NACE - National Association of Corrosion Engineers Corrosion Theory and Forms Questions and Answers — Questions and Answers
Question 1: For a corrosion cell to exist, which four elements must be present?
- Anode, cathode, metallic pathway, and electrolyte (Correct answer)
- Anode, conductor, acidic solution, and current
- Two dissimilar metals, water, and a protective coating
- Cathode, noble metal, ionic path, and voltage source
Correct answer: Anode, cathode, metallic pathway, and electrolyte
A corrosion cell is an electrochemical cell that requires four essential components to function: an anode where oxidation occurs, a cathode where reduction occurs, a metallic pathway for electron flow between the anode and cathode, and an electrolyte to carry ionic current. [2, 26, 29, 35]
Question 2: A steel pipeline is connected with brass fittings. In the presence of an electrolyte, which phenomenon is most likely to occur and why?
- The brass fittings will corrode because they are smaller in surface area.
- The steel pipeline will corrode because it is more anodic than brass. (Correct answer)
- Both the steel and brass will corrode at an equal rate.
- The steel will be cathodically protected by the brass fittings.
Correct answer: The steel pipeline will corrode because it is more anodic than brass.
When two dissimilar metals like steel and brass are in contact in an electrolyte, galvanic corrosion occurs. [1, 42] Steel is less noble (more anodic) than brass in the galvanic series. Therefore, the steel will act as the anode and corrode preferentially, while the more noble brass acts as the cathode. [1]
Question 3: Which form of corrosion is characterized by a localized attack that creates small holes or cavities in the metal, often initiated by a localized breakdown of a passive film?
- Uniform corrosion
- Intergranular corrosion
- Pitting corrosion (Correct answer)
- Erosion corrosion
Correct answer: Pitting corrosion
Pitting corrosion is a localized form of corrosion that produces pits or holes in a material. [5, 10, 11] It is particularly insidious because it can cause failure with only a small amount of metal loss. It typically starts when a protective passive film is compromised at specific points. [5]
Question 4: A technician inspects a stainless steel component that was welded and notices severe corrosion along the heat-affected zone next to the weld. This is most characteristic of which form of corrosion?
- Galvanic corrosion
- Crevice corrosion
- Stress corrosion cracking
- Intergranular corrosion (Correct answer)
Correct answer: Intergranular corrosion
Intergranular corrosion is a localized attack along the grain boundaries of a metal. [8, 22] In some stainless steels, welding can cause sensitization, where chromium carbides precipitate at the grain boundaries, depleting the adjacent regions of chromium and making them susceptible to corrosion. [12, 25] This is often referred to as 'weld decay'. [12, 25]
Question 5: Crevice corrosion is a form of localized corrosion that occurs in stagnant microenvironments. Which condition is a primary driver for the initiation of crevice corrosion?
- High fluid velocity across the surface
- A differential in oxygen concentration between the crevice and the bulk environment (Correct answer)
- The presence of two dissimilar metals in direct contact
- Uniform tensile stress across the entire component
Correct answer: A differential in oxygen concentration between the crevice and the bulk environment
Crevice corrosion initiates due to a difference in the chemical environment inside the crevice compared to the outside. [6] A key factor is the depletion of oxygen within the stagnant solution in the crevice, which creates an oxygen concentration cell. The area inside the crevice becomes anodic, while the area outside with more oxygen becomes cathodic, driving corrosion within the crevice. [18, 16]
Question 6: Which of the following describes environmentally assisted cracking (EAC)?
- A purely mechanical failure due to cyclic loading.
- Uniform loss of material due to a chemical reaction with the environment.
- Brittle fracture of a normally ductile material resulting from the combined action of a corrosive environment and tensile stress. (Correct answer)
- Accelerated corrosion caused by the impingement of a high-velocity fluid.
Correct answer: Brittle fracture of a normally ductile material resulting from the combined action of a corrosive environment and tensile stress.
Environmentally assisted cracking (EAC) is a general term for cracking that occurs when a susceptible material is subjected to a combination of a corrosive or aggressive environment and tensile stress. [31, 34] This includes phenomena like stress corrosion cracking (SCC), corrosion fatigue, and hydrogen embrittlement. [4, 31, 32]
NACE - National Association of Corrosion Engineers Corrosion Theory and Forms Questions and Answers
For a corrosion cell to exist, which four elements must be present?