Caustic Corrosion | Materials And Corrosion Control
| Damage Mechanism | Caustic Corrosion |
| Damage Description | Localized corrosion due to the concentration of caustic or alkaline salts that usually occurs under evaporative or high heat transfer conditions. However, general corrosion can also occur depending on alkali or caustic solution strength. |
| Affected Materials | Carbon Steel, low alloy steels and 300 Series Stainless Steel. |
| Control Methodology | · In steam generating equipment, caustic corrosion is best prevented through proper design. Damage can be minimized by reducing the amount of free caustic, by ensuring adequate water flooding and water flow, by ensuring proper burner management to minimize hot spots on heater tubes, and by minimizing the ingress of alkaline producing salts into condensers.
· In process equipment, caustic injection facilities should be designed to allow proper mixing and dilution of caustic in order to avoid the concentration of caustic on hot metal surfaces. · Carbon steel and 300 Series SS have serious corrosion problems in high strength caustic solutions above about 150°F (66°C). Alloy 400 and some other nickel base alloys exhibit much lower corrosion rates. |
| Monitoring Techniques | · For process equipment, UT thickness gauging is useful to detect and monitor general corrosion due to caustic. However, localized losses due to caustic corrosion may be difficult to locate.
· Injection points should be inspected in accordance with API STD 570. · UT scans and radiography can be used. · Steam generation equipment may require visual inspection with the use a boroscope. |
| Inspection Frequency | · UT, coupons and visual inspection at T&I |
| KPIs | Exposure to high solution strength caustic can result in general corrosion of carbon steel above 175°F (79°C) and very high corrosion rates above 200°F (93°C). |
| Reference Resources (Standards/GIs/BPs) | · API RP 571 (DM #19) |
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