NACE - National Association of Corrosion Engineers Corrosion Theory and Forms Questions and Answers

Question 1

For a corrosion cell to exist, which four elements must be present?

Accepted Answer

Anode, cathode, metallic pathway, and electrolyte

Answer

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?

Accepted Answer

The steel pipeline will corrode because it is more anodic than brass.

Answer

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?

Accepted Answer

Pitting corrosion

Answer

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?

Accepted Answer

Intergranular corrosion

Answer

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?

Accepted Answer

A differential in oxygen concentration between the crevice and the bulk environment

Answer

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]

NACE - National Association of Corrosion Engineers Certification Practice Test

NACE - National Association of Corrosion Engineers Certification Practice Test

NACE - National Association of Corrosion Engineers Corrosion Theory and Forms Questions and Answers