API 510 Corrosion Mechanisms and Monitoring 5 — Questions and Answers
Question 1: Which inspection technique is specifically recommended for detecting stress corrosion cracking (SCC) that may be tightly closed and not visible by conventional ultrasonic shear wave methods?
- Magnetic flux leakage (MFL) testing
- Time-of-flight diffraction (TOFD) or phased array UT (Correct answer)
- Infrared thermography
- Eddy current testing with surface probes
Correct answer: Time-of-flight diffraction (TOFD) or phased array UT
TOFD and phased array UT are more effective than conventional shear wave UT for detecting tight SCC because they use diffraction signals from crack tips rather than reflection from crack faces.
Question 2: Caustic stress corrosion cracking (caustic SCC) in pressure vessels most commonly occurs at what location and under what condition?
- Base metal away from welds when caustic concentration exceeds 5%
- Weld and HAZ regions under tensile stress with concentrated caustic above 120°F (50°C) (Correct answer)
- External surfaces exposed to atmospheric moisture and cleaning agents
- Low-alloy steel components operating above 700°F (370°C) in caustic service
Correct answer: Weld and HAZ regions under tensile stress with concentrated caustic above 120°F (50°C)
Caustic SCC preferentially attacks weld and HAZ regions where residual tensile stresses are highest, and requires concentrated caustic (typically >5%) above approximately 120°F (50°C).
Question 3: When reviewing a corrosion monitoring program, an inspector finds that ultrasonic thickness measurements at permanent monitoring points show readings jumping erratically between successive inspections. What is the most likely cause?
- Accelerating corrosion rate due to process change
- Measurement error from inconsistent probe placement or surface preparation (Correct answer)
- Hydrogen blistering causing internal delamination of the steel
- Thermal expansion differences between measurement sessions
Correct answer: Measurement error from inconsistent probe placement or surface preparation
Erratic UT readings at permanent monitoring points most commonly result from inconsistent probe placement, coupling variation, or surface condition differences between inspection rounds.
Question 4: In the context of API 510, what is 'injection point corrosion' and why does it require special inspection attention?
- Corrosion at flanged connections where chemical injection fittings are installed
- Accelerated, localized corrosion downstream of chemical or water injection nozzles due to mixing turbulence and concentration gradients (Correct answer)
- General wall thinning at injection ports from high-velocity fluid impingement
- Galvanic corrosion between the injection fitting material and the vessel shell
Correct answer: Accelerated, localized corrosion downstream of chemical or water injection nozzles due to mixing turbulence and concentration gradients
Injection point corrosion is accelerated, localized attack downstream of injection locations where turbulence, incomplete mixing, and concentration gradients create highly aggressive conditions.
Question 5: Which alloying element is most effective at improving the resistance of carbon steel to high-temperature sulfidic (H₂S) corrosion above 500°F (260°C)?
- Nickel additions above 3%
- Chromium additions, with 5-9% Cr providing significantly better resistance (Correct answer)
- Molybdenum additions above 2%
- Silicon additions above 1%
Correct answer: Chromium additions, with 5-9% Cr providing significantly better resistance
Chromium is the primary alloying element for sulfidic corrosion resistance; the Modified McConomy curves show that 5-9% Cr steels have dramatically lower corrosion rates than carbon steel.
Question 6: A pressure vessel operating in wet CO₂ service develops general thinning with a pH-dependent corrosion rate. What corrosion product is responsible for the partial protective film that forms?
- Magnetite (Fe₃O₄) scale formed at temperatures above 500°F
- Iron carbonate (FeCO₃) scale, also called siderite, formed at higher pH and temperature (Correct answer)
- Ferrous sulfide (FeS) from trace H₂S contamination in the CO₂ stream
- Chromium oxide (Cr₂O₃) from chromium content in the steel
Correct answer: Iron carbonate (FeCO₃) scale, also called siderite, formed at higher pH and temperature
In wet CO₂ corrosion (sweet corrosion), iron carbonate (FeCO₃/siderite) forms at higher pH and temperatures, providing partial protection that reduces corrosion rate when the film is stable.
Question 7: What corrosion mechanism is described as 'liquid metal embrittlement' (LME) and what is a common example relevant to pressure vessel inspection?
- Degradation of steel by dissolved hydrogen causing internal blistering at inclusions
- Brittle fracture of a solid metal caused by contact with a liquid metal, such as zinc (galvanizing) on carbon steel at elevated temperatures (Correct answer)
- Intergranular attack of aluminum alloys by mercury from process streams
- Embrittlement of copper alloys by ammonia in the presence of moisture
Correct answer: Brittle fracture of a solid metal caused by contact with a liquid metal, such as zinc (galvanizing) on carbon steel at elevated temperatures
LME is brittle fracture of solid metal in contact with a liquid metal; zinc from galvanized coatings or fasteners contacting carbon or stainless steel at elevated temperatures is a common refinery example.
API 510 Corrosion Mechanisms and Monitoring 5
Which inspection technique is specifically recommended for detecting stress corrosion cracking (SCC) that may be tightly closed and not visible by conventional ultrasonic shear wave methods?