Knowledge and Expertise

🧠 API 571 — Knowledge and Expertise in Corrosion Processes

Based on American Petroleum Institute API 571 – Damage Mechanisms Affecting Fixed Equipment in the Refining Industry

👉 API 571 is the core corrosion/damage mechanism reference used for:

• RBI (API 580/581)
• API 510
• API 570
• Fitness-for-service
• Inspection planning

 

 

 

🔥 WHAT API 571 IS ABOUT

API 571 explains:

✔ How equipment fails
✔ Why damage occurs
✔ Where damage appears
✔ What conditions cause it
✔ How inspectors detect it

 

 

 

 

📘 MAIN CORROSION / DAMAGE CATEGORIES

🔴 1. GENERAL CORROSION

 

Characteristics

• Uniform metal loss
• Predictable thinning
• Easier to monitor

Common Detection

✔ UT thickness readings

 

 

 

 

🔴 2. PITTING CORROSION

 

Characteristics

• Localized attack
• Deep pits
• Dangerous because hard to detect

Common Cause

✔ Chlorides

 

 

 

 

🔴 3. CORROSION UNDER INSULATION (CUI)

 

Characteristics

• Hidden corrosion
• Moisture trapped under insulation
• Very common RBI topic

High-Risk Temperature Range

Typically:

• ~25°C–175°C (varies)

 

 

 

🔴 4. STRESS CORROSION CRACKING (SCC)

 

Characteristics

• Branching cracks
• Requires:
  • Tensile stress
  • Corrosive environment

Common Example

✔ Chloride SCC in stainless steel

 

 

 

 

🔴 5. HYDROGEN-INDUCED CRACKING (HIC)

 

Characteristics

• Internal stepwise cracking
• Wet H₂S environments

Common Material

✔ Carbon steel

 

 

 

 

🔴 6. EROSION/CORROSION

 

Characteristics

• High flow velocity
• Turbulence damage
• Common at:
  • Elbows
  • Reducers
  • Injection points

 

 

🔴 7. FATIGUE

 

Characteristics

• Cyclic loading
• Vibration/thermal cycling
• Beach marks

 

 

 

🔴 8. CREEP

 

Characteristics

• High temperature damage
• Long-term exposure
• Grain boundary voids/cracks

 

 

 

🔥 API 571 EXPERT THINKING

What damage mechanism exists?
↓
What conditions cause it?
↓
Where will it occur?
↓
How can it be detected?
↓
How severe is it?

 

 

 

📊 MOST IMPORTANT API 571 KNOWLEDGE

Damage	Typical Cause
CUI	Moisture under insulation
SCC	Stress + corrosive environment
HIC	Wet H₂S
Erosion	Velocity/turbulence
Fatigue	Cyclic stress
Creep	High temperature

 

🔍 COMMON HIGH-RISK LOCATIONS

✔ Dead legs
✔ Elbows
✔ Weld HAZ
✔ Injection points
✔ Low points
✔ Under insulation

 

 

 

🧪 COMMON INSPECTION METHODS

Damage	Method
General corrosion	UT
SCC	PT/MT/UT
HIC	UT/advanced UT
CUI	UT + insulation removal
Fatigue cracks	PT/MT

 

⚠️ COMMON API 571 EXAM TRAPS

❌ Confusing SCC vs HIC
❌ Ignoring operating conditions
❌ Using wrong inspection method
❌ Missing hidden corrosion (CUI)

 

 

 

🎯 GOLDEN API 571 RULE

“Damage mechanisms are driven by environment, material, stress, and temperature.”

 

 

 

⚡ ULTRA-FAST MEMORY

• CUI = hidden moisture corrosion
• SCC = branching cracks
• HIC = wet H₂S cracking
• Erosion = elbows/velocity
• Fatigue = cyclic loading
• Creep = high temperature

 

 

 

🏁 FINAL ONE-LINE

“API 571 teaches why equipment fails and how inspectors recognize damage mechanisms before failure occurs.”