Measurement of the natural potential of metals is an electrochemical experiment used to investigate what potential a metal surface exhibits when the metal is immersed in a solution.
In solution, metals are affected by oxidation reactions in which metal atoms become metal ions and reduction reactions in which substances such as oxygen or hydrogen ions accept electrons.
As a result, a naturally determined potential is observed for each metal.
Natural potential is an important indicator when considering how readily a metal corrodes.
In general, metals with more active natural potentials tend to be more easily oxidized and more susceptible to corrosion.
In contrast, metals with more noble natural potentials tend to be more difficult to oxidize and less susceptible to corrosion.
However, actual corrosion is affected not only by potential but also by surface films, solution pH, oxygen concentration, chloride ions, temperature, and other factors.
This article clearly explains, as examples of discussions that can be used in laboratory reports on natural-potential measurements of metals, the meaning of natural potential, its relationship with reference electrodes, the difference from standard electrode potential, corrosion susceptibility, potential differences, galvanic corrosion between dissimilar metals, surface condition, solution conditions, causes of error, and points for improvement.
Note:
This article is a reference intended to assist with discussions of natural-potential measurement results for metals obtained in electrochemistry experiments, materials chemistry experiments, inorganic chemistry experiments, and basic chemistry experiments at universities and similar institutions.
For the actual metal samples, electrolyte, reference electrode, measuring equipment, measurement time, surface treatment, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Is the Natural Potential of a Metal?
- Main Items to Include in the Results
- Reference Experimental Values and Calculation Examples for Natural-Potential Measurements
- Reference Experimental Conditions
- Change in Natural Potential Over Time
- How to Read Natural Potentials
- Example Calculation of Potential Difference
- Corrosion Tendency When Dissimilar Metals Are in Contact
- Natural Potential When the Chloride-Ion Concentration Is Changed
- Differences in Natural Potential Caused by Surface Treatment
- Example of How to Write the Results
- Points for Connecting the Results to the Discussion
- Example Discussion
- Summary
- Difference Between Natural Potential and Standard Electrode Potential
- Role of the Reference Electrode
- Natural Potential and Corrosion Susceptibility
- Potential Difference and Galvanic Corrosion
- Discussion of Metals Showing Active Potentials
- Discussion of Metals Showing Noble Potentials
- Relationship with the Ionization Tendency of Metals
- Effect of Surface Films
- Measurement Time and Potential Stabilization
- Effect of pH
- Effect of Dissolved Oxygen
- Effect of Chloride Ions
- Effect of Pretreatment of the Metal Surface
- Effect of Temperature
- When the Natural Potential Becomes More Noble Over Time
- When the Natural Potential Becomes More Active Over Time
- Causes of Error in Natural-Potential Measurements
- When the Results Can Be Considered Good
- Example Discussions When the Experiment Did Not Go Well
- How to Write Points for Improvement
- Difference Between a Superficial Discussion and a Good Discussion
- Examples of Expressions That Can Be Used in Reports
- Points to Check When Discussing Natural-Potential Measurements of Metals
- Summary
What Is the Natural Potential of a Metal?
Natural potential is the electrode potential that a metal naturally exhibits when immersed in a solution without external current being applied.
It may also be called the natural immersion potential, open-circuit potential, or corrosion potential.
At the metal surface, reactions such as metal dissolution, oxygen reduction, and hydrogen evolution occur simultaneously, and the potential is determined by the balance among these reactions.
Natural potential provides a clue for considering how readily a metal is oxidized in a particular solution.
However, natural potential changes not only with the metal itself but also with the type of solution, pH, oxygen concentration, temperature, and the presence or absence of a surface film.
Therefore, it is important to understand natural potential as a value that reflects the condition of a metal surface under specific conditions.
Example Discussion:
Natural potential is the potential measured when a metal is immersed in a solution without external current being applied.
This potential is determined as a result of the balance between the oxidation reaction of the metal and reactions such as oxygen reduction in the solution.
Therefore, natural potential is an important indicator for discussing the corrosion susceptibility and surface condition of a metal.
Main Items to Include in the Results
In the results of natural-potential measurements, organize the type of metal sample, surface treatment, type and concentration of electrolyte, pH, temperature, type of reference electrode, measurement time, change in potential over time, and potential after stabilization.
Because the potential is measured relative to a reference electrode, always record which reference electrode was used.
