Discussion Examples for Polarization Curves | Corrosion Current, Corrosion Potential, and Reaction Rate

What Is a Polarization Curve?

A polarization curve is a curve that records the current flowing when the potential of a metal electrode is changed.
Potential is often plotted on the horizontal axis and current or current density on the vertical axis, and the curve is used to investigate how readily corrosion reactions proceed and the characteristics of electrode reactions.
In corrosion studies, the dissolution reaction of the metal and reduction reactions can be discussed by examining polarization behavior in the anodic and cathodic directions.

When a metal is immersed in a solution, oxidation of the metal and reduction reactions in the solution proceed simultaneously.
In a polarization measurement, the potential is shifted away from this natural state and changes in the reaction rate are observed as current.
Therefore, a polarization curve can be regarded as a visualization of corrosion reactions in terms of current.

Main Items to Include in the Results

In the results of polarization-curve measurements, organize the type of metal sample, electrolyte, pH, temperature, reference electrode, counter electrode, potential scan range, scan rate, natural potential, corrosion potential, corrosion current density, anodic current, cathodic current, passive region, limiting current, and other information.
Because the shape of the curve changes greatly depending on the measurement conditions, it is important to clearly describe those conditions.

Main Items to Include in the Results

• Type of metal sample
• Pretreatment of the sample surface
• Type of electrolyte
• Electrolyte concentration
• pH
• Temperature
• Presence or absence of dissolved oxygen
• Type of reference electrode
• Type of counter electrode
• Potential scan range
• Scan rate
• Natural potential or open-circuit potential
• Corrosion potential
• Corrosion current density
• Trend of the anodic polarization curve
• Trend of the cathodic polarization curve
• Presence or absence of a passive region
• Presence or absence of limiting current
• Causes of error and points for improvement

Reference Experimental Values and Calculation Examples for Polarization-Curve Measurements

Here, the process of measuring a polarization curve of a metal sample and determining the corrosion potential, corrosion current density, Tafel extrapolation, and corrosion rate is organized using reference experimental values.

A polarization curve is obtained by measuring the current that flows when the electrode potential is changed and is used to investigate the corrosion reaction of a metal and how readily electrode reactions proceed.
Near the corrosion potential, the metal-dissolution reaction and reduction reaction are balanced, and by determining the current density at this point through extrapolation, the tendency for corrosion to proceed can be evaluated.

Reference Experimental Conditions

Example of Natural-Potential Measurement

Before measuring the polarization curve, the sample is immersed in the solution and the natural potential is measured without externally applying current.
Performing the polarization measurement after the natural potential has stabilized to some extent makes the results easier to compare.

In this reference example, the natural potential after 30 minutes was -0.667 V.
In the polarization-curve measurement, the current responses on the anodic and cathodic sides are measured using the region around this natural potential as a reference.

Example of Polarization-Curve Measurement Results

The following is an example in which current density was measured while the potential was changed.
The current density is small near the corrosion potential and increases when the potential is shifted toward either the anodic or cathodic side.

How to Determine Corrosion Potential and Corrosion Current Density

The corrosion potential Ecorr is the potential at which the anodic and cathodic reactions are balanced.
In a polarization curve, the linear Tafel regions on the anodic and cathodic sides are extrapolated, and the corrosion potential and corrosion current density are determined from the point at which the two lines intersect.

It is important that the corrosion current density is not obtained by directly reading the current flowing at the corrosion potential, but instead by extrapolating the Tafel region.
Near the corrosion potential, the anodic and cathodic currents cancel each other, so even if the measured net current appears small, this does not necessarily mean that corrosion is not occurring.

Concept of Tafel Extrapolation

In Tafel extrapolation, regions in which the relationship between potential and log|current density| is approximately linear are selected, and straight lines on the anodic and cathodic sides are extended toward the corrosion potential.
Their intersection gives the estimated corrosion potential Ecorr and corrosion current density icorr.

Example Calculation of Corrosion Rate

The corrosion rate can be approximately calculated from the corrosion current density.
Here, an example is shown in which the corrosion rate of carbon steel is determined using the following simplified equation.

