As we have discussed in our
previous post about the “Corrosion Engineering” where we have seen the basic of corrosion
mechanism, we have also discussed the engineering materials that may suffer
from corrosion in our previous post “corrosion failure examples”.
Let us go ahead with new
post in respect of corrosion engineering, where we will see the basic technique
for determining the corrosion rate.
Corrosion measurement principle
Mixed potential theory will yield the
two electrochemical processes for determining the rate of corrosion in an
engineering material, Let us discuss the both electrochemical method as
mentioned below.
Current will be measured with the help of
ammeter A and potential of electrode will be measured w. r .t to reference
electrode by potentiometer electrometer circuit. When we reduce the resistance,
current will be increased and we will measure the current and potential at
various setting.
Tafel extrapolation techniques
The Tafel extrapolation process for calculating corrosion rate was given by Wagner and Traud in order to verify the concept of mixed potential theory. According to this concept, data will be obtained from anodic or cathodic polarization measurements. Cathodic polarization data will be preferred as these data will be much easier to measure experimentally. Let us draw a diagram, as shown below, in order to measure the data for anodic or cathodic polarization.
Figure 1:
Electric circuit for anodic or cathodic polarization measurements
The metal sample will be considered as the
working electrode, and anodic current or cathodic current will be supplied to
it with the help of an auxiliary electrode which will be composed of inert
material such as Platinum
A Tafel plot will be performed for a metal workpiece
by polarizing the workpiece approximate 300 mV anodically (positive-going
potential) and cathodically (negative-going potential) from the corrosion
potential, ECORR. The resulting current will be plotted on a
logarithmic scale as shown in below figure.
Figure 2: Tafel Extrapolation Techniques
With the help of above curve, we may secure the
corrosion current i.e. ICORR, by using the following equations.
ICORR = corrosion current density, μ A/cm2
M = molecular weight
n = number of electrons involved in the
electrochemical reaction
E.W. = equivalent weight of corroding species, g
d = density of corroding species, g/cm3
With the help of this procedure it is
feasible to determine corrosion rates which will be extremely low, and it might
be used for continuous monitoring the corrosion rates of a system.
Let us consider the curve in order to
determine the rate of corrosion
Figure 3: Corrosion rate measurement
Linear Polarization Techniques
There were few disadvantage of earlier
discussed technique i.e. Tafel extrepolation method, hence in order to overcome
the disadvantage of using Tafel extrepolation method, new technique was
analyzed and that is linear-polarization analysis method. It was concluded that
applied current density will be a linear function of electrode potential as
displayed in following figure.
Figure 4: Applied-current
linear-polarization curve
This curve will display the first 20 mV
polarization of curve. The slope of this linear-polarization curve will be
related to kinetic parameters of the system as indicated below.
ba
and bc
: Tafel slops of the anodic and cathodic
reactions respectively
Rp: Corrosion resistance
Rp = DE/Diapp
We first have to secure the values of
beta and then we will secure the corrosion rate with the help of first equation
that we have discussed during first technique.
Comparison of corrosion rates
Let us draw a table to understand the
corrosion rate whether corrosion rate is good or bad, so let us see the
following table as mentioned here.
Figure 5: Comparison rate corrosion
Where, m p y means mils penetration per
year
Keep
reading.....
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