Expressions and Measures of Corrosion Rates

There are three main methods that are used to express the corrosion rate  

Thickness reduction per unit time is the measure of most practical signifrcance and interest. In the metric system this measure is usually expressed in mm/year. In some literature one ean still find the unit mils per year (mpy) = 1/1000 inches per year, possibly also inches per year (ipy). 

Weight loss per unit area and unit time was commonly used in earlier times, mainly because weight loss was usually the directly determined quantity in corrosion testing. Here the test specimens were weighed before and after the exposure to the corrosion medium. On this basis one could calculate the thickness reduction as weight loss per unit area/density. 

Corrosion current density is a particularly suitable measure of corrosion rate when treating corrosion theory and in connection with electrochemical corrosion testing. Current density is also directly applicable for cathodic and anodic protection (Sections 10.4 and 10.5). In corrosion testing the unit pA/cm is most often used. When dealing with cathodic protection the units mA/m and A/m are used for the cathode (sfructure to be protected) and the anode, respectively. 

The relationship between thickness reduction per time unit ds/dt (on each corroding side of the specimen/component) and the corrosion current density icoir is determined from Faraday s equations  

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