API API-571 - Corrosion and Materials Professional

Comprehensive and Detailed Explanation From Exact Extract:

Per API RP 571, deaerators often operate in environments where concentrated caustic solutions can form locally due to oxygen scavengers and water chemistry control additives.

When postweld heat treatment (PWHT) is not performed:

High residual welding stresses remain

Conditions are ideal for caustic stress corrosion cracking (Caustic SCC)

API RP 571 identifies deaerators as classic examples of equipment that has experienced caustic SCC when PWHT was omitted.

Referenced Documents (Study Basis):

API RP 571 – Section on Caustic Stress Corrosion Cracking

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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.

Comprehensive and Detailed Explanation From Exact Extract:

Hydrochloric acid (HCl) corrosion in atmospheric crude units is primarily associated with the overhead system, where chloride salts hydrolyze to form HCl in the presence of condensed water. This damage mechanism is well documented in API RP 571 under “Chloride Stress Corrosion Cracking and Hydrochloric Acid Corrosion”.

The primary indicator of HCl corrosion risk is the acidity of the condensed water in the overhead system. The overhead accumulator boot water collects condensed water where dissolved HCl will be present. Monitoring the pH of this water provides a direct, real-time indication of acid formation and corrosion severity.

Low pH values (typically below 5.5) indicate active HCl corrosion conditions.

API RP 571 states that pH monitoring is a key operational control and surveillance tool for managing overhead corrosion.

Why the other options are incorrect:

Option A (UT at water injection points) does not address overhead acid formation and is not a susceptibility monitoring method.

Option C (Corrosion probes or coupons) provide general corrosion rate data, but they do not directly measure HCl formation or acid severity and often respond too slowly for operational control.

Option D (UT scanning or radiography) is a damage detection method, not a susceptibility or early-warning monitoring technique.

API RP 571 emphasizes that effective monitoring of HCl corrosion requires controlling and monitoring water chemistry, especially pH in the overhead accumulator boot, to prevent severe localized corrosion and ammonium chloride salt deposition.

Referenced Documents (Study Basis):

API RP 571 – Section on Hydrochloric Acid Corrosion in Crude Unit Overhead Systems

API Corrosion and Materials Study Guides – Atmospheric Crude Unit Overhead Corrosion

API RP 571 under Oxidation:

“Carbon steels and low alloy steels, such as 9 Cr, have poor oxidation resistance at temperatures above 1000°F (540°C), rapidly losing thickness.”

“Stainless steels with higher Cr and Ni content (like 310) have superior oxidation resistance.”

Thus, 9 Cr steel will corrode fastest at 1350°F, making option D correct.

API RP 571 under Thermal Fatigue highlights:

“Differential thermal expansion between dissimilar materials (e.g., carbon steel and stainless steel) in welds causes high cyclic stresses that can result in thermal fatigue cracking.”

“These stresses typically occur during transient thermal cycling such as startup/shutdown operations.”

(Reference: API RP 571, Section 4.2.1.5 – Thermal Fatigue)

Thus, the correct mechanism resulting from differential expansion in bimetallic welds is thermal fatigue, making option B correct.

API RP 571 explains that both Sulfide Stress Cracking (SSC) and SOHIC are:

“Strongly influenced by material hardness. Carbon and low alloy steels with hardness exceeding 22 HRC (248 Brinell) are most susceptible.”

“They occur in hardened steels under tensile stress, in the presence of wet H₂S environments.”

(Reference: API RP 571, Sections 4.2.2.1 and 4.2.2.7 – SSC and SOHIC)

Therefore, hardened steels are the common factor for both mechanisms, making option C the correct choice.

API RP 571 on HTHA states:

“Acoustic emission testing is not currently a proven or reliable method for identifying HTHA in refinery components.”

“HTHA typically occurs internally, not as surface blisters, and is not limited to welds. Austenitic (300 series) stainless steels are generally resistant at normal refinery conditions.”

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

Thus, option A is the most accurate statement.

From API RP 571 Section 5.3.2.1 (Creep and Stress Rupture):

“Short-term creep rupture is often associated with localized overheating and results in bulging and thinning, often with a characteristic ‘fish-mouth’ rupture appearance. These failures occur under sustained load in a short time (hours or days), typically in high-temperature service due to a sudden increase in temperature or poor heat distribution.”

Thus, Option B accurately describes the mechanism and visual indications of short-term stress rupture.

Comprehensive and Detailed Explanation From Exact Extract:

Wet Hâ‚‚S damage mechanisms include hydrogen blistering, hydrogen-induced cracking (HIC), stepwise cracking (SWC), and stress-oriented hydrogen-induced cracking (SOHIC). These mechanisms are primarily internal and subsurface, as described in API RP 571.

Shear Wave Ultrasonic Testing (SWUT) is specifically highlighted in API RP 571 as a preferred screening and detection method for wet Hâ‚‚S damage because it can detect subsurface planar defects with minimal surface preparation. Typically, only basic coupling and access are required.

Why the other options are incorrect:

Option A (ACFM) is a surface crack detection technique and requires relatively clean surfaces; it is not suitable for subsurface hydrogen damage.

Option C (WFMT – Wet Fluorescent Magnetic Particle Testing) requires extensive surface preparation, coating removal, and direct access to the surface, and only detects surface-breaking flaws.

Option D (LPT – Liquid Penetrant Testing) also requires clean, bare metal surfaces and cannot detect subsurface damage.

API RP 571 explicitly notes that ultrasonic techniques, particularly shear wave UT, are widely used for detecting internal hydrogen-related cracking with minimal disruption and surface preparation, making it the most effective choice for wet Hâ‚‚S damage screening.

Referenced Documents (Study Basis):

API RP 571 – Section on Wet H₂S Damage (HIC, SOHIC, Blistering)

API Corrosion and Materials Inspection Study Guide

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571 and API RP 941 (Nelson Curves), 1 Cr–0.5 Mo steel is no longer recommended for services susceptible to High-Temperature Hydrogen Attack (HTHA).

API RP 941 documents industry experience showing that 1 Cr–0.5 Mo steels have suffered HTHA damage below previously assumed safe operating limits, due to insufficient carbide stability. As a result, this material has been removed from the Nelson Curves as an acceptable choice for new construction in hydrogen service.

By contrast:

Mn–0.5 Mo, C–0.5 Mo, and 1.25 Cr–0.5 Mo steels retain higher resistance due to more stable carbide structures.

Referenced Documents (Study Basis):

API RP 571 – Section on High-Temperature Hydrogen Attack

API RP 941 – Updated Nelson Curves and Material Recommendations

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