Free API API-571 Practice Exam with Questions & Answers

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571 and corrosion control best practices, maximum protection of buried or soil-exposed carbon steel is achieved by combining:

A protective coating (primary barrier)

Cathodic protection (CP) (secondary, active protection)

This dual system:

Minimizes current demand

Protects coating defects

Extends service life significantly

Neither coatings nor CP alone provide equivalent long-term reliability.

Referenced Documents (Study Basis):

API RP 571 – Section on Soil-Side Corrosion

API Corrosion Control Study Guide

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Comprehensive and Detailed Explanation From Exact Extract:

Per API RP 571, sulfidation corrosion rates are significantly accelerated by the presence of hydrogen. Hydrogen promotes:

Breakdown of protective sulfide scales

Increased diffusion of sulfur into the metal

Higher metal loss rates

This phenomenon is often referred to as hydrogen-assisted sulfidation and is commonly observed in refinery process units.

Moisture is more closely associated with oxidation mechanisms rather than high-temperature sulfidation.

Referenced Documents (Study Basis):

API RP 571 – Section on Sulfidation and High-Temperature Corrosion

API RP 571 discusses Hydrogen-Induced Cracking (HIC) and Blistering:

“Postweld heat treatment (PWHT) does not significantly reduce susceptibility to HIC, as the mechanism is primarily driven by hydrogen charging in wet H₂S environments and steel cleanliness, not residual stresses.”

“PWHT is more effective for SSC, SOHIC, and other stress-driven mechanisms.”

Thus, HIC will not benefit much from PWHT, making option C correct.

From API RP 571 Section 5.1.2.7 (Amine Stress Corrosion Cracking) and API RP 945 (Avoiding Environmental Cracking in Amine Units):

MEA (Monoethanolamine) systems are most aggressive due to:

“Their high reactivity and degradation potential producing corrosive by-products that promote cracking.”

API RP 945:

“SCC in carbon steel is most frequently reported in MEA units, especially where localized concentrations of heat-stable salts exist.”

Thus, Option C (MEA) is the correct answer due to its high SCC risk.

API RP 945, dedicated to environmental cracking in amine units, states:

“Carbon steel is widely used in amine systems but is particularly susceptible to localized corrosion, under-deposit corrosion, and stress corrosion cracking, especially in high-pressure or rich amine environments.”

“Proper control of amine quality, oxygen ingress, and temperature is critical to avoid accelerated corrosion in carbon steel equipment.”

Thus, carbon steels are the most commonly affected materials, making option C correct.

From API RP 571 Section 4.2.3 (Galvanic Corrosion):

“Galvanic corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte. The more anodic metal corrodes preferentially.”

Contact without insulation and in presence of an electrolyte (like water or brine) is key to galvanic action.

Insulating or coating prevents electrical connection or electrolyte contact, reducing corrosion risk.

Hence, Option D correctly describes the condition that increases galvanic corrosion potential.

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 describes temper embrittlement as a subtle metallurgical change that reduces impact toughness without obvious macrostructural change:

“Temper embrittlement is not readily apparent by visual or standard tensile testing. It is confirmed by Charpy impact testing which measures toughness at low temperatures.”

“It typically affects Cr-Mo steels exposed to intermediate temperatures (650°F to 1100°F) for extended times.”

(Reference: API RP 571, Section 4.2.1.4 – Temper Embrittlement)

Thus, the condition is not visually or physically apparent, but is confirmed through impact testing, making option D correct.

API RP 571 – Polythionic Acid Stress Corrosion Cracking:

“Polythionic acid SCC may be mistaken for wet H₂S damage because both involve cracking in austenitic stainless steels under certain conditions.”

Thus, option D is correct.

API RP 571 describes under Caustic Corrosion:

“One of the significant contributing factors is caustic accumulation in low-flow or stagnant areas, especially in heat-traced lines or around steam injection points.”

“Localized heating can concentrate caustic solutions, promoting corrosion even in carbon or stainless steels.”

Thus, option C (heat-traced equipment) is the correct and technically accurate choice.