Corrosion rate (mm/year) = 0.0116 × icorr

In this equation, icorr is expressed in μA/cm².
When the corrosion current density is 12.0 μA/cm², the corrosion rate is calculated as follows.

Corrosion rate = 0.0116 × 12.0 = 0.139 mm/year

Therefore, in this reference example, the corrosion rate of carbon steel is estimated to be approximately 0.14 mm/year.

Comparison of Polarization Curves Under Different Solution Conditions

Corrosion behavior changes depending on the chloride-ion concentration, pH, amount of dissolved oxygen, presence or absence of a corrosion inhibitor, and other factors in the solution.
The following shows examples of corrosion potential and corrosion current density when the solution conditions are changed.

In this reference example, the higher the NaCl concentration, the more the corrosion potential shifts in the active direction and the larger the corrosion current density becomes.
On the other hand, under conditions where a corrosion inhibitor was added, the corrosion current density became smaller and the corrosion rate also decreased.

Example Calculation of Inhibition Efficiency

When evaluating the effect of a corrosion inhibitor, the inhibition efficiency may be calculated by comparing the corrosion current density without the inhibitor with that in the presence of the inhibitor.

Inhibition efficiency (%) = (icorr, without inhibitor − icorr, with inhibitor) ÷ icorr, without inhibitor × 100

In a 3.5 mass% NaCl aqueous solution, if icorr without the inhibitor is 12.0 μA/cm² and icorr with the inhibitor is 2.6 μA/cm², the inhibition efficiency is calculated as follows.

Inhibition efficiency = (12.0 − 2.6) ÷ 12.0 × 100 = 78.3%

From this result, the addition of the corrosion inhibitor greatly reduced the corrosion current density and was considered to suppress the corrosion reaction.

Example of How to Write the Results

A carbon-steel specimen was immersed in a 3.5 mass% NaCl aqueous solution, and the natural potential was measured.
After 30 minutes, the natural potential had stabilized at approximately -0.667 V.
The polarization curve was then measured, and extrapolation of the anodic and cathodic Tafel linear regions gave a corrosion potential Ecorr of -0.660 V and a corrosion current density icorr of 12.0 μA/cm².

The corrosion rate calculated from the corrosion current density was 0.139 mm/year.
In addition, comparison of the solution conditions showed that icorr was 1.8 μA/cm² in pure water, whereas it was 12.0 μA/cm² in 3.5 mass% NaCl aqueous solution, confirming that corrosion proceeded more readily in an environment containing chloride ions.

Points for Connecting the Results to the Discussion

In a discussion of polarization curves, it is important not only to present the values of the corrosion potential and corrosion current density but also to explain how those values are related to the tendency of the corrosion reaction to proceed.

• Did the natural potential stabilize over time?
• Can the reason why the corrosion potential shifted in the noble or active direction be explained?
• Was the corrosion rate also larger under conditions with a larger corrosion current density?
• Did the corrosion current density increase under conditions with a higher NaCl concentration?
• Did the addition of a corrosion inhibitor reduce the corrosion current density?
• Were the linear regions used for Tafel extrapolation appropriate?
• Could solution resistance, polishing condition of the electrode surface, oxide films, dissolved oxygen, or temperature have affected the results?

Example Discussion

In this experiment, the polarization curve of carbon steel was measured, and the corrosion potential and corrosion current density were determined by Tafel extrapolation.
In a 3.5 mass% NaCl aqueous solution, the corrosion potential Ecorr was -0.660 V and the corrosion current density icorr was 12.0 μA/cm².
The corrosion rate calculated from the corrosion current density was 0.139 mm/year, indicating that corrosion readily proceeds in an environment containing chloride ions.

Comparison of the solution conditions showed that icorr was 1.8 μA/cm² in pure water, 5.5 μA/cm² in 0.5 mass% NaCl aqueous solution, and 12.0 μA/cm² in 3.5 mass% NaCl aqueous solution.
This was considered to result from the increase in NaCl concentration increasing the electrical conductivity of the solution and, furthermore, making chloride ions more likely to destroy the protective film on the metal surface.

