Corrosion & Damage Mechanisms Flashcards

Which damage mechanism in API 571 involves the time-dependent deformation of metal under sustained stress at elevated temperatures, eventually leading to rupture?

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.

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?

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.

API 571 describes temper embrittlement as a concern for which class of materials in which temperature range?

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.

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?

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.

In API 571, amine stress corrosion cracking (amine SCC) most commonly affects which material and under what condition?

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.

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?

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.

According to API 571, sigma phase embrittlement is a primary concern for which class of alloys and in what temperature range?

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.