API 510 Corrosion Mechanisms and Monitoring 4 — Questions and Answers
Question 1: According to API 510, when calculating remaining life using both short-term and long-term corrosion rates, which rate should be used for the next inspection interval?
- Always the long-term rate as it reflects true service history
- Always the short-term rate as it reflects current process conditions
- The more conservative (higher) of the two rates (Correct answer)
- The average of the short-term and long-term rates
Correct answer: The more conservative (higher) of the two rates
API 510 requires using the more conservative (higher) corrosion rate between short-term and long-term calculations to ensure adequate safety margin in remaining life assessments.
Question 2: What is the primary purpose of installing corrosion-resistant alloy (CRA) cladding or weld overlay on the internal surface of a pressure vessel?
- To increase the structural strength of the vessel shell
- To provide a corrosion-resistant surface while using lower-cost base material (Correct answer)
- To reduce heat transfer and prevent external corrosion
- To allow operation at higher pressures than the base material permits
Correct answer: To provide a corrosion-resistant surface while using lower-cost base material
CRA cladding or weld overlay provides corrosion resistance at the wetted surface while the lower-cost carbon or low-alloy steel base material provides structural integrity.
Question 3: Erosion-corrosion in a pressure vessel causes wall loss that is typically characterized by which appearance?
- Uniform, even thinning across all internal surfaces
- Deep, narrow pits with smooth walls distributed randomly
- Directional grooving or horseshoe-shaped depressions aligned with flow (Correct answer)
- Intergranular attack concentrated at weld fusion lines
Correct answer: Directional grooving or horseshoe-shaped depressions aligned with flow
Erosion-corrosion produces directional damage — grooves, horseshoe pits, and streamlined depressions — that reflect the local flow pattern and turbulence direction.
Question 4: Which monitoring technique directly measures electrochemical corrosion rate in real time by applying a small voltage perturbation to a probe in a conducting electrolyte?
- Electrical resistance (ER) probe monitoring
- Linear polarization resistance (LPR) monitoring (Correct answer)
- Hydrogen flux monitoring
- Acoustic emission monitoring
Correct answer: Linear polarization resistance (LPR) monitoring
LPR monitoring applies a small polarization to a probe and measures the current response to calculate instantaneous corrosion rate based on the Stern-Geary relationship.
Question 5: In sour service (H₂S-containing) environments, which steel hardness level is specified in NACE MR0175/ISO 15156 as the maximum to prevent sulfide stress cracking (SSC)?
- 200 HBW maximum
- 22 HRC (237 HBW) maximum (Correct answer)
- 30 HRC (286 HBW) maximum
- 35 HRC (327 HBW) maximum
Correct answer: 22 HRC (237 HBW) maximum
NACE MR0175/ISO 15156 limits carbon and low-alloy steel hardness to 22 HRC (approximately 237 HBW) to prevent SSC in H₂S-containing environments.
Question 6: A vessel inspector notices orange-brown tubercles on the internal surface of a carbon steel water storage vessel. What corrosion mechanism do these indicate?
- Caustic gouging from concentrated sodium hydroxide
- Microbiologically influenced corrosion (MIC) with iron-related bacteria (Correct answer)
- Wet H₂S blistering from dissolved hydrogen sulfide
- Carbonate cracking from bicarbonate-rich water
Correct answer: Microbiologically influenced corrosion (MIC) with iron-related bacteria
Orange-brown tubercles are characteristic of MIC involving iron-oxidizing or iron-reducing bacteria, which create mounds of corrosion product over active pit sites beneath.
Question 7: What is the significance of the 'critical pitting temperature' (CPT) for stainless steels used in pressure vessel applications?
- The minimum temperature at which sensitization begins during welding
- The temperature below which pitting corrosion cannot initiate in a given chloride environment (Correct answer)
- The maximum service temperature before high-temperature oxidation accelerates
- The threshold temperature for intergranular corrosion in austenitic grades
Correct answer: The temperature below which pitting corrosion cannot initiate in a given chloride environment
The CPT is the minimum temperature at which pitting corrosion can initiate and propagate in a specific chloride-containing environment, used to select appropriate alloys for service.
API 510 Corrosion Mechanisms and Monitoring 4
According to API 510, when calculating remaining life using both short-term and long-term corrosion rates, which rate should be used for the next inspection interval?