, temperature, pH and flow velocity, and on the disturbances that piping and fitting layouts add (NRC, Issue 139: Thinning of Carbon Steel Piping in LWRs).
- The wrong alloy for the duty. Carbon steel, cast iron and aluminum alloys have low velocity tolerance in aggressive water, and among copper alloys the tolerance differs severalfold (see the table below). Per AMPP, hardness is not a reliable predictor of erosion corrosion resistance, so a harder grade is not automatically the answer.
How do you diagnose erosion corrosion on a failed or returned part?
Map the wall loss against the flow path, measure the remaining wall, calculate the local velocity at the narrowest section, and confirm the fluid, the solids content and the alloy as delivered. If the thinning sits just downstream of a step, burr, seat or throttling point, the geometry is the root cause.
Use this checklist before raising a nonconformance against the parts:
- Section or split the part along the flow path. Photograph the attack with an arrow showing flow direction. Directional pits and grooves that start at an edge and fade downstream point to erosion corrosion; a uniform loss over the whole wetted area points to the fluid chemistry.
- Measure the remaining wall. Ultrasonic thickness readings on a grid, or a sectioned profile, show the depth and the shape of the loss and whether sister parts in service are near their minimum wall.
- Calculate the local velocity. Divide the actual flow rate by the smallest cross-section the fluid passes through, such as a seat bore, a port intersection or a partly open disc, not the nominal line size. Because velocity scales with the inverse square of the bore, a seat bore half the line diameter carries the same flow about four times faster than the line itself. Compare the local figure with the alloy limits in the next section.
- Check when it failed. A copper-alloy part that thinned in its first weeks points to start-up conditions. Tuthill notes that the protective film takes 2 to 4 weeks to form fully and that only after that can somewhat higher velocities be tolerated, and then for no more than 15% of operating time (Tuthill, Table 3). High-velocity flushing, debris or sand at commissioning can strip a film that never matured.
- Check for cavitation. Rough, spongy damage on the low-pressure side of a vane, seat or orifice, plus noise and vibration in service, means the pressure is dropping below vapor pressure. That is a hydraulic problem first; ASTM G32 covers vibratory cavitation erosion testing for ranking materials.
- Check the fluid. Solids content, entrained air, temperature, pH, dissolved oxygen, chlorine dosing and whether the system sits stagnant between runs.
- Confirm the alloy. Compare the mill or foundry certificate with the drawing and run a positive material identification check on the failed part; a substituted leaded brass or a lower grade stainless can fail where the specified alloy would not. The PMI guide covers what XRF can and cannot confirm.
To qualify a replacement alloy or coating for a slurry or particle-laden duty, a wear-corrosion test beats judgment. ASTM G119, written for metals in liquids and slurries, gives a way to compute how much of the total loss comes from wear and corrosion acting together rather than separately, and notes that reducing either the wear or the corrosion component may significantly reduce total loss (ASTM G119-09(2021)).
Which materials tolerate higher flow velocity before erosion corrosion starts?
In seawater and similar chloride waters, velocity tolerance rises from copper and brass through aluminum brass and the copper-nickels to nickel-aluminum bronze, and austenitic, duplex and super-austenitic stainless steels and titanium are far more tolerant still. Carbon steel, cast iron and aluminum alloys sit at the bottom.
The table collects the published seawater guidelines for copper alloys, which are the alloys where velocity most often decides the material choice for pump, valve and fitting parts. Note that the permitted velocity in piping rises with pipe diameter and with long-radius bends, because small bores and tight bends raise wall shear stress at the same average velocity.
