API 510 Corrosion Mechanisms and Monitoring 3 — Questions and Answers
Question 1: Which nondestructive examination (NDE) method is best suited for detecting hydrogen-induced cracking (HIC) in plate steel?
- Magnetic particle testing (MT)
- Dye penetrant testing (PT)
- Automated ultrasonic testing (AUT) with phased array (Correct answer)
- Visual inspection (VT)
Correct answer: Automated ultrasonic testing (AUT) with phased array
AUT with phased array ultrasonics can detect the internal, planar HIC blisters and step-wise cracking that occur within the steel plate, which surface methods cannot find.
Question 2: Naphthenic acid corrosion in refinery equipment is characterized by which distinctive feature?
- Uniform thinning with a smooth, polished surface appearance
- Pitting and grooving at areas of high turbulence and velocity (Correct answer)
- Intergranular cracking along weld heat-affected zones
- Scale buildup with underlying metal loss in a layered pattern
Correct answer: Pitting and grooving at areas of high turbulence and velocity
Naphthenic acid corrosion produces characteristic pitting and grooving at high-turbulence locations because the corrosion product (iron naphthenate) is soluble and is swept away by flow.
Question 3: What does the term 'short-term corrosion rate' mean in the context of API 510 remaining life calculations?
- Corrosion rate measured over the last inspection interval only (Correct answer)
- Rate calculated from the first and most recent thickness readings
- Average corrosion rate over the entire vessel service life
- Rate extrapolated from coupon data over a 30-day test period
Correct answer: Corrosion rate measured over the last inspection interval only
The short-term corrosion rate in API 510 is calculated from the most recent inspection interval, reflecting current operating conditions more closely than the long-term rate.
Question 4: Which condition is required for chloride stress corrosion cracking (Cl-SCC) to occur in austenitic stainless steel?
- Temperature above 600°F (315°C) and presence of hydrogen sulfide
- Tensile stress, chloride ions, and temperature typically above 140°F (60°C) (Correct answer)
- Cold working to greater than 30% reduction in area before service
- Sensitization at grain boundaries combined with oxidizing acid exposure
Correct answer: Tensile stress, chloride ions, and temperature typically above 140°F (60°C)
Cl-SCC requires the simultaneous presence of tensile stress, chloride ions, and elevated temperature (typically above 140°F/60°C) to initiate and propagate cracks.
Question 5: In high-temperature service above 750°F (400°C), what corrosion mechanism causes carbon steel to lose strength and ductility through carbon removal from the steel matrix?
- High-temperature oxidation (scaling)
- High-temperature sulfidation
- High-temperature hydrogen attack (HTHA) (Correct answer)
- Carburization
Correct answer: High-temperature hydrogen attack (HTHA)
HTHA occurs when hydrogen reacts with carbon in the steel to form methane gas, which decarburizes the steel and creates internal fissures, reducing mechanical properties.
Question 6: A corrosion monitoring program uses electrical resistance (ER) probes and weight loss coupons. What is the primary advantage of ER probes over coupons?
- ER probes measure localized pitting more accurately than coupons
- ER probes provide continuous, real-time corrosion rate data without retrieval (Correct answer)
- ER probes are more resistant to biofouling than metal coupons
- ER probes can distinguish between different corrosion mechanisms
Correct answer: ER probes provide continuous, real-time corrosion rate data without retrieval
ER probes provide continuous online monitoring without requiring retrieval and re-exposure periods, allowing real-time tracking of corrosion rate changes with process upsets.
Question 7: Which corrosion mechanism is specifically associated with amine treating units in gas processing plants and is identified by thin-edged, sharp cracks typically in weld HAZ regions?
- Amine corrosion (general thinning from amine degradation products)
- Amine stress corrosion cracking (amine SCC) (Correct answer)
- Carbonate stress corrosion cracking
- Wet H₂S sulfide stress cracking (SSC)
Correct answer: Amine stress corrosion cracking (amine SCC)
Amine SCC produces sharp, thin-edged cracks preferentially in the weld HAZ of carbon steel equipment due to residual tensile stresses combined with amine environment exposure.
API 510 Corrosion Mechanisms and Monitoring 3
Which nondestructive examination (NDE) method is best suited for detecting hydrogen-induced cracking (HIC) in plate steel?