Free Practice Questions for API API-571 Exam

According to API RP 571 and API RP 751 (HF Alkylation Units):

“HF acid corrosion rates increase with elevated temperature and higher water content. This is because water acts as an ionizing agent that facilitates the acid’s attack on steel.”

“Corrosion is minimized when water content is strictly controlled, usually between 0.5–1.5%.”

“Contrary to some assumptions, increasing HF acid concentration actually decreases corrosivity.”

Hence, option B is correct.

According to API RP 571 Section 5.3.2.3 (Spheroidization):

“Spheroidization is the transformation of the microstructure of carbon and low alloy steels when exposed to elevated temperatures for long durations. The rate of spheroidization depends on temperature, prior microstructure, and exposure time... The microstructure becomes less effective at carrying loads, and strength is reduced.”

Key influencing factors for the rate of spheroidization are:

Temperature (higher accelerates the process),

Microstructure (initial phase distribution and morphology).

Pressure and hydrogen partial pressure are not relevant for this transformation, nor is stress a primary driver.

Therefore, Option D (temperature and microstructure) 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.

API RP 571 notes in the context of Hydrochloric Acid Corrosion in overhead condensers:

“Titanium is highly resistant to low pH acidic aqueous phases, including hydrochloric acid formed during condensation in overhead systems.”

“Stainless steels like 316 and 410 are not suitable in the presence of free chlorides at low pH and elevated temperatures.”

(Reference: API RP 571, Section 4.3.3.3 – Hydrochloric Acid Corrosion)

Thus, Titanium is the preferred alloy under the described acidic and high-temperature conditions, making option B correct.

API RP 571 states under Galvanic Corrosion:

“Because galvanic corrosion usually manifests as thinning of the more anodic metal, ultrasonic thickness measurements taken near dissimilar metal joints are an effective way to detect this type of internal corrosion.”

“Surface NDE techniques like liquid penetrant or magnetic particle testing are not effective for detecting wall loss.”

Thus, option D (ultrasonic thickness testing) is correct.

Galvanic corrosion occurs when two dissimilar metals are electrically connected in the presence of an electrolyte such as brackish water or seawater. The rate and severity of galvanic corrosion depend on:

The relative position of the metals on the galvanic series (potential difference)

Area ratio between anode and cathode

Conductivity of the electrolyte

API RP 571 explains:

“The more active (anodic) metal in the galvanic couple corrodes preferentially. The more noble (cathodic) metal is protected.”

“The greater the difference in potential between the two metals, the more aggressive the galvanic reaction.”

“Aluminum is highly anodic compared to steel. In seawater or brackish conditions, aluminum will corrode rapidly when electrically coupled to steel, especially if the aluminum surface area is small relative to the steel.”

(Reference: API RP 571, Section 4.3.5.1 – Galvanic Corrosion)

Referring to the galvanic series presented in your table:

Aluminum is much higher (more anodic) than steel.

This makes aluminum the sacrificial metal in such a pair.

This is not the case for brass-to-nickel or cast iron-to-Ni-Resist, which are much closer together on the galvanic scale.

Therefore, among all the listed pairs, Aluminum coupled to Steel creates the most significant galvanic potential difference and is most likely to result in galvanic corrosion in brackish or seawater service.

Correct Answer: B

According to API RP 571, under Sulfuric Acid Corrosion:

“The most aggressive corrosion typically occurs below 65% H₂SO₄, particularly between 20% to 50% concentration range.”

“Above 95% concentration, sulfuric acid becomes less corrosive to carbon steel due to its dehydrating nature.”

“Dilute sulfuric acid is highly ionized and conducts electricity well, increasing corrosion rates.”

(Reference: API RP 571, Section 4.3.3.2 – Sulfuric Acid Corrosion)

Hence, the most severe corrosion occurs below 65% concentration, making option B the accurate answer.

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.

According to API RP 571 Section 5.1.2.6 (Carbonate Stress Corrosion Cracking - CSCC):

“The cracking is usually intergranular and is most commonly detected using wet fluorescent magnetic-particle testing (WFMT) after surface cleaning. PT may not be sensitive enough to detect fine cracks formed by this mechanism.”

Because CSCC cracks are typically surface-connected and very fine, WFMT offers the best visibility, especially after proper surface prep.

Therefore, Option D is correct.

API RP 571 describes Temper Embrittlement as:

“A metallurgical degradation mechanism that leads to a reduction in fracture toughness due to long-term exposure in the temperature range of 650°F to 1070°F (345°C to 575°C).”

“It affects Cr-Mo steels and is not easily detected except by impact testing.”

Therefore, option C most accurately defines the condition based on the recognized API description.