List three kinds of general corrosion rates.

• A. Linear; Decreasing with Time; Increasing with Time.
• B. Static; Dynamic; Variable.
• C. Exponential; Logarithmic; Quadratic.
• D. Constant; Uniform; Pitting.

Answer: (A)

General corrosion rate describes how fast metal is being lost over time under a fairly uniform attack. The three common ways this rate can behave with time are linear, decreasing with time, and increasing with time.

Linear means the metal loss per unit area is constant, so the total loss grows directly with time. This happens when the environment and the surface conditions keep delivering the same amount of attack and there’s no developing barrier or changing chemistry to slow it down.

Decreasing with time occurs when protective effects develop on the surface, such as the growth of a scale or oxide film that blocks further reaction, or when diffusion of reactants to the metal becomes more limited. As these barriers form, the rate diminishes as time goes on.

Increasing with time happens when the protective features break down or the environment becomes more aggressive, causing the attack to speed up over time. This can happen if films crack or detach, or if conditions shift to favor faster corrosion.

Other terms like uniform or pitting describe the nature or pattern of corrosion rather than how the rate changes with time, so they don’t capture these time-related rate trends.