On the other hand, in the 3.5 mass% NaCl aqueous solution containing a corrosion inhibitor, icorr decreased to 2.6 μA/cm².
The inhibition efficiency compared with the condition without the corrosion inhibitor was 78.3%, and the addition of the inhibitor was considered to have greatly suppressed the corrosion reaction.
This was considered to result from the corrosion inhibitor adsorbing onto the metal surface and hindering the anodic reaction or cathodic reaction.

However, in estimating the corrosion current density by Tafel extrapolation, the result may change depending on which range is treated as the linear region.
The polishing condition of the sample surface, oxide films, solution resistance, dissolved oxygen, temperature, and other factors also affect the polarization curve.
Therefore, it is important to standardize the measurement conditions and perform multiple measurements to confirm reproducibility.

Summary

In polarization-curve measurements, the susceptibility of a metal to corrosion can be electrochemically evaluated by determining the corrosion potential and corrosion current density.

In this reference example, the corrosion current density increased as the NaCl concentration increased, and the corrosion rate also increased.
On the other hand, the corrosion current density decreased when a corrosion inhibitor was added, suppressing corrosion.
In a report, it is useful to discuss the corrosion potential, corrosion current density, corrosion rate, Tafel extrapolation, and corrosion-inhibition effect in relation to one another.

What Is the Corrosion Potential?

The corrosion potential is the potential at which the anodic and cathodic reactions are balanced when a metal is in a solution without external current flowing.
It may be used in a sense close to natural potential or open-circuit potential.
At this potential, the anodic current caused by dissolution of the metal and the cathodic current caused by oxygen reduction or hydrogen evolution are equal in magnitude.

A more active corrosion potential may in some cases indicate that the metal is in a state in which it is more easily oxidized.
However, the corrosion rate cannot be directly judged from the corrosion potential alone.
To consider the corrosion rate, the corrosion current must also be evaluated.

What Is the Corrosion Current?

The corrosion current is the current corresponding to the corrosion reaction of a metal at the corrosion potential.
At the corrosion potential, the total current observed externally is approximately 0, but in reality, the oxidation reaction of the metal and the reduction reaction proceed simultaneously.
The current corresponding to these internal reactions is the corrosion current.

The larger the corrosion current density, the faster the metal-dissolution reaction per unit area is considered to proceed.
Therefore, corrosion current density is used as an indicator of corrosion rate.
In a polarization curve, the corrosion current density is estimated by methods such as Tafel extrapolation.

Discussion of Anodic Polarization

Anodic polarization means changing the electrode potential in a direction more noble than the corrosion potential.
The metal becomes more likely to lose electrons, and the dissolution reaction of the metal proceeds.
Therefore, in the anodic polarization curve, an increase in current associated with oxidative dissolution of the metal is observed.

For example, in iron, the reaction in which Fe dissolves as Fe2+ corresponds to the anodic reaction.
However, depending on the metal, an oxide film may form above a certain potential and passivation may occur, keeping the current at a low level.
The shape of the anodic polarization curve reflects the ease of metal dissolution and the formation of surface films.

Fe → Fe2+ + 2e-

M → Mn+ + ne-

Discussion of Cathodic Polarization

Cathodic polarization means changing the electrode potential in a direction more active than the corrosion potential.
At this time, reduction reactions such as oxygen reduction and hydrogen evolution become more favorable on the metal surface.
In the cathodic polarization curve, current corresponding to these reduction reactions is observed.

In neutral aqueous solutions, the reduction of dissolved oxygen is often important.
In acidic solutions, hydrogen evolution caused by reduction of H+ becomes more likely.
Which cathodic reaction is dominant changes depending on pH, dissolved oxygen, potential range, and the condition of the metal surface.

O2 + 2H2O + 4e- → 4OH-

2H+ + 2e- → H2

Concept of Tafel Extrapolation

Tafel extrapolation is a representative analytical method used to determine the corrosion current density and corrosion potential from a polarization curve.
A region in which a linear relationship is observed between potential and the logarithm of current density is called the Tafel region.
The linear portions on the anodic and cathodic sides are extrapolated, and the corrosion potential and corrosion current density are estimated from their intersection.

In Tafel extrapolation, the range selected as the linear region greatly affects the result.
If a diffusion-controlled region, a passive region, or a region with large noise is used, the corrosion current density may not be estimated correctly.
Therefore, it is important to carefully examine the shape of the curve when selecting the analysis range.

