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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.

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

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.

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

Let us go ahead toward next post
"Effect of corrosion"

Keep reading.....

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