Erosion – Corrosion | Materials And Corrosion Control
| Damage Mechanism | Erosion/Erosion – Corrosion |
| Damage Description | · Erosion is the accelerated mechanical removal of surface material as a result of relative movement between, or impact from solids, liquids, vapor or any combination thereof.
· Erosion-corrosion is a description for the damage that occurs when corrosion contributes to erosion by removing protective surface films or scales, or by exposing the metal surface to accelerated corrosion under the combined action of erosion and corrosion. · Metal loss rates depend on the velocity and concentration of impacting medium (i.e., particles, liquids, droplets, slurries, and two-phase flow), the size and hardness of impacting particles, the hardness and corrosion resistance of material subject to erosion and the angle of impact. · Damage is characterized by localized loss in thickness, as pits, grooves, gullies, waves, rounded holes and valleys, often exhibiting a directional pattern. Failures can occur in a relatively short time. |
| Affected Materials | · All metals, alloys and refractories
· Some alloys have recognized fluid & gas velocity limits to minimize erosion/erosion-corrosion, e.g., alloys for seawater service |
| Control Methodology | · Consider design improvements involving changes in shape, geometry and materials selection, e.g., increasing the pipe diameter to decrease velocity; streamlining bends to reduce impingement; increasing the wall thickness; and using replaceable impingement baffles · Improved resistance to erosion is usually achieved through increasing substrate hardness using harder alloys, hardfacing or surface-hardening treatments. In FCCUs, erosion resistant refractories in cyclones and slide valves have been successful · Erosion-corrosion is best mitigated by using more corrosion-resistant alloys and/or altering the process environment to reduce corrosivity, for example, deaeration, condensate injection or the addition of inhibitors. · Heat exchangers utilize impingement plates and occasionally tube ferrules to minimize erosion problems. |
| Monitoring Techniques | · Monitor vulnerable locations in piping systems, such as bends, elbows, tees and reducers; injection points, downstream of letdown valves and block valves; pumps; blowers; impellers; agitators; heat exchanger tubing; measuring device orifices; turbine blades; nozzles; ducts and vapor lines; scrapers; cutters; and wear plates. · Visual examination of suspected or troublesome areas, as well as UT or RT checks to map the extent of metal loss · Specialized corrosion coupons and on-line corrosion monitoring electrical resistance probes have been used in some applications · IR scans are used to detect refractory loss on stream |
| Inspection Frequency | · OSI TMLs for piping |
| KPIs | · Do not exceed recognized erosional velocity limits in piping (SAES-L-132) · Determine if solids present (e.g., sand, corrosion products/black powder) that can accelerate damage rates. |
| Reference Resources (Standards/GIs/BPs) | · API RP 571 (DM #20)
· API STD 570 |
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