Upload
dung-bo
View
118
Download
16
Embed Size (px)
DESCRIPTION
Corrosion rate
Citation preview
Lecture 3
Corrosion rates Faraday law
Michael Faraday (1791-1867)
For example: to calculate how many gram of iron will be oxidized by a direct current charge
from metals surface into the surrounding electrolyte at a current flow of one ampere for one
year.
Corrosion current density
Corrosion current values may be obtained from galvanic cells and polarization
measurements
nF
irnF
M
WLi
)3600.24(
Where:
n is the number of electrons associated with the ionization of each metal atom,
F is 96,500 C/mol, I current density in A/cm2, M molecular weight of metal, WL in
g/cm2.day, (24x3600) days time in second. r is the reaction rate
Corrosion current density can be also determine by expression as below
Equivalent Weight
Equivalent Weight
For alloy: consider a unit mass of alloy oxidized
Therefore, the alloy equivalent weight, EW, is the reciprocal of this quantity:
Calculation of corrosion rate
Penetration rate (CR)
Mass loss rate (MR)
Weight loss (WL)
The corrosion rate or the rate of material removal as a consequence of the chemical
action, is an important corrosion parameter. It may be present by weight loss WL:
Where:
W is the weight loss after exposure corrode time t [days],
A represent the exposed specimen area [cm2].
The WL is conveniently expressed in terms of [g/cm2.day] or [g/cm2.year].
At
WWL
Corrosion penetration rate
This may be expressed as the corrosion penetration rate (CPR), or the thickness loss
of material per unit of time. The formula for this calculation is
where W is the weight loss after exposure time t; = density of metal and A = exposed
specimen area, respectively. K is a constant to change unit.
The CPR is conveniently expressed in terms of either mils per year (mpy) or
millimeters per year (mm/yr).
In the first case, K = 534 to give CPR in mpy (where 1 mil = 0.001 in.),
KWL
At
KWCPR
Classification of
corrosion resistance
after ASTM
Sign CPR, mm/yrCPR.
m/yr
Very good A 0,02 0,1 25 100
Good B > 0,1 0,5 > 100 500
Average C > 0,5 1 > 500 1000
Poor D > 1 5 > 1000 5000
Very poor E > 5 > 5000
Classification of materials base on CPR
Base on corrosion penetration rate (CPR), materials can be divided into 5 groups as
below:
Classification SignCPR
mm/yr
Outstanding < 0.02
Very good A 0.02 0.1
Good B 0.1 0.5
Average C 0.5 1
Poor D 1 5
Very poor E > 5