API Corrosion & Damage Mechanisms 2 — Questions and Answers
Question 1: Which damage mechanism in API 571 involves the time-dependent deformation of metal under sustained stress at elevated temperatures, eventually leading to rupture?
- Thermal fatigue
- Creep (Correct answer)
- Stress corrosion cracking
- Erosion-corrosion
Correct answer: Creep
Creep is the time-dependent, permanent deformation of a material under sustained stress at elevated temperatures, and eventually leads to creep rupture if not mitigated.
Question 2: According to API 571, which damage mechanism is specifically associated with the formation of a hard, brittle iron carbide phase in carbon and C-0.5Mo steels exposed to elevated temperatures over long periods?
- Temper embrittlement
- Graphitization (Correct answer)
- Sigma phase embrittlement
- 885°F (475°C) embrittlement
Correct answer: Graphitization
Graphitization is the transformation of iron carbide (pearlite) into graphite nodules in carbon and C-0.5Mo steels at temperatures generally above 800°F (427°C), reducing toughness and ductility.
Question 3: API 571 describes temper embrittlement as a concern for which class of materials in which temperature range?
- Austenitic stainless steels at temperatures above 1000°F
- Low-alloy steels (Cr-Mo) cooled slowly through the 650–1070°F range (Correct answer)
- Carbon steels at temperatures below 32°F
- Duplex stainless steels exposed above 500°F
Correct answer: Low-alloy steels (Cr-Mo) cooled slowly through the 650–1070°F range
Temper embrittlement affects low-alloy Cr-Mo steels when they are slowly cooled through or held in the 650–1070°F (343–577°C) range, causing tramp element segregation to grain boundaries and loss of toughness.
Question 4: Which form of localized corrosion described in API 571 initiates at occluded areas such as under gaskets, deposits, or between mating surfaces due to restricted oxygen and ion transport?
- Pitting corrosion
- Crevice corrosion (Correct answer)
- Galvanic corrosion
- Erosion corrosion
Correct answer: Crevice corrosion
Crevice corrosion initiates in confined spaces where the local chemistry becomes depleted of oxygen and enriched in corrosive species, creating an aggressive micro-environment.
Question 5: In API 571, amine stress corrosion cracking (amine SCC) most commonly affects which material and under what condition?
- Austenitic stainless steel in lean amine service at high temperatures
- Carbon steel welds and HAZ in amine service that have not been post-weld heat treated (PWHT) (Correct answer)
- Copper alloys in rich amine piping at ambient temperatures
- Duplex stainless steel in high-concentration amine systems
Correct answer: Carbon steel welds and HAZ in amine service that have not been post-weld heat treated (PWHT)
Amine SCC primarily affects carbon steel welds and heat-affected zones with high residual stresses in amine service; PWHT is the primary mitigation to reduce residual stresses below the cracking threshold.
Question 6: Which API 571 mechanism involves the localized removal of material due to the combined action of corrosion and mechanical wear from flowing fluids or entrained particles?
- Cavitation
- Erosion-corrosion (Correct answer)
- Liquid-metal embrittlement
- Microbiologically influenced corrosion
Correct answer: Erosion-corrosion
Erosion-corrosion is a synergistic mechanism where the mechanical removal of protective corrosion films by flow or particles accelerates the overall metal loss rate beyond what either mechanism would cause alone.
Question 7: According to API 571, sigma phase embrittlement is a primary concern for which class of alloys and in what temperature range?
- Carbon steels above 900°F (482°C)
- 300-series austenitic and duplex stainless steels in the 1000–1700°F (538–927°C) range (Correct answer)
- Titanium alloys in sour service above 600°F
- Nickel-base alloys in carburizing environments above 1500°F
Correct answer: 300-series austenitic and duplex stainless steels in the 1000–1700°F (538–927°C) range
Sigma phase, a hard and brittle iron-chromium intermetallic, forms in austenitic and duplex stainless steels held or slowly cooled through approximately 1000–1700°F, severely reducing impact toughness.
API Corrosion & Damage Mechanisms 2
Which damage mechanism in API 571 involves the time-dependent deformation of metal under sustained stress at elevated temperatures, eventually leading to rupture?