API 570 EXAM STUDY GUIDE [MODULE 2 : PART 12] CAUSTIC CORROSION

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API 570 EXAM STUDY GUIDE
API 571 : Caustic Corrosion
Module 2 : Part 12

Clause 4.3.10 Caustic Corrosion in API 571 is a damage mechanism due to the concentration of Caustic or Alkaline. This post contains important points of caustic corrosion based on API 571 (Clause 4.3.10 ; Caustic Corrosion ; API 571 Second Edition)

Description Of Damage

  • Localized corrosion
  • Due to concentration of Caustic salts
  • Due to concentration of alkaline salts
  • Occurs under evaporative or high heat transfer conditions
  • General corrosion can also occur depending on alkali or caustic strength.
That means when there is caustic and alkaline salts in a fluid, their concentration increases due to evaporation or heat transfer. Due to that concentration corrosion of material occurs.
Two condition are needed for caustic corrosion to occur, 
  1. Presence of Caustic of alkaline salts
  2. A condition for increasing its concentration

Affected Materials

  • CS
  • Low Alloy Steel
  • 300 Series SS

Critical Factors

Caustic Salts

Caustic is intentionally added to process streams.

Alkaline Salts

Alkaline salts enter process streams through leaks in condensers or process equipment.
Concentrating Mechanisms: Departure from Nucleate Boiling (DNB), evaporation and deposition.

Affected Units or Equipment

Most often associated with,
  1. Boilers
  2. Steam Generating equipment
  3. Heat exchangers
Other equipment's,
  • Preheat exchangers
  • Furnace tubes
  • Transfer Lines
Units
  1. Crude discharge unit
  2. Units which use caustic for removing sulphur compounds from product streams.

Appearance or Morphology of Damage

  • Localized metal loss
  • Appear as groove in boiler tube
  • Locally thinned areas under insulating deposits
  • Localized gouging may result along a waterline where corrosives concentrate
  • In vertical tubes - Circumferential groove
  • In horizontal or sloped tubes - grooving at the top of the tube or as longitudinal grooves on opposite sides of the tube.

Prevention / Mitigation

In steam generating equipment, this can be best prevented through proper design. 
Other methods are,
1. Reduce free caustic by,
    • ensuring adequate water flooding and water flow
    • Ensuring proper burner management to minimize hot spots on heater tubes
    • Minimizing the ingress of alkaline producing salts into condensers.
2. In process equipment, caustic injection facilities should avoid the concentration of caustic on hot metal surfaces.
Alloy 400 and some other nickel base alloys exhibit much lower corrosion rates.

Inspection and Monitoring

  • Localized losses due to caustic corrosion may be difficult to locate.
  • UT scans and radiography may be used
  • Steam generating equipment : Visual inspection using boroscope
  • Process equipment: UT thickness gauging

Related Mechanisms

  1. Caustic gouging or ductile gouging
  2. Departure from Nucleate Boiling (DNB)
QUESTIONS

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