Environmental Engineering Reference
In-Depth Information
therefore, based on either of these two manifestations, i.e., weight loss and pene-
tration into the metal.
The most widely used rate expression based on weight loss is mg/dm
2
/day
(mdd) and the rate expressions based on penetration are inch penetration/year
(ipy) and mils penetration/year (mpy). One mil is one-thousandth of an inch. The
last expression is very convenient as it does not involve a decimal point or zeros.
Thus 0.002 ipy is simply expressed as 2 mpy. The expression is readily calculated
from the weight loss of the metal specimen during the corrosion test by the empir-
ical formula:
534
W
DAt
mpy
(2.45)
where
W
is weight loss, mg
D
is density of metal, g/cm
3
A
is area of specimen, in.
2
t
is exposure time, h
mdd and mpy are convertible through the following multiplying factors:
mdd
1.44
density
mpy
(2.46)
mdd
mpy
0.696
density
(2.47)
Corrosion rates are often measured electrochemically (discussed later) in terms
of current density. However, the corrosion data are rarely presented in terms of
current density; they are converted to mdd or mpy.
1.117
n
at. wt.
1 mdd
10
5
A/cm
2
(2.48)
2.5.2 Exposure Tests
The simplest of corrosion rate measuring tests is the exposure test. A specimen
of known initial weight is exposed to the corrosive environment for a specified
length of time at the end of which it is taken from the solution, weighed to
ascertain the weight loss, and the corrosion rate is then calculated by dividing
with the area of specimen and the time of exposure. However, specimens of any
size or form cannot be used for the sake of standardization and comparison of
data from different sources. Surface preparation also should follow standardized
practices, usually a finish with no. 120 abrasive cloth or paper. The details of
specimen preparation are provided in ASTM Standard No. G1-88.
The exposure of the specimens varies from simple hanging in the test solution
