1. Standard Expressions for Corrosion Rate
CHAPTER 2
Standard Expressions for Corrosion Rate

2. A good corrosion rate expression should involve
familiar units
easy calculation with minimum opportunity for error
ready conversion to life in years
penetration
whole number without cumbersome decimals.

3. where W = weight loss, mg
D = density of specimen, g/cm3
A = area of specimen, sq in.
T = exposure time, hr

4. Conversion from other units to obtain mils per year is
Multiply By
in./yr
1000
in./month
12,100
mg/dm2/day (mdd)
1.44/specific gravity

5. Some equivalent metric penetration rates are :
The corrosion rates of usefully resistant materials generally range between 1 and 200 mpy.

6. where W is the weight loss in milligrams, D is the density in g/cm3, A is the area in cm2, and T is the time exposure in hours.

7. Corrosion rates determined by linear polarization and other electrochemical techniques are expressed in terms of current density. These expressions can be converted into penetration rates by the following expression based on Faraday’s law :

8. Corrosion penetration rate =
where
a is the atomic weight of the metal
i is the current density in A/cm2
n is the number of electrons lost (valence change)
D is the density in g/cm3
K is a constant depending on the penetration rate desired.

9. For mpy, m/yr, and mm/yr, the values of K are 0. 129, 3. 27, and 0
For mpy, m/yr, and mm/yr, the values of K are 0.129, 3.27, and , respectively. The parameter n is determined by analysis of the solution or by measuring the potential and pH and referring to appropriate thermodynamic data.

10. The atomic weight a and n values for alloys can be calculated from weighted averages of their components.

11. Consider iron corroding in air - free acid at an electrochemical corrosion rate of 1 A/cm2. It dissolves as ferrous ions (Fe2+), and thus, n = 2. The corrosion rate in mpy is obtained by substitution :

12. Table 2 - 1 Comparison of Corrosion - rate Expressions for Engineering applications
Comment
Weight loss, g or mg
Poor - sample shape and exposure time
Percent weight change
influence results.
Milligrams per square decimeter per day (mdd)
Good - but expressions do not give
Grams per square decimeter per day
penetration rates.
Grams per square centimeter per hour

13. Table 2 - 1 Comparison of Corrosion - rate Expressions for Engineering applications (continue)
Comment
Grams per square meter per hour
Grams per square inch per hour
Moles per square centimeter per hour

14. Table 2 - 1 Comparison of Corrosion - rate Expressions for Engineering applications (continue)
Comment
Inches per year
Better - expressions give penetration rates.
Inches per month
Millimeters per year
Mils per year (mpy)
Best - expresses penetration without
decimals or large numbers.