| Alloy (UNS) | Part or duty | Guideline velocity, m/s (ft/s) |
|---|---|---|
| Aluminum brass (C68700) | Condenser tube, once-through | 2.0 (6.5) average maximum |
| 90-10 copper-nickel (C70600) | Condenser tube, once-through | 2.3 (7.5) average maximum |
| 70-30 copper-nickel (C71500) | Condenser tube, once-through | 2.9 (9.5) average maximum |
| 90-10 copper-nickel (C70600) | Pipe 3 in (76 mm) and smaller | 1.5 (5) nominal |
| 90-10 copper-nickel (C70600) | Pipe 4 in (101 mm) and larger, long-radius bends | 3.35 (11) nominal |
| 70-30 copper-nickel (C71500) | Pipe 4 in (101 mm) and larger, long-radius bends | 3.7 (12) nominal |
| Leaded red brass (C83600), silicon bronze (C87600) | Pump impeller or propeller, peripheral speed | 9.1 (30) |
| Tin bronze (C90300), M bronze (C92200) | Pump impeller or propeller, peripheral speed | 10.7 (35) |
| Aluminum bronze (C95200), manganese bronze (C86500) | Pump impeller or propeller, peripheral speed | 15.2 (50) |
| Nickel-aluminum bronze (C95500, C95800), Mn-Al bronze (C95700) | Pump impeller or propeller, peripheral speed | 22.9 (75) |
| Any copper alloy | Any tube, minimum to keep sediment moving | 0.9 (3) minimum |
| Austenitic and super-austenitic stainless | Piping | About 10 (33) practical maximum, set by noise and pump cost, not by erosion corrosion |
Stainless steel behaves differently. The Nickel Institute reports a depth of attack of only 0.06 mm on 316L and none on a 6% molybdenum austenitic grade after 30 days at 30 m/s and 30 °C, and says the practical limit for stainless seawater pipework is about 10 m/s because of noise and pump cost (Nickel Institute, Copper alloys versus stainless steels for seawater cooling systems). The risk for stainless is crevice corrosion, not velocity: the same publication says all stainless steels are susceptible in seawater, cites a PREN above 40 as the usual requirement, and reports that only the 6% molybdenum and superduplex grades were free of crevice attack in the tests it shows (Nickel Institute 11027). Stagnant periods and deposits make crevices worse, so drain and flush stainless parts for standby. For cavitation, Tuthill's ranking puts cobalt-base hard facing, titanium, 300-series and precipitation-hardened stainless, and nickel alloys 625, 718 and C at the top; nickel-aluminum bronze and alloys 400 and K-500 close behind; manganese bronze and G and M bronze lower; and carbon steel, cast iron and aluminum in the group not used where cavitation may occur unless heavily protected.
Upgrading the alloy has knock-on effects. A stainless or titanium part installed in a copper-alloy system becomes a large cathode and can speed up attack on the copper parts around it, which is covered in the galvanic corrosion guide. And moving from 304 to 316 helps with chlorides but does not change velocity tolerance much; the 304 vs 316 guide explains when 316 is worth it.
Which design and machining changes stop erosion corrosion, and which fits your part?
Remove the local turbulence first, then bring the velocity at the narrowest section within the alloy's limit, then add a sacrificial or replaceable wear element at the unavoidable impingement point, and only then upgrade the alloy. AMPP's design advice follows the same order: lower velocity, avoid abrupt direction changes and flow restrictions, align joints, and thicken the wall where attack is expected.
Geometry fixes are cheap when they are on the drawing before the part is machined and expensive afterward. Use the table to match the symptom to the change.
| Where the wall is lost | Preferred fix | If that is not possible |
|---|---|---|
| Just downstream of a step between fitting bore and pipe | Match the fitting bore to the mating pipe inside diameter, or add a taper transition | Add wall allowance downstream and a thickness check point |
| At intersecting drilled or bored ports in a valve or manifold body | Blend the intersection with a radius and remove all burrs; avoid blind dead ends in the flow path | Relocate or enlarge the port so the jet does not hit a wall directly |
| Valve seat and disc lip, throttling valve body | Size the valve so it does not run nearly closed; use a trim or seat designed for throttling | Hard-faced or cobalt-alloy overlay seat, replaceable seat ring or a higher-velocity alloy for the trim |
| Outside radius of an elbow or tee | Long-radius elbow; avoid closely spaced bends | Thicker wall or replaceable wear pad on the outer radius |
| Downstream of an orifice, reducer or control valve | Increase the straight run after the restriction; use a gradual reducer | Low-alloy or stainless spool for carbon steel lines in hot water service; scheduled thickness checks |
| Pump volute cutwater, impeller vane tips | Run the pump near its best efficiency point; check suction pressure for cavitation | Nickel-aluminum bronze, duplex stainless or hard-faced wear rings and impeller |
| Tank or vessel wall opposite an inlet | Point the inlet toward the center, away from the wall | Replaceable impingement plate |
| Everywhere, with sand or slurry | Filter or settle the solids | Larger diameter to cut velocity; wear-resistant lining or coating tested for the slurry |
Two cautions apply to every row. A coating or lining only helps while it is intact: AMPP notes that protective coatings and cathodic protection may reduce the rate of attack, but a coating chipped at a port edge exposes bare metal exactly where the flow is most aggressive, so edges that carry a coating need a radius the coating can cover. And for heat exchanger tube inlets, AMPP describes tapered replaceable ferrules as a way to take the turbulent entry zone off the tube itself. Edge breaks on internal intersections are covered in the fillet vs chamfer guide.
What should the drawing, RFQ and PO say so erosion corrosion does not recur?
State the fluid, the flow rate or design velocity and the solids content, name the exact alloy by UNS number, and mark the wetted flow-path features where edges, steps and finish matter. A supplier can hold a blend radius or a matched bore only if the drawing calls it out.
- Service line. For example, “SERVICE: SEAWATER, CONTINUOUS, ENTRAINED SAND POSSIBLE” or “SERVICE: BOILER FEEDWATER.” Add the design flow rate so the shop and your reviewer can check the velocity at the narrowest section.
