Free API Corrosion and Materials API-571 Exam Questions

Comprehensive and Detailed Explanation From Exact Extract:

According to API RP 571, carbon steel exhibits a minimum corrosion rate in sulfuric acid at concentrations around 65--70 wt% due to formation of a protective iron sulfate film.

When sulfuric acid concentration drops below approximately 65%, this protective film becomes unstable and corrosion rates increase dramatically.

Referenced Documents (Study Basis):

API RP 571 -- Section on Sulfuric Acid Corrosion

API RP 578 (Material Verification Program), referenced in API RP 571, specifically calls out sulfidation failures as a major driver for Positive Material Identification (PMI):

''PMI programs are particularly important in services prone to sulfidation where incorrect alloying elements (e.g., carbon steel used in place of low Cr alloy) have led to catastrophic failures.''

''Many failures have occurred when carbon steel was mistakenly installed in place of specified Cr-alloys in sulfidation-prone environments.''

(Reference: API RP 578 and API RP 571, Section 4.2.1.1 -- Sulfidation)

Thus, option C is the most appropriate answer.

API RP 571 on Thermal Fatigue:

''Mixing of hot and cold streams, particularly where hydrogen is present, can induce thermal cycling stresses, leading to thermal fatigue cracking, especially at fittings like tees.''

''Hydrogen embrittlement typically affects stressed zones, but this pattern strongly indicates thermal fatigue.''

So the correct answer is C -- Thermal fatigue.

API RP 751, which governs HF alkylation units, specifies:

''The total residuals of chromium, nickel, and copper in carbon steel used in HF service should be less than 0.15 wt.% to minimize corrosion risk.''

''Higher residuals increase the reactivity and corrosion rate of carbon steel in the presence of HF acid.''

(Reference: API RP 751, Section 5.3 -- Materials Specification for HF Units)

Therefore, option A is the correct choice.

API RP 571 on Naphthenic Acid Corrosion (NAC) details:

''NAC is most aggressive in two-phase flow where the shear stress is high, such as in elbows, reducers, and at orifices.''

''High velocity and turbulence in two-phase flow strip protective layers and accelerate corrosion.''

Therefore, option A is the correct and most critical phase.

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

According to API RP 571 and API RP 583:

''The most effective mitigation of CUI is the use of appropriate coatings underneath the insulation, which are resistant to moisture and chloride ingress.''

''Inspection and insulation selection are also important but are secondary controls.''

(Reference: API RP 571, Section 4.3.4 -- Corrosion Under Insulation; API RP 583)

Hence, option D is correct.

According to API RP 571, under the section 'Corrosion in Aqueous Environments -- Cooling Water Corrosion', one of the key contributors to corrosion in carbon steel and other materials used in heat exchangers is the presence of dissolved oxygen. API RP 571 states:

'Oxygen is a primary contributor to corrosion in cooling water systems. Systems open to the atmosphere are typically more corrosive than closed systems due to the continual replenishment of oxygen.'

'Corrosion rates are highest where oxygen concentration is the greatest, especially in systems using untreated or poorly treated water.'

'Carbon steel corrodes in the presence of oxygen and water, forming corrosion products that may or may not adhere to the surface.'

(Reference: API RP 571, Section 4.3.1.1 -- Cooling Water Corrosion)

Therefore, increasing oxygen content directly increases corrosion activity in exchanger tubes, making option C the correct and documented answer.

Comprehensive and Detailed Explanation From Exact Extract:

Naphthenic Acid Corrosion (NAC) occurs in high-temperature refinery streams (typically 450--750 F / 230--400 C) containing organic acids. According to API RP 571, NAC is particularly aggressive toward carbon steels and low-alloy steels, and resistance improves with increasing chromium and molybdenum content, but is best with titanium.

Titanium exhibits exceptional resistance to NAC due to the formation of a stable, protective oxide film that is not attacked by naphthenic acids. API RP 571 identifies titanium as one of the most resistant materials available for severe NAC environments.

Why the other options are less suitable:

Option B (9 Cr-1 Mo steel) has limited resistance and is still susceptible at higher TAN and velocities.

Option C (317 stainless steel) offers improved resistance but can still experience attack under severe NAC conditions.

Option D (321 stainless steel) has lower molybdenum content than 317 SS, making it less resistant.

API RP 571 explicitly notes that titanium provides superior resistance compared to stainless steels and Cr-Mo alloys in NAC service.

Referenced Documents (Study Basis):

API RP 571 -- Section on Naphthenic Acid Corrosion

API Corrosion and Materials Selection Study Guide

As per API RP 571 Section 5.3.2.2 (Sigma Phase Embrittlement):

''Sigma phase embrittlement occurs in austenitic stainless steels (like 300 series) and duplex stainless steels exposed to 1000F to 1800F (540C to 980C)... This embrittlement results in a significant loss of toughness and ductility.''

Sigma phase is not a concern in Monel (nickel-copper alloy) or 5Cr steels.

400 series are ferritic and not typically affected by sigma phase.

Hence, Option A (300 series stainless steel) is correct.