API Corrosion and Materials API-571 Exam Questions

Answer : D

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.

Answer : C

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.

Answer : B

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

Answer : C

From API RP 571 Section 5.1.1.3 (Amine Corrosion):

''Amine corrosion is a form of general or localized corrosion caused by the presence of amine solutions used to remove acid gases (HS and CO)... Corrosion is most severe in areas where acid gas loading is high.''

While temperature and concentration are contributing factors, the primary driver is dissolved acid gases such as CO and HS.

Thus, the correct answer is Option C.

Answer : B

According to API RP 571, under Sulfuric Acid Corrosion:

''The most aggressive corrosion typically occurs below 65% HSO, 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.

Answer : C

API RP 571, under Chloride Stress Corrosion Cracking, notes:

''Cracking often appears as highly branched or 'crazed' networks on the surface, especially in austenitic stainless steels exposed to chlorides under tensile stress.''

''These cracks often initiate in the heat-affected zones or cold-worked areas and are intergranular in nature.''

This signature 'crazed' appearance with branching is diagnostic of Cl-SCC, making option C the correct and most descriptive answer.

Answer : B

Comprehensive and Detailed Explanation From Exact Extract:

Caustic stress corrosion cracking (Caustic SCC) occurs when carbon and low-alloy steels are exposed to caustic environments in the presence of tensile stress.

Per API RP 571, the most effective and proven prevention method is postweld stress-relief heat treatment, which:

Reduces residual welding stresses

Eliminates one leg of the SCC triangle (stress)

Why other options are incorrect:

300 series stainless steels are also susceptible under some caustic conditions.

SCC can occur even at low caustic concentrations.

SCC has been observed below 150 F in high-stress areas.

Referenced Documents (Study Basis):

API RP 571 -- Section on Caustic Stress Corrosion Cracking