The numerical result obtained by fitting corrosion data to a model is generally a corrosion current. More practical, however, is to think about corrosion rates in more useful terms, such as a corrosion rate in millimeters per year. How is corrosion current used to generate a corrosion rate?
Assume an electrolytic dissolution reaction involving a chemical species, S:
S → S^{n+} + ne^{–}
You can relate current flow to mass via Faraday's Law.
Q_{S} = n F M_{S}
where
Q_{S} is the charge in coulombs resulting from the reaction of species S,
n is the number of electrons transferred per molecule or atom of S,
F is Faraday's constant = 96 485.34 coulombs/mole,
M_{S} is the number of moles of species S reacting.
A more useful form of Faraday’s Law requires the idea of equivalent weight. The equivalent weight (EW_{S}) is the mass of species S that will react with one faraday of charge. For an atomic species, EW = AW/n (where AW is the atomic weight of the species). For a complex alloy that undergoes uniform dissolution, the equivalent weight is a weighted average of the equivalent weights of the alloy components. Mole fraction, not mass fraction, is used as the weighting factor. If the dissolution is not uniform, you may have to measure the corrosion products to calculate EW.
Substituting into Faraday’s Law we get:
W_{S} = EW_{S} Q_{S} / F
where
W_{S} is the mass of species S that has reacted.
In cases where the corrosion occurs uniformly across a metal's surface, you can calculate the corrosion rate in units of distance per year. Be careful: this calculation underestimates the problem when localized corrosion occurs!
Conversion from a weight loss to a corrosion rate (CR) is straightforward. We need to know the density, d, and the sample area, A. Charge is given by Q = I t, where t is the time in seconds and I is a current. We can substitute in the value of Faraday's constant. Modifying the previous equation,
CR = I_{corr} K EW / d A
CR | The corrosion rate. Its units are given by the choice of K |
I_{corr} | The corrosion current in amperes. |
K | A constant that defines the units for the corrosion rate. |
EW | The equivalent weight in grams/equivalent |
d |
Density in grams/cm³ |
A | Area of the sample in cm² |
The following table shows the value of K used in the corrosion rate equation for corrosion rates in the units of your choice.
Units for Corrosion Rate |
K |
Units |
mm/year |
3272 |
mm/(A cm year) |
µm/year |
3.272 ×10^{6} |
µm/(A cm year) |
mils/year (Gamry default) |
1.288 × 10^{5} |
mils/(A cm year) |
See ASTM Standard G 102, Standard Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements, for further information. The standard can be obtained from ASTM, 1916 Race St., Philadelphia, Pennsylvania 19013-1187 (USA) www.astm.org.