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The Corrosion Rate - Introduction

The Corrosion Rate and the Penetration Rate are two important concepts in corrosion control.

The Corrosion Rate is the mass loss per unit area per unit time.

The Corrosion Rate is frequently reported as
Milligrams Per Square Decimeter Per Day (mdd).

The Penetration Rate is the average depth of metal loss.

The most common unit for Penetration Rate used in the USA is
Mils Per Year (mpy).
mil is equivalent to 1/1000 inch. The most common metric unit is Millimeters Per Year (mmpy).

In general, an acceptable Penetration Rate is less than 20 mpy.

There is more than one way to determine the Corrosion Rate or the Penetration Rate of a metal or alloy. This Section outlines procedures to determine the Corrosion Rate or the Penetration Rate using data from either an
Exposure Test or an Electrochemical Test.

It is important to remember that neither the Corrosion Rate nor the Penetration Rate defined by Mass Loss can account for localized corrosion effects. The reason for this is that many localized corrosion processes do not result in significant mass loss. For example, a component can be destroyed as a result of Stress Corrosion Cracking (
SCC) - even if the mass loss is negligible.

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