Relationship Between Corrosion Current Density and Reaction Rate

Corrosion current density is an indicator of how rapidly a metal dissolves per unit area.
Because electric current represents the amount of electron transfer, a larger current density indicates that metal atoms are being oxidized to metal ions at a faster rate.
Therefore, a metal with a larger corrosion current density can be evaluated as having a higher corrosion rate.

However, to determine the actual corrosion rate from the corrosion current density, the number of electrons involved in the reaction, the atomic weight of the metal, its density, and other factors must be considered.
In addition, if localized corrosion is occurring, the average current density alone may not sufficiently represent locally deep corrosion.
It is useful to judge the results of the polarization curve together with surface observations.

When the Corrosion Potential Shifts in the Noble Direction

When the corrosion potential shifts in the noble direction, the metal may have become more difficult to oxidize, or the cathodic reaction may have changed.
If an oxide film or protective film forms on the metal surface, metal dissolution may be suppressed and the corrosion potential may shift in the noble direction.
The corrosion potential may also change when a corrosion inhibitor is added.

However, a nobler corrosion potential does not necessarily mean that the corrosion rate has decreased.
To judge the corrosion rate, it is also necessary to confirm whether the corrosion current density has decreased.
This is because the corrosion potential represents the balance point of reactions and does not directly indicate the speed of corrosion.

When the Corrosion Potential Shifts in the Active Direction

When the corrosion potential shifts in the active direction, the metal may have entered a state in which it dissolves more actively.
If a surface film is destroyed or localized corrosion is promoted by chloride ions, the corrosion potential may shift in the active direction.
A shift in the active direction may also be observed when metal dissolution proceeds under acidic conditions.

However, this also does not directly indicate the corrosion rate itself.
Even if the corrosion potential becomes more active, the corrosion current may be small if the cathodic reaction is suppressed.
Therefore, the corrosion potential and corrosion current are interpreted together.

Discussion When Passivation Is Observed

Passivation is a phenomenon in which a dense oxide film or similar layer forms on the metal surface and suppresses the metal-dissolution reaction.
In a polarization curve, it may be observed as a region where the current remains at a low value even when the potential is increased in the anodic direction.
In stainless steel, aluminum, and similar metals, passive films are strongly related to corrosion resistance.

A metal with a wide passive region and low passive current density is considered to have its dissolution suppressed by a surface film.
However, in the presence of chloride ions, the passive film may be destroyed and pitting corrosion may occur.
If a sudden increase in current is observed in the polarization curve, breakdown of the passive film or transpassive oxidation should be considered.

Discussion of Diffusion-Limited Current

In a cathodic polarization curve, the current may approach a constant value even when the potential is shifted further in the active direction.
This is a state in which the reaction is limited by the rate at which reactants are supplied to the electrode surface and is called diffusion-limited current.
In an oxygen-reduction reaction, the rate at which dissolved oxygen diffuses to the electrode surface may become the limiting factor.

When a diffusion-limited current is observed, the reaction rate is considered to be controlled by mass transfer rather than charge transfer.
The limiting current may change if stirring or the dissolved-oxygen concentration is changed.
Therefore, diffusion-limited current provides an important clue for considering mass transfer and oxygen concentration in the solution.

Discussion When a Corrosion Inhibitor Is Added

When a corrosion inhibitor is added, it may adsorb onto the metal surface to suppress reactions or form a protective film and thereby inhibit corrosion.
In a polarization curve, this may appear as a decrease in corrosion current density, a change in corrosion potential, or suppression of the anodic or cathodic reaction.
The way the curve changes differs depending on which reaction the inhibitor mainly suppresses.

With an inhibitor that suppresses the anodic reaction, the metal-dissolution current becomes smaller.
With an inhibitor that suppresses the cathodic reaction, the oxygen-reduction or hydrogen-evolution current becomes smaller.
With a mixed-type inhibitor that suppresses both reactions, the overall corrosion current density decreases.

Effect of pH

pH greatly affects the shape of a polarization curve.
Under acidic conditions, hydrogen evolution caused by reduction of H+ occurs readily, and the cathodic current may become large.
The stability of oxide films and hydroxides on the metal surface also changes depending on pH.