- Alloy by UNS number and product form. “C95800 NICKEL-ALUMINUM BRONZE CASTING” or “S32205 DUPLEX BAR,” never “bronze” or “stainless”; require certificates and a PMI check if a substitution would be costly.
- Flow-path edge note. “ALL INTERSECTIONS IN FLOW PATH BLENDED, NO BURRS, NO SHARP STEPS”, with the blend radius dimensioned on the features that matter. The general break sharp edges note is not enough on its own for internal ports.
- Bore matching. Dimension the fitting bore to the mating pipe schedule and call out any taper transition, so the joint does not create a step.
- Wetted surface finish. Specify roughness on bores, seats and volutes in the flow path; the surface finish chart covers how to call it out.
- Wall allowance and check points. Where impingement cannot be designed out, add erosion allowance to the minimum wall and mark the ultrasonic thickness check points for service inspection.
- Castings. For cast bodies, state the radiographic or other acceptance level for wetted walls, because porosity opened by machining in the flow path is a turbulence starter.
How is an erosion-sensitive pump, valve or fitting part handled on a Saola order?
Bodies, seats, bores and ports are machined by CNC turning and milling at our lead CNC plant in Vietnam, from bar or from castings. Any castings and coatings come from partner plants we manage, and the quote names the production site and country of origin for each step.
Machined valve and manifold bodies, fitting bores, seat pockets and port intersections run on CNC turning and CNC milling at T.K Viet Nam Precision, our lead CNC plant. When a drawing carries a wetted flow path, the engineering review looks for the details in this guide, such as a port intersection with no blend callout, a fitting bore that steps against the mating pipe, or a generic alloy name, and asks before quoting rather than after. Brass and bronze castings for valve and fitting bodies come through sand casting at partner plants we manage (for other cast alloys, ask before you design around them), and any coating or finish goes to a managed surface finishing partner; those plants may not be in Vietnam, and the quote states where each step is done. The alloy, the certificates, any PMI check and the inspection plan for the flow-path features are agreed in writing. To start, send the drawing with the service line and flow-path notes through the quote form.
Frequently asked
Is erosion corrosion the same as cavitation?
No, though they often appear together and both are flow-driven. Erosion corrosion comes from shear and impingement stripping the protective film; cavitation comes from vapor bubbles forming in a low-pressure zone and collapsing against the surface, hammering it. Cavitation damage is rough and spongy and sits on the low-pressure side of vanes, seats and orifices. The cure for cavitation is mainly hydraulic: more suction pressure, less pressure drop or a better trim.
Will a harder material stop erosion corrosion?
Not reliably. AMPP cautions that hardness alone does not predict erosion corrosion resistance; in most water services what matters more is how adherent and quick to reform the protective surface film is. Hard facing does help where solid particles dominate, and at valve seats that see both particles and high velocity.
Is 316 stainless enough for a seawater pump or valve part?
For velocity, usually yes: austenitic stainless has a very erosion-resistant film. The weak point is crevices: under seals, gaskets and deposits, and worst when the pump stands idle with seawater inside. The Nickel Institute cites PREN above 40 for seawater and found 316L attacked in crevice tests, so either drain and flush during standby or move to a superduplex or 6% molybdenum grade (Nickel Institute 11027).
Why did the leak appear just downstream of a fitting or valve?
Because the fitting or valve raised the local velocity and turbulence above the line average. A step in bore, a protruding gasket, a weld bead, a burr or a partly open valve creates a jet and a reattachment zone a short distance downstream, and the wall shear there is far higher than in the straight pipe. Remove the disturbance or add wall where it lands.
Is erosion corrosion a supplier defect?
Usually it is a duty or design problem, not a manufacturing defect. The supplier side to check is a substituted alloy, burrs or unblended intersections the drawing asked to be removed, a bore that does not match the mating pipe, and casting porosity opened in a wetted wall. If none of those is present, look at velocity, solids and the operating point.
Sources
- Erosion, AMPP (Association for Materials Protection and Performance), accessed October 2026
- Guidelines for the use of copper alloys in seawater, Arthur H. Tuthill, Materials Performance (NACE), September 1987, hosted by the Copper Development Association
- Copper alloys versus stainless steels for seawater cooling systems, Nickel Institute publication 11027, accessed October 2026
- Information Notice 2006-08: Secondary piping rupture at the Mihama Power Station in Japan, U.S. Nuclear Regulatory Commission, March 16, 2006
- Issue 139: Thinning of Carbon Steel Piping in LWRs (NUREG-0933), U.S. Nuclear Regulatory Commission, accessed October 2026
- ASTM G119-09(2021), Standard Guide for Determining Synergism Between Wear and Corrosion (scope), ASTM International, 2021