Free API API-571 Practice Exam with Questions & Answers | Set: 5

API RP 571 details that:

“Sulfide Stress Cracking (SSC) susceptibility increases significantly with increased hardness of the steel.”

“NACE MR0175/ISO 15156 provides hardness limits for carbon and low-alloy steels to avoid SSC in sour environments. These are typically limited to 22 HRC or 248 Brinell.”

“High hardness promotes crack initiation under tensile stress in sour environments (i.e., containing H₂S).”

(Reference: API RP 571, Section 4.2.2.1 – Sulfide Stress Corrosion Cracking)

While hydrogen-induced cracking (HIC) and blistering are related to hydrogen charging, SSC is the only mechanism directly and critically impacted by hardness, making option B correct.

In API RP 571, Carbonate SCC and Alkaline SCC are both discussed under mechanisms that occur in basic (high pH) environments, often with similar contributing conditions and prevention methods. The document highlights:

“Carbonate SCC is considered a subcategory of alkaline stress corrosion cracking. It results from the concentration of alkaline carbonates under thermal insulation or deposits.”

“Both mechanisms involve the same key parameters: elevated temperatures, tensile stress, and high pH solutions.”

(Reference: API RP 571, 3rd Edition, Section 4.2.2.4 and 4.2.2.3)

Because these mechanisms share similar material susceptibility and operating conditions, they are indeed closely related—option B is therefore the accurate and supported answer.

According to API RP 571, under High Temperature Hydrogen Attack (HTHA):

“HTHA occurs when hydrogen diffuses into steel at elevated temperatures and reacts with carbides in the steel matrix to form methane (CH₄).”

“The methane is unable to diffuse out of the steel and forms internal pressures, leading to fissuring, decarburization, and eventual failure.”

“HTHA typically affects carbon steels and low alloy steels exposed to high temperature hydrogen services, particularly above 400°F (204°C) depending on partial pressure of hydrogen.”

(Reference: API RP 571, Section 4.2.1.3 – High Temperature Hydrogen Attack)

Hence, option D is the correct and technically supported answer.

From API RP 571 Section 5.1.2.5 (Polythionic Acid Stress Corrosion Cracking):

“PTASCC typically occurs on the surface of stainless steels that have been sensitized. Since cracks are open to the surface, liquid penetrant testing (PT) is the most effective detection method.”

Magnetic particle testing is ineffective for non-magnetic materials like austenitic stainless steels.

UT is less effective for fine surface-connected cracks.

Hardness testing is unrelated to detecting cracking.

Thus, Option C (Liquid penetrant testing) is the correct and preferred method for detecting PTASCC.