Under neutral or basic conditions, oxygen-reduction reactions and hydroxide formation become important.
When conditions with different pH values are compared, the corrosion potential, corrosion current density, and ease of passivation may change.
Therefore, the pH conditions must always be checked when discussing polarization curves.

Effect of Chloride Ions

Chloride ions, Cl-, promote corrosion of many metals.
Cl- may destroy protective films on the surface and cause localized corrosion such as pitting.
In polarization curves, the passive region may become narrower or the increase in current associated with passive-film breakdown may appear at a lower potential.

If the corrosion current density becomes larger in a solution containing NaCl, chloride ions may have promoted metal dissolution.
In addition, increasing the electrolyte concentration lowers the solution resistance and may make corrosion current flow more easily.
In chloride environments, not only uniform corrosion but also localized corrosion should be considered.

Effect of Surface Treatment

Polarization curves are strongly affected by the condition of the metal surface.
Differences in polishing condition, oxide films, oil, contamination, scratches, and pretreatment can change the corrosion potential and corrosion current density.
If the surface is clean and uniform, relatively reproducible curves are easier to obtain.

On the other hand, scratches or contamination on the surface may form local galvanic cells and make localized corrosion more likely.
In addition, the natural potential and polarization behavior may differ immediately after polishing and after an oxide film has formed.
It is important to standardize surface treatment before measurement.

Effect of Scan Rate

In polarization-curve measurements, the rate at which the potential is changed, that is, the scan rate, affects the results.
If the scan rate is too fast, reactions and mass transfer at the electrode surface cannot keep up with the potential change, making it difficult for the measurement to reflect a state close to equilibrium.
As a result, errors may occur in reading the corrosion potential and current values.

A slower scan rate makes it easier to measure a state in which the surface reactions have proceeded sufficiently, but the measurement takes longer and the surface condition may change excessively.
It is important to use a scan rate appropriate for the experimental conditions and to use the same scan rate for samples being compared.

Effect of Solution Resistance

Because the electrolyte has resistance, the measured potential may include a voltage drop within the solution.
This is called IR drop.
When the solution resistance is large, a difference may arise between the actual potential at the electrode surface and the potential measured or controlled by the instrument.

If the IR drop is large, errors may occur in the shape of the polarization curve and in the estimation of corrosion current density by Tafel extrapolation.
Solution conductivity, the position of the reference electrode, and the distance between electrodes affect this phenomenon.
IR compensation may be performed when necessary.

Causes of Error in Polarization-Curve Measurements

Causes of error in polarization-curve measurements include differences in metal-surface pretreatment, instability of the reference electrode, deviations in electrolyte concentration or pH, temperature changes, differences in dissolved-oxygen concentration, inappropriate scan rate, IR drop, adhesion of bubbles, noise, poor electrical connections, and selection of the Tafel region.
Because polarization curves are sensitive to many conditions, it is important to record the measurement conditions.

Causes of overestimating or underestimating the corrosion current density include incorrect selection of the linear region, variation in surface condition, current noise, use of diffusion-controlled regions, and the influence of passive regions.
It is necessary to examine not only the numerical values but also the overall shape of the curve in the discussion.

When the Results Can Be Considered Good

Polarization-curve results can be considered good when the measurement is performed after the open-circuit potential has stabilized, the anodic and cathodic curves are smooth, and Tafel regions can be identified.
In addition, results can be considered reasonable when measurements under the same conditions are reproducible and the trends in corrosion current density and corrosion potential do not greatly contradict surface observations or known corrosion resistance.

If the corrosion current density decreases in a sample subjected to corrosion-protection treatment, a protective effect can be considered to have occurred.
If the corrosion current density increases in a solution containing chloride ions, the corrosion-promoting effect can be judged to have been reflected in the polarization curve.
It is important to clearly describe the correspondence between the results and the conditions.

Example Discussions When the Experiment Did Not Go Well

When a polarization-curve measurement does not go well, possible causes should be considered from results such as a noisy curve, unstable current, difficulty identifying a Tafel region, large differences in corrosion potential among measurements, or corrosion current densities different from expected values.
Organizing the causes according to surface treatment, electrolyte, reference electrode, scan rate, bubbles, and analysis range makes the discussion easier.

