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API 571: 4.3.10 Caustic Corrosion

Terms in this set (15)

What is caustic corrosion?
Caustic corrosion is a corrosion mechanism caused by sodium or potassium hydroxide and alkaline salts in high conentrations. Typically characterized by localized metal loss
Where is caustic corrosion likely to occur?
In high heat situations and evaporative environments
Sodium hydroxide in boilers are present in normal controlled conditions, therefore, for caustic corrosion to occur a mechanism of concentration must be present, what are mechanisms of concentration?
Departure from nucleate boiling (DNB)
Deposition (wick boiling)
Evaporation
What is DNB?
A depositing action where at nucleate points, water will bubble and rise towards the fluid flow... at this interface, the difference in temperature of the fluid flow and the bubbles will cause the bubbles to collapse and deposits of sodium hydroxide and other dissolved solids can begin to form. This will start to corrode (inside a boiler) the thin magnetite protective covering and cause metal loss.
The deposit mechanism that aids caustic corrosion?
Deposits can begin to form over the sufrace and stop the bulk water from accessing thesurface, due to the heat input, steam will form under the deposits and escape thus creating channels, by capillary attraction can attract more deposits
The mechanism for evaporation?
Im horozontal/sloping tubes, critical heat flux is lower (heats up quicker) than in vertical tubes. The relationship between critical heat flux and water flow rates can affect the rate of evaporation.
What are the affected materials?
Carbon steel, low alloy steel and 300 series SS
What is the major contriburing factor for caustic corrosion?
The presence of sodium and potassium hydroxide.
Caustic (sodium hydroxide) is intentionally added at controlled concentrations to boiler feed water in many treatment programs to maintain proper pH. This ensures the formation and retention of passive, protective
magnetite layers on boiler tube surfaces.
At low concentration there is no issue however, concentration mechanisms can act as areas where caustic can become highly concentrated thus corrosive
A concentrating mechanisms must exist to build up caustic strength, name 3 mechanisms?
DNB, deposition and evaporation
How does caustic corrosion appear on horizontal/sloping tubes?
Parallel or converging longitudinal trenches opposite each other may form. If the pipe is full, then the parallel trenches will merge into one trench at the crown (12oclock) position. If the pipe is vertical, then damage can appear circumferentially.
At what temperature does carbon steel and 300 series SS have serious corrosion problems?
above about 66 degrees/150F
Inspection....
UT/RT, injection points in accordance with API 570, steam generation with use of internal boroscope.
What are the caustic solutions?
sodium and potassium hydroxide.
In boiler tubes sodium and potassium hydroxide can affect what protective layer?
the magnetite layer.
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