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APPENDIX B

EXAMPLE CALCULATION OF CORROSION RATE FROM POLARIZATION RESISTANCE

Corrosion rate, CR, was calculated from the measured polarization resistance, PR, using a modified form of the Stern-Geary equation,

Modified form of the Stern-Geary equation expressing the inverse relationship between corrosion rate and polarization resistance. Corrosion rate equals 3.15 times 10 to the power of seven; that times the quotient of the atomic weight of the corroding metal divided by the product of the number of electrical equivalents, Faraday’s constant, and metal density; that times the quotient of a term that depends on the Tafel constant divided by the measured polarization resistance.
(3.2)

where Z is atomic weight, n is the number of electrical equivalents, F is Faraday’s constant, ρ is alloy density, and B is a term the value for which depends on the Tafel constants. For the case of black bar reinforcing steel, a typical measured value for PR was 103 Ω·cm2. Thus, with values for the other constants as,

Z = 56 grams/mol,
n = 2 equivalents,
F = 96,500 Coulombs/mol·equivalent,
ρ = 7.87 grams/cm3, and
B = 0.026 V,
appendix b
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This page last modified on 09/19/07
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