How to Write Points for Improvement

In a discussion of polarization curves, writing not only the causes of error but also points for improvement makes the report easier to organize.
Points for improvement can be organized according to the sample surface, measurement conditions, electrode system, and analytical method.

Improvements to the Sample Surface

• Polish the metal surface under the same conditions
• Remove oil and contamination
• Determine the measurement area accurately
• Keep the standing time after polishing constant
• Prevent bubbles from adhering to the surface
• Allow the open-circuit potential to stabilize sufficiently before measurement

Improvements to Measurement Conditions

• Prepare the electrolyte concentration accurately
• Keep the pH constant
• Keep the temperature constant
• Standardize dissolved-oxygen conditions
• Set an appropriate scan rate
• Position the reference electrode appropriately
• Consider IR compensation when necessary

Improvements to the Analytical Method

• Select the Tafel region carefully
• Avoid regions with large noise
• Do not use diffusion-controlled regions for linear extrapolation
• Do not confuse passive regions with Tafel regions
• Perform multiple measurements to confirm reproducibility
• Consider surface observations together with electrochemical data

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of polarization curves, simply writing that “the current increased” or “the corrosion current was large” results in a superficial discussion.
A good discussion relates the anodic reaction, cathodic reaction, corrosion potential, corrosion current, reaction rate, surface film, and solution conditions.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of polarization curves.
Adjust the necessary parts according to your own experimental results.

• A polarization curve shows the current response when the potential is changed.
• During anodic polarization, oxidative dissolution of the metal becomes more favorable.
• During cathodic polarization, reduction reactions such as oxygen reduction and hydrogen evolution become more favorable.
• The corrosion potential is the potential at which the anodic and cathodic reactions are balanced.
• Corrosion current density is an important indicator for evaluating corrosion rate.
• The larger the corrosion current density, the greater the metal-dissolution rate is considered to be.
• The corrosion potential and corrosion current density can be estimated by Tafel extrapolation.
• In the passive region, the metal-dissolution current remains low because of a protective film.
• Diffusion-limited current indicates that the supply rate of reactants is limiting the reaction.
• Polarization curves are affected by surface condition, pH, chloride ions, dissolved oxygen, and scan rate.

Points to Check When Discussing Polarization Curves

Checking the following points before writing the report makes the discussion easier to write.

• Is the definition of a polarization curve explained?
• Is the meaning of corrosion potential described?
• Is corrosion current density related to corrosion rate?
• Are anodic polarization and cathodic polarization distinguished?
• Are the reaction equations for the anodic and cathodic reactions written?
• Is the concept of Tafel extrapolation explained?
• If passivation is present, is its meaning explained?
• If diffusion-limited current is present, is it related to mass transfer?
• Are the effects of pH, chloride ions, and oxygen considered?
• Are the effects of surface treatment and scan rate considered?
• Are causes of error considered separately in terms of measurement and analysis conditions?
• Do the points for improvement correspond to the causes of error?

Summary

A polarization curve is a curve obtained by measuring the current flowing when the potential of a metal electrode is changed and is used to evaluate how readily corrosion reactions and electrode reactions proceed.
The corrosion potential is the potential at which the anodic and cathodic reactions are balanced, and the corrosion current density is an important indicator for evaluating the corrosion rate of the metal.
The larger the corrosion current density, the faster the metal-dissolution reaction is considered to proceed.

During anodic polarization, oxidative dissolution of the metal proceeds, while during cathodic polarization, reduction reactions such as oxygen reduction and hydrogen evolution proceed.
Tafel extrapolation can be used to estimate the corrosion potential and corrosion current density from the polarization curve.
However, the results are affected by the selection of the analysis range, surface condition, solution conditions, scan rate, IR drop, and other factors.

In a report, rather than simply writing that “the current was large or small,” organize and discuss the corrosion potential, corrosion current, anodic reaction, cathodic reaction, Tafel extrapolation, reaction rate, passivation, diffusion limitation, solution conditions, causes of error, and points for improvement.
Polarization curves are important experimental data for electrochemically understanding the corrosion behavior of metals.