API Corrosion & Damage Mechanisms 3

Question 1

API 571 identifies microbiologically influenced corrosion (MIC) as being caused primarily by which type of organism in stagnant or low-flow aqueous systems?

Accepted Answer

Sulfate-reducing bacteria (SRB) that generate hydrogen sulfide

Answer

Aerobic bacteria that produce nitric acid

Answer

Fungi that produce organic acids at low temperatures

Answer

Algae that block cathodic protection current

Question 2

In API 571, which damage mechanism is described as the preferential corrosion of welds due to differences in microstructure, residual stress, or composition between the weld metal and base metal?

Accepted Answer

Weld decay (weld corrosion)

Answer

Selective leaching

Answer

Dealloying

Answer

Intergranular corrosion

Question 3

According to API 571, dealloying (selective leaching) most commonly affects which alloy system in petroleum industry service?

Accepted Answer

Copper-zinc alloys (brass) where zinc is selectively removed (dezincification)

Answer

Carbon steel in dilute hydrochloric acid

Answer

Titanium alloys in hydrofluoric acid service

Answer

Nickel-base alloys in high-temperature sulfidation

Question 4

Which damage mechanism described in API 571 can occur when a liquid metal contacts a stressed or susceptible solid metal, causing rapid embrittlement and cracking?

Accepted Answer

Liquid metal embrittlement (LME)

Answer

Cavitation damage

Answer

Thermal shock cracking

Answer

Sulfide stress cracking

Question 5

In API 571, high-temperature hydrogen attack (HTHA) is most accurately described as what type of damage to carbon and low-alloy steels?

Accepted Answer

Methane formation from hydrogen reacting with carbides, causing fissuring and decarburization

Answer

Hydrogen blistering from atomic hydrogen recombining at internal voids

Answer

Surface oxidation from hydrogen reacting with the iron oxide scale

Answer

Hydrogen embrittlement from dissolved hydrogen reducing ductility

Question 6

API 571 recommends using Nelson Curves (from API RP 941) for determining safe operating limits against which damage mechanism?

Accepted Answer

High-temperature hydrogen attack (HTHA)

Answer

Sulfide stress cracking (SSC)

Answer

Naphthenic acid corrosion

Answer

Creep rupture

Question 7

According to API 571, which damage mechanism is characterized by cyclic stresses from temperature fluctuations causing crack initiation and propagation in metals, typically at stress concentration points?

Accepted Answer

Thermal fatigue

Answer

Creep fatigue

Answer

Corrosion fatigue

Answer

Mechanical fatigue