Main Items to Include in the Results
- Type of metal sample
- Pretreatment of the metal surface
- Sample area
- Type of electrolyte
- Electrolyte concentration
- pH
- Temperature
- Presence or absence of dissolved oxygen
- Presence or absence of chloride ions
- Type of reference electrode
- Potential immediately after measurement begins
- Potential change over time
- Natural potential after stabilization
- Potential difference between metals
- Observed corrosion results
- Causes of error and points for improvement
Example of How to Write the Results:
Several metal samples were immersed in the same electrolyte, and their natural potentials relative to a reference electrode were measured.
The potentials fluctuated immediately after measurement began, but approached constant values over time.
Comparison of the stabilized natural potentials showed differences among the metals, allowing differences in corrosion susceptibility to be discussed.
Reference Experimental Values and Calculation Examples for Natural-Potential Measurements
Here, several types of metals are immersed in an electrolyte solution, and changes in natural potential over time are organized using reference experimental values.
Natural potential is the potential exhibited by a metal when no external current is applied.
It is determined by the balance between oxidation and reduction reactions occurring at the metal surface and provides a clue for considering the corrosion susceptibility of metals and the potential difference when dissimilar metals are brought into contact.
Reference Experimental Conditions
| Item | Details |
|---|---|
| Metals measured | Zinc, iron, copper, aluminum, stainless steel |
| Test solution | 3.5 mass% NaCl aqueous solution |
| Reference electrode | Ag/AgCl electrode |
| Measurement temperature | 25°C |
| Immersion time | 0–30 min |
| Evaluation items | Natural potential, potential stability, potential difference between metals, corrosion tendency |
Change in Natural Potential Over Time
Immediately after a metal is immersed in solution, the natural potential may fluctuate greatly because the surface condition changes.
Therefore, the natural potential is measured at fixed time intervals to confirm whether the potential has stabilized.
| Metal | 0 min | 5 min | 10 min | 20 min | 30 min | Condition after 30 min |
|---|---|---|---|---|---|---|
| Zinc | -0.990 V | -1.025 V | -1.038 V | -1.045 V | -1.047 V | Stabilized at an active potential |
| Iron | -0.570 V | -0.620 V | -0.645 V | -0.658 V | -0.662 V | Gradually shifted in the active direction |
| Copper | -0.090 V | -0.075 V | -0.068 V | -0.064 V | -0.063 V | Stabilized at a relatively noble potential |
| Aluminum | -0.710 V | -0.785 V | -0.830 V | -0.855 V | -0.862 V | Changed while being affected by the surface film |
| Stainless steel | -0.210 V | -0.180 V | -0.165 V | -0.158 V | -0.156 V | Stabilized at a relatively noble potential |
How to Read Natural Potentials
Comparing the natural potentials after 30 minutes, zinc shows the most active potential and copper the most noble potential.
In general, metals showing more active potentials tend to be more easily oxidized and more susceptible to corrosion.
| Metal | Natural Potential after 30 min | Potential Trend | How to Interpret the Corrosion Tendency |
|---|---|---|---|
| Zinc | -1.047 V | Most active | Easily oxidized and commonly used for sacrificial protection |
| Aluminum | -0.862 V | Active | Easily oxidized, but strongly affected by the oxide film |
| Iron | -0.662 V | Somewhat active | Readily corrodes in chloride environments |
| Stainless steel | -0.156 V | Relatively noble | Less susceptible to corrosion because of the passive film |
| Copper | -0.063 V | Most noble | Relatively difficult to oxidize |
Example Calculation of Potential Difference
When different metals are electrically connected, galvanic corrosion may occur because of the difference in their natural potentials.
The potential difference is determined as the difference between the natural potentials of the two metals.
Potential difference = Natural potential of the noble metal − Natural potential of the active metal
For example, when iron and copper are combined, the natural potential after 30 minutes is -0.662 V for iron and -0.063 V for copper.
Potential difference = -0.063 − (-0.662) = 0.599 V
Therefore, a potential difference of approximately 0.60 V is considered to exist between iron and copper.
In this combination, iron, which shows the more active potential, is likely to become the anode, and corrosion of the iron may be accelerated.
Corrosion Tendency When Dissimilar Metals Are in Contact
When dissimilar metals are brought into contact, the metal showing the more active potential tends to become the anode and dissolve, while the metal showing the more noble potential tends to act as the cathode and be protected.
| Combination | Active Metal | Noble Metal | Potential Difference | Side More Susceptible to Corrosion |
|---|---|---|---|---|
| Zinc − Iron | Zinc | Iron | 0.385 V | Zinc |
| Iron − Copper | Iron | Copper | 0.599 V | Iron |
| Aluminum − Copper | Aluminum | Copper | 0.799 V | Aluminum |
| Stainless Steel − Iron | Iron | Stainless steel | 0.506 V | Iron |
| Zinc − Copper | Zinc | Copper | 0.984 V | Zinc |
In this reference example, the combination of zinc and copper has the largest potential difference.
