API API-571 Practice Test Questions Answers

API RP 571 states under the section Phosphoric Acid Corrosion:

“Corrosion usually occurs when the acid dries out and forms concentrated phosphoric acid deposits, which are very aggressive to carbon steel.”

“Drying or concentration occurs due to poor flow distribution or operational upsets. Localized areas may experience acid concentration due to vaporization.”

(Reference: API RP 571, Section 4.3.3.8 – Phosphoric Acid Corrosion)

Therefore, the most aggressive corrosion occurs when the acid dries out, making option D correct.

Erosion and erosion-corrosion are mechanical degradation mechanisms enhanced by fluid motion, and typically result in directional or flow-oriented metal loss patterns.

According to API RP 571 Section 5.4.2 (Erosion and Erosion/Corrosion):

“Erosion typically results in localized thinning, often described as grooves, gullies, or horseshoe-shaped patterns... The metal loss is directional and may occur in elbows, tees, reducers, and pumps.”

Hence, Option C (grooves and gullies) most accurately describes erosion and erosion-corrosion characteristics.

According to API RP 571 Section 5.1.2.1 (Hydrogen-Induced Cracking) and Publ 939-A, blistering in carbon and low alloy steels exposed to wet H2S environments can be exacerbated by the presence of cyanides. These accelerate hydrogen entry into steel, increasing the chance of hydrogen blistering.

API RP 571 states:

“HIC and blistering are forms of hydrogen damage that occur in wet H2S environments. The presence of cyanides can significantly increase hydrogen entry and the risk of blistering.”

Publ 939-A (Research on Cracking in Wet H2S Service) also notes:

“Cyanides act as catalytic agents in hydrogen charging, which promotes internal pressure build-up and increases blistering.”

Thus, cyanides (Option B) are the most critical in enhancing blistering in H2S environments.

API RP 571, under Organic Acid Corrosion, highlights:

“Corrosive organic acids in overhead systems are those that condense at temperatures above the water dew point. These acids condense independently of water and can form highly concentrated corrosive films.”

“When acids condense first, they aggressively corrode carbon steel prior to any water wash mitigation.”

(Reference: API RP 571, Section 4.3.3.9 – Organic Acid Corrosion)

Hence, option D correctly identifies the most aggressive corrosion scenario.

Hydrogen Stress Cracking (HSC) is a subclass of environmental cracking that occurs in certain materials when hydrogen is present. According to API RP 571, HSC typically affects high-strength low alloy steels and carbon steels, particularly under tensile stress in a hydrogen-containing environment. This includes areas like weld-affected zones and hard spots in steels.

From API RP 571 Section 5.1.2.1 (Hydrogen-Induced Cracking):

"Hydrogen stress cracking (HSC) can occur in carbon steel and low alloy steels in environments containing hydrogen at ambient to moderately elevated temperatures (up to 200°F or 93°C)... Hardness is a critical factor for HSC. High hardness in welds and heat affected zones (HAZ) are most susceptible."

Additionally, API RP 939-C does not directly define HSC but reinforces the relationship between hydrogen exposure and metal integrity, particularly under conditions not exceeding 450 °F (230 °C) for hydrogen services.

Thus, option A is correct, as it directly reflects the metal types most susceptible to HSC in operational environments defined by API RP 571.