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In what damage mechanism does hydrogen combine with carbides in steel to form bubbles or cavities of CH?
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 (CH).''
''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 400F (204C) 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.
An inspector observes sharp-edged pitting in piping immediately downstream from an orifice. This damage has most likely resulted from which damage mechanism?
According to API RP 571 Section 5.4.2 (Erosion and Erosion/Corrosion):
''Erosion can occur downstream of flow disturbances such as control valves, orifice plates, elbows... It is characterized by localized metal loss with a directional pattern such as grooves or sharp-edged pits in areas of high velocity or turbulence.''
Cavitation and flashing cause damage with different signatures like pitting and spongy surface texture, while sharp-edged pitting strongly indicates erosion, especially downstream of flow restriction devices.
Thus, the correct answer is Option C (Erosion).
(Which of the following would be most useful in finding carbolic acid corrosion?)
Comprehensive and Detailed Explanation From Exact Extract:
Carbolic acid corrosion (phenol corrosion) is described in API RP 571 as a process-specific corrosion mechanism that may not produce distinct or easily detectable wall-loss patterns using conventional NDE methods.
API RP 571 emphasizes that confirmation of carbolic acid corrosion often requires:
Chemical identification of corrosion products
Metallurgical examination
Verification of acid presence and metal interaction
A boat sample (also known as a coupon or scoop sample) removed from the affected area and analyzed in a laboratory allows:
Identification of corrosion morphology
Chemical analysis of deposits
Confirmation of phenol-related attack
Why the other options are incorrect:
UT and RT detect wall loss but cannot identify the corrosion mechanism.
Acoustic emission detects active cracking, not specific corrosion chemistry.
Angle beam UT is for crack detection, not corrosion identification.
Therefore, laboratory analysis of a removed sample is the most useful method.
Referenced Documents (Study Basis):
API RP 571 -- Section on Organic Acid Corrosion (Phenols)
API Corrosion Failure Analysis Study Guide
What alloy element most improves naphthenic acid corrosion (NAC) resistance?
API RP 571, Naphthenic Acid Corrosion:
''Molybdenum significantly improves resistance to naphthenic acid attack. Alloys such as 317, 316Mo, and Alloy 20 are used in severe NAC environments due to higher Mo content.''
''Chromium and nickel play secondary roles; molybdenum is the key element.''
Answer is A -- Molybdenum.
Why are high-cycle fatigue cracks difficult to detect with nondestructive examination (NDE)?
According to API RP 571, high-cycle fatigue (HCF) is characterized by very tight, narrow cracks that can escape detection using typical NDE methods due to their minimal opening displacement and fine geometry.
From API RP 571 Section 5.2.1 (Fatigue - High Cycle):
'The cracks are usually tight and may be difficult to detect using conventional NDE techniques such as PT or MT. UT and RT may also have difficulty identifying these small, tight flaws, especially in early stages of propagation.'
The primary issue in NDE is not the geometry (such as 90 corners) or ID location, but rather the tightness and early-stage subtlety of the cracks which results in reduced detectability. Therefore, option B is correct as it aligns most accurately with API RP 571's detailed characterization of HCF detection challenges.