The greater the potential difference, the greater the driving force for corrosion caused by contact between dissimilar metals is considered to become.
Natural Potential When the Chloride-Ion Concentration Is Changed
The natural potential of a metal also changes depending on the type and concentration of the solution.
Here, the natural potentials of an iron sample are compared when the NaCl concentration is changed.
| Sample | Solution Condition | Natural Potential after 30 min | Surface Observation | Corrosion Tendency |
|---|---|---|---|---|
| A | Pure water | -0.420 V | No major change | Relatively small |
| B | 0.5 mass% NaCl | -0.545 V | Partial discoloration | Somewhat more susceptible to corrosion |
| C | 3.5 mass% NaCl | -0.662 V | Reddish-brown corrosion products observed | Susceptible to corrosion |
| D | 10 mass% NaCl | -0.701 V | Clear increase in corrosion products | Very susceptible to corrosion |
As the NaCl concentration increases, the natural potential of iron shifts in the active direction.
Because chloride ions may destroy the protective film on the metal surface and promote localized corrosion, corrosion is considered to proceed more readily in chloride environments.
Differences in Natural Potential Caused by Surface Treatment
Even for the same metal, the natural potential may change depending on the polishing condition of the surface and the presence or absence of an oxide film.
Here, an example of measurements obtained by changing the surface condition of an iron sample is shown.
| Surface Condition | Natural Potential after 30 min | Surface Characteristics | Possible Reason |
|---|---|---|---|
| Immediately after polishing | -0.690 V | Fresh metal surface exposed | A reactive surface is exposed and shows an active potential |
| Left in air for 1 day | -0.620 V | Thin oxide film formed | Surface film shifts the potential somewhat in the noble direction |
| After corrosion-protection treatment | -0.410 V | Surface protected | Corrosion reaction is suppressed and a noble potential is shown |
Natural potential is not determined only by the type of metal but is also strongly affected by the surface condition and solution conditions.
Therefore, when comparing natural potentials, it is important to standardize the pretreatment of the sample surfaces.
Example of How to Write the Results
The natural potentials of each metal were measured in a 3.5 mass% NaCl aqueous solution.
After 30 minutes, the natural potentials were -1.047 V for zinc, -0.862 V for aluminum, -0.662 V for iron, -0.156 V for stainless steel, and -0.063 V for copper.
From these results, zinc showed the most active potential among the metals measured in this experiment, while copper showed the most noble potential.
The potential difference between iron and copper was calculated as -0.063 − (-0.662) = 0.599 V.
Therefore, when iron and copper are electrically connected, iron, which shows the more active potential, may become the anode and corrosion on the iron side may be accelerated.
In addition, when the NaCl concentration was varied for the iron sample, the natural potential was -0.420 V in pure water and -0.662 V in a 3.5 mass% NaCl aqueous solution.
Under conditions with higher NaCl concentration, the natural potential shifted in the active direction and the amount of corrosion products on the surface also increased.
Points for Connecting the Results to the Discussion
In a discussion of natural-potential measurements, it is important not merely to arrange the potentials in order but to explain them in relation to the corrosion tendencies of the metals, dissimilar-metal contact, solution conditions, and surface condition.
- Did the natural potential of each metal stabilize over time?
- Can metals showing more active potentials be explained as more easily oxidized?
- Can metals showing more noble potentials be considered relatively less susceptible to corrosion?
- When dissimilar metals are brought into contact, which metal is more likely to become the anode?
- Can a greater potential difference between metals be considered to increase the likelihood of galvanic corrosion?
- Did the natural potential shift in the active direction as the NaCl concentration increased?
- Could polishing condition, oxide films, contamination, dissolved oxygen, or temperature have affected the natural potential?
Example Discussion
In this experiment, the natural potentials of several metals were measured in a 3.5 mass% NaCl aqueous solution.
After 30 minutes, the natural potentials were -1.047 V for zinc, -0.662 V for iron, and -0.063 V for copper, with zinc showing the most active potential and copper the most noble potential.
In general, metals showing more active potentials are more easily oxidized, so zinc was considered to be a metal that is more susceptible to corrosion under the conditions of this experiment.
