API 571 Acidic Process Stream Corrosion 3

Question 1

Microbiologically influenced corrosion (MIC) in hydrocarbon piping systems is most commonly caused by which type of microorganism?

Accepted Answer

Sulfate-reducing bacteria (SRB)

Answer

Sulfate-reducing bacteria (SRB) are the predominant MIC-causing organisms in hydrocarbon systems; they produce H2S under anaerobic conditions, which is corrosive to carbon steel.

Question 2

What is the characteristic visual evidence of active MIC attack found during internal inspection of carbon steel piping?

Accepted Answer

Tubercles with underdeposit pitting and black sulfide deposits

Answer

MIC produces characteristic tubercles (raised mounds of corrosion products) with localized pitting beneath them, often accompanied by black iron sulfide (FeS) deposits from SRB metabolic activity.

Question 3

Which temperature range is considered the primary zone of concern for Corrosion Under Insulation (CUI) on carbon steel equipment per API 583?

Accepted Answer

25°F to 250°F (–4°C to 121°C)

Answer

API 583 identifies 25°F to 250°F (–4°C to 121°C) as the primary CUI temperature range for carbon steel, where liquid water can be present and trapped beneath insulation to drive corrosion.

Question 4

Austenitic stainless steel equipment under insulation (CUI) is primarily vulnerable to which form of degradation?

Accepted Answer

Chloride stress corrosion cracking and pitting

Answer

Austenitic stainless steels under insulation are primarily vulnerable to chloride-induced pitting and SCC, as chlorides in trapped moisture concentrate at the metal surface.

Question 5

Which insulation type is generally considered most problematic for CUI due to its high water retention capacity and potential to leach chlorides?

Accepted Answer

Calcium silicate insulation

Answer

Calcium silicate insulation has high porosity, absorbs and retains water readily, and can leach chlorides, making it particularly aggressive for CUI on both carbon steel and stainless steel.

API 571 - API 571 Corrosion and Materials Practice Test

API 571 - API 571 Corrosion and Materials Practice Test

API 571 Acidic Process Stream Corrosion 3