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CORROSION MONITORING AND MEASUREMENT TECHNIQUE :red_flag: - Coggle Diagram
CORROSION MONITORING AND MEASUREMENT TECHNIQUE
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Weight Loss measurement
The Weight Loss technique is the best known and simplest of all corrosion monitoring techniques.
The method involves exposing a specimen of material (the coupon) to a process environment for a given duration, then removing the specimen for analysis.
Advantages of weight loss coupons are that:
The technique is applicable to all environments - gases, liquids, solids/particulate flow.
Visual inspection can be undertaken.
Corrosion deposits can be observed and analyzed.
Weight loss can be readily determined and corrosion rate easily calculated.
Localized corrosion can be identified and measured.
Inhibitor performance can be easily assessed
Potentiodynamic polarization curves
Potentiodynamic polarization is the characterization of a metal specimen by its current - potential relationship.
When a specimen is in contact with a corrosive liquid and the specimen is not connected to any instrumentation — as it would be "in service"— the specimen assumes a potential (relative to a reference electrode) termed the corrosion potential
A potentiodynamic anodic polarization plot such as Figure 1 can yield important information such as:
The ability of the material to spontaneously passivate in the particular medium
The potential region over which the specimen remains passive
The corrosion rate in the passive region
Cell Design
(Requirements)
Working electrode
Representative
free of crevices
free of edge effects
free of galvanic effects
reproducible
free of water-line effects
Mass transport
Methods of controlling mass transport
rotating disk or cylinder
flow channel
jet impingement
gas bubbling
Linear polarization resistance
The Linear Polarization technique is useful for determining the effect on the corrosion rate of a number of possible corrosion inhibitors added at different concentrations.
Polarization Resistance Plot
Polarization resistance is an extremely rapid procedure
for determining corrosion rates. At a scan rate of 0.1 mV/sec, a potential range of 50 mV requires less than ten minutes.
The rapidity of the measurement also makes polarization resistance useful for less rigorous experiments, such as qualitative evaluation of inhibitors
Electrochemical impendance measurement
Impedance is an opposition to the flow of electrons or current.
In direct current (dc) circuits, only resistors produce this effect. However, in alternation current (ac) circuits, two other circuit elements, capacitors and inductors, impede the flow of electrons.
In dc theory (a special case of ac theory where the frequency equals 0 Hz) resistance is defined by Ohm's Law:
E = I R
In ac theory, where the frequency is non-zero, the analogous equation is:
E = I Z
EIS concept
Electrochemical impedance is usually measured by applying an AC potential to an electrochemical cell and measuring the current through the cell. Suppose that we apply a sinusoidal potential excitation.
Most of the circuit elements in the model are common electrical elements such as resistors, capacitors, and inductors.
The impedance of an inductor increases as frequency increases
The impedance versus frequency behavior of a capacitor is opposite to that of an inductor
Open circuit potential decay
