CHAPTER 2
Standard Expressions
for Corrosion Rate
A good corrosion rate expression
should involve
(1) familiar units
(2) easy
calculation
with
minimum
opportunity for error
(3) ready conversion to life in years
(4) penetration
(5)whole number without cumbersome
decimals.
534 W
Mils per year =
DAT
where W = weight loss, mg
D = density of specimen, g/cm3
A = area of specimen, sq in.
T = exposure time, hr
Conversion from other units to
obtain mils per year is
Multiply
By
in./yr
1000
in./month
12,100
mg/dm2/day
1.44/specific
(mdd)
gravity
• Some equivalent metric penetration
rates are :
mm
m
nm
pm
1 mpy = 0.0254
= 25.4
= 2.90
= 0.805
yr
yr
hr
sec
• The
corrosion
rates
of
usefully
resistant materials generally range
between 1 and 200 mpy.
m
yr
W
= 87,600
DAT
mm
W
= 87.6
yr
DAT
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.
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 :
ai
Corrosion penetration rate = K
nD
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.
For mpy, m/yr, and mm/yr, the values of
K are 0.129, 3.27, and 0.00327, respectively.
The parameter n is determined by analysis
of the solution or by measuring the
potential
and
pH
and
referring
appropriate thermodynamic data.
to
The atomic weight a and n values for
alloys can be calculated from weighted
averages of their components.
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 :
(55.8) (1)
Penetration rate = 0.129
= 0.46 mpy
(2) (7.86)
Table 2 - 1 Comparison of Corrosion - rate
Expressions for Engineering applications
Expression
Weight loss, g or mg
Percent weight change
Milligrams per square
decimeter per day (mdd)
Grams
per
square
decimeter per day
Grams
per
square
centimeter per hour
Comment
Poor - sample shape and
exposure time
influence results.
Good - but expressions
do not give
penetration rates.
Table 2 - 1 Comparison of Corrosion - rate
Expressions for Engineering applications (continue)
Expression
Grams per square meter
per hour
Grams per square inch
per hour
Moles
per
square
centimeter per hour
Comment
Table 2 - 1 Comparison of Corrosion - rate
Expressions for Engineering applications (continue)
Expression
Inches per year
Comment
Better - expressions
give penetration rates.
Inches per month
Millimeters per year
Mils per year (mpy)
Best
expresses
penetration without
decimals
or
large
numbers.