How it works
Most metals in service survive because a thin surface film — an oxide, a carbonate scale, a corrosion-product layer — slows the reaction between metal and fluid. Erosion-corrosion begins when the flow removes that film mechanically, by shear stress, turbulence, impinging droplets or suspended particles. The bare metal corrodes, the film tries to reform, the flow strips it again. Neither mechanism alone explains the rate: the synergy between mechanical removal and electrochemical attack produces losses much larger than the sum of the two acting separately.
Velocity is the dominant variable, but the threshold depends on the alloy, the fluid chemistry, the temperature and the solids content. Copper alloys, carbon steel and cast iron are the classic victims; alloys whose passive film reforms quickly tolerate higher velocities.
Where it shows up in plants
Damage concentrates wherever the flow changes direction or speed:
- pump volutes, cutwaters, impeller outlets and discharge nozzles;
- elbows, tees, reducers and the pipe immediately downstream of control valves and orifice plates;
- heat exchanger tube inlets, where flow contracts entering the tube;
- impingement zones opposite inlet nozzles in vessels.
The surface is typically clean and smooth, with grooves, gullies or horseshoe-shaped pits aligned with the flow, the rounded end pointing upstream. That appearance separates it from cavitation damage, which is rough and sponge-like, and from particle abrasion, which leaves polished scoring without a corrosion component.
A related mechanism, flow-accelerated corrosion of carbon steel in water and wet steam, dissolves the protective magnetite layer chemically rather than mechanically; it is managed through water chemistry and material selection and should not be confused with erosion-corrosion in a failure analysis.
How it is measured and managed
Wall loss is tracked by ultrasonic thickness readings at condition monitoring locations placed at the flow disturbances listed above, not on straight runs. Remedies act on the cause or on the surface:
- reduce velocity or turbulence — larger lines, long-radius elbows, relocating throttling points;
- upgrade to an alloy with a faster-healing passive film;
- fit sacrificial or replaceable wear items such as tube inlet ferrules or inserts;
- rebuild lost metal and apply a smooth, chemically resistant coating that isolates the metal from the fluid.
Restoring the original hydraulic profile matters as much as the material: a rebuilt surface left rough or stepped generates the very turbulence that started the attack.