On the other hand, copper and stainless steel showed relatively noble potentials.
This was considered to result from the fact that copper is relatively difficult to oxidize and that a passive film forms on the surface of stainless steel.
However, natural potential is affected not only by the intrinsic properties of the metal but also by the surface condition and solution conditions, so the measured values depend on the experimental conditions.
Regarding combinations of dissimilar metals, the potential difference between iron and copper was approximately 0.60 V.
When these two metals are brought into contact in an electrolyte solution, iron, which shows the more active potential, becomes the anode and dissolution of iron may be accelerated.
In this way, the difference in natural potential is an important indicator when considering the likelihood of galvanic corrosion.
In addition, for the iron sample, the natural potential shifted in the active direction as the NaCl concentration increased, and the amount of corrosion products also increased.
This was considered to result from chloride ions tending to destroy the protective film on the metal surface and promote the corrosion reaction.
Therefore, in natural-potential measurements, it is necessary to consider not only the type of metal but also the ion concentration in the solution and the condition of the surface film.
Summary
In natural-potential measurements, measuring the potential exhibited by a metal without applying external current makes it possible to discuss the corrosion tendency of the metal and the corrosion risk when dissimilar metals are in contact.
In this reference example, zinc and aluminum showed active potentials, while copper and stainless steel showed relatively noble potentials.
In addition, as the NaCl concentration increased, the natural potential of iron shifted in the active direction, indicating a state in which corrosion was more likely to occur.
In a report, it is useful to discuss natural potential, potential difference, corrosion tendency of the metal, solution conditions, and surface condition in relation to one another.
Difference Between Natural Potential and Standard Electrode Potential
Standard electrode potential is an electrode potential measured under standard-state conditions in which conditions such as metal-ion concentration and temperature are specified.
In contrast, natural potential is the potential that a metal naturally exhibits in an actual solution without external current being applied.
Therefore, natural potential does not necessarily completely agree with the standard electrode potential.
Natural potential includes the effects of oxide films on the metal surface, oxygen in the solution, pH, chloride ions, metal-ion concentration, and other factors.
Therefore, standard electrode potential can be regarded as a value indicating the fundamental tendency of a metal toward oxidation and reduction, whereas natural potential can be regarded as a value reflecting the condition of the metal in an actual environment.
Example Discussion:
Unlike standard electrode potential, natural potential reflects the actual solution conditions and the surface condition of the metal.
Standard electrode potential indicates the tendency toward oxidation and reduction under standard-state conditions, whereas natural potential is affected by oxygen concentration, pH, surface films, chloride ions, and other factors.
Therefore, natural-potential results must be discussed not only from the magnitude of the standard electrode potential but also in relation to the experimental conditions.
Role of the Reference Electrode
Electrode potential cannot be measured as an absolute value by itself.
Therefore, in natural-potential measurements, the potential of the metal sample is measured relative to a stable reference electrode.
Reference electrodes such as silver/silver chloride electrodes and saturated calomel electrodes may be used.
The measured natural potential is a value relative to the reference electrode used.
Therefore, when comparing values from different experiments or literature, the same type of reference electrode must be used or conversion must be performed when necessary.
If the reference electrode is unstable, errors arise in all measured values.
Example Discussion:
Because natural potential is measured relative to a reference electrode, the type of reference electrode used must be clearly stated.
If the reference electrode is stable, the potential differences among metal samples can be compared.
However, if the liquid junction of the reference electrode is clogged or the internal solution is in poor condition, the measured potential may shift.
Natural Potential and Corrosion Susceptibility
Metals with active natural potentials generally tend to lose electrons easily and be readily oxidized.
Therefore, when compared in the same solution, metals with lower natural potentials may be considered more susceptible to corrosion.
In contrast, metals with noble natural potentials tend to be difficult to oxidize and less susceptible to corrosion.
However, corrosion rate cannot be completely judged from natural potential alone.
Corrosion rate is affected not only by how readily oxidation occurs but also by oxygen-reduction reactions, surface films, solution resistance, current density, and other factors.
Natural potential is an indicator of corrosion susceptibility, but it is desirable to evaluate it together with corrosion amount, polarization curves, and other measurements.
Example Discussion:
Under the same solution conditions, metals with lower natural potentials tend to be more easily oxidized and more susceptible to corrosion.
The metal that showed the most active natural potential in this experiment may lose electrons more readily than the other metals and therefore may corrode more easily.
However, because natural potential does not directly indicate the corrosion rate itself, it must be evaluated together with observations of the surface condition and corrosion products.
Potential Difference and Galvanic Corrosion
When metals having different natural potentials are brought into contact in an electrolyte solution, current may flow because of the potential difference and corrosion of one of the metals may be accelerated.
This is called galvanic corrosion or dissimilar-metal contact corrosion.
In general, the metal with the more active natural potential becomes the anode and is preferentially oxidized.
The greater the potential difference, the greater the driving force for galvanic corrosion becomes.
However, the actual corrosion rate is also affected by the conductivity of the electrolyte, area ratio of the metals, surface films, oxygen supply, and other factors.
Natural-potential measurements provide a clue for considering corrosion risk when dissimilar metals are combined.
Example Discussion:
When two metals having different natural potentials are brought into contact, a battery is formed because of the potential difference.
In this case, the metal with the more active natural potential becomes the anode and is more readily oxidized, causing corrosion to be accelerated.
Therefore, when metals with a large measured natural-potential difference are brought into contact, attention must be paid to galvanic corrosion.
Discussion of Metals Showing Active Potentials
Metals showing active natural potentials are considered to be in a state in which they readily lose electrons in solution.
For example, metals such as zinc and iron tend to show more active potentials than copper or silver and are more readily oxidized.
This property is also used in sacrificial corrosion protection.
However, showing an active potential does not necessarily mean that severe corrosion will occur within a short period.
If a protective film forms on the surface or if little oxygen is present in the solution, the corrosion rate may remain small.
When interpreting natural potential, the potential should be related to actual surface changes.
Example Discussion:
A metal showing an active natural potential is considered to be in a state in which it loses electrons more readily than other metals and is more easily oxidized.
Therefore, under the same solution conditions, corrosion may proceed more readily.
However, if a protective film forms on the surface, the corrosion rate may remain small even when the natural potential is active, so it is necessary to make a judgment together with surface observations.
Discussion of Metals Showing Noble Potentials
Metals showing noble natural potentials are considered to be relatively stable and difficult to oxidize.
Copper, silver, gold, and similar metals tend to show more noble potentials than iron and zinc.
Noble metals tend to become the cathodic side when in contact with dissimilar metals, and their own corrosion may be suppressed.
However, when a metal showing a noble potential is brought into contact with an active metal, corrosion of the active metal may be accelerated.
Therefore, even if the noble metal itself is resistant to corrosion, it may promote corrosion of surrounding metals depending on the combination.
Material selection that considers potential differences is important.
Example Discussion:
A metal showing a noble natural potential is considered to be difficult to oxidize and less susceptible to corrosion.
However, when a noble metal is brought into contact with an active metal, the active metal may become the anode and its corrosion may be accelerated.
Therefore, although a noble natural potential indicates the stability of the metal itself, the corrosion risk caused by potential differences must be considered when dissimilar metals are in contact.
Relationship with the Ionization Tendency of Metals
The natural potential of a metal is related to its ionization tendency.
Metals with a large ionization tendency readily lose electrons and become positive ions, and their natural potentials tend to be more active.
In contrast, metals with a small ionization tendency are less likely to lose electrons and tend to show more noble potentials.
However, natural potential is not determined solely by ionization tendency.
Actual metal surfaces may contain oxide films, adsorbed substances, and corrosion products, and these can change the potential.
Therefore, ionization tendency is a basic concept for considering natural potential, but surface condition must also be considered when interpreting experimental values.
Example Discussion:
Metals with larger ionization tendencies are more easily oxidized and tend to show more active natural potentials.
If zinc and iron showed lower natural potentials than copper in this experiment, the result can be considered consistent with the ionization tendency of the metals.
However, because natural potential changes when oxide films or corrosion products form, the measured values cannot be completely explained by ionization tendency alone.
Effect of Surface Films
When oxide films or corrosion products form on a metal surface, the natural potential changes.
In metals such as aluminum and stainless steel that form dense oxide films, the surface may be protected and the natural potential may shift in a relatively noble direction.
Such a state may be called passivation.
On the other hand, when the surface film is destroyed, the underlying metal is exposed and the natural potential may shift in the active direction.
Chloride ions may destroy protective films and cause localized corrosion such as pitting.
If the potential changes over time during measurement, formation or destruction of a surface film can be discussed.
Example Discussion:
If the natural potential shifted in the noble direction during measurement, an oxide film or corrosion products may have formed on the metal surface and changed the surface toward a more protective state.
On the other hand, if the natural potential shifted in the active direction, destruction of the surface film or exposure of the underlying metal may have produced a state in which corrosion