API 510 Corrosion Mechanisms & Damage Assessment 2 — Questions and Answers
Question 1: Which type of corrosion is characterized by a localized attack that produces small pits or cavities on the metal surface?
- Uniform corrosion
- Pitting corrosion (Correct answer)
- Galvanic corrosion
- Crevice corrosion
Correct answer: Pitting corrosion
Pitting corrosion is a localized form that creates small pits or cavities, often making it more dangerous than uniform corrosion because it is harder to detect.
Question 2: In API 510 inspection, what is 'stress corrosion cracking' (SCC)?
- Cracking from mechanical overload combined with cyclic fatigue
- Cracking from the combined action of tensile stress and a corrosive environment (Correct answer)
- Cracking caused by rapid cooling of hot metal surfaces
- Cracking due to hydrogen embrittlement alone
Correct answer: Cracking from the combined action of tensile stress and a corrosive environment
SCC results from the simultaneous presence of tensile stress and a specific corrosive environment, producing cracks that may not be visible without NDE.
Question 3: Which of the following environments most commonly causes stress corrosion cracking in carbon steel pressure vessels?
- Dry nitrogen gas
- Caustic (sodium hydroxide) solutions (Correct answer)
- Liquid propane
- Clean water below 50°F
Correct answer: Caustic (sodium hydroxide) solutions
Caustic stress corrosion cracking (caustic embrittlement) occurs in carbon steel exposed to concentrated sodium hydroxide solutions, especially at elevated temperatures.
Question 4: What is 'hydrogen-induced cracking' (HIC) as it relates to pressure vessel inspection?
- Cracking caused by external hydrogen flame impingement
- Planar cracks in steel caused by hydrogen diffusing into the metal and collecting at inclusions (Correct answer)
- Cracking from hydrogen gas pressure exceeding design limits
- Surface oxidation from hydrogen sulfide exposure
Correct answer: Planar cracks in steel caused by hydrogen diffusing into the metal and collecting at inclusions
HIC forms when atomic hydrogen diffuses into steel and recombines at internal defects, creating stepwise planar cracks without external stress being required.
Question 5: Erosion-corrosion in pressure vessels is best described as:
- Galvanic attack accelerated by fluid turbulence
- Accelerated corrosion from the combined effect of corrosive fluid and mechanical erosion (Correct answer)
- Uniform thinning caused by high-velocity steam only
- Corrosion limited to weld heat-affected zones
Correct answer: Accelerated corrosion from the combined effect of corrosive fluid and mechanical erosion
Erosion-corrosion occurs when the mechanical removal of protective films by flowing fluid is combined with chemical corrosion, accelerating metal loss.
Question 6: During an API 510 inspection, an inspector finds corroded areas showing a rough, uneven surface with deeply pitted areas and smooth surrounding metal. What corrosion type is most likely?
- Uniform corrosion
- Intergranular corrosion
- Pitting corrosion (Correct answer)
- Galvanic corrosion
Correct answer: Pitting corrosion
Pitting corrosion produces localized attack with deep pits surrounded by relatively smooth base metal, distinguishing it from uniform corrosion.
Question 7: Which non-destructive examination (NDE) technique is most effective for detecting and measuring pitting corrosion in a pressure vessel shell?
- Visual inspection (VT) alone
- Magnetic particle testing (MT)
- Ultrasonic thickness measurement (UT) (Correct answer)
- Liquid penetrant testing (PT)
Correct answer: Ultrasonic thickness measurement (UT)
Ultrasonic thickness measurement can accurately detect and quantify remaining wall thickness at pitted areas, making it the preferred method for pitting assessment.
API 510 Corrosion Mechanisms & Damage Assessment 2
Which type of corrosion is characterized by a localized attack that produces small pits or cavities on the metal surface?