What is the difference between corrosion rate and corrosion depth?

• A. Rate and depth are the same.
• B. Rate is the maximum depth reached; depth is the average annual loss.
• C. Rate measures color change; depth measures surface roughness.
• D. Rate is metal loss per unit time; depth is total loss over a period.

Answer: (D)

The difference being tested is what each quantity represents: how fast something changes versus how much has actually changed over time. Corrosion rate tells you the metal loss per unit time (for example, millimeters per year or mils per year), while corrosion depth (total loss) is the cumulative amount of metal that has been removed after a defined period. If you lose 0.2 mm in one year, the corrosion rate is 0.2 mm/year and the depth after that year is 0.2 mm. Over multiple years, depth builds up and represents the total thickness lost, while the rate could stay the same or vary over time. The other choices mix up what each term means or describe unrelated phenomena like color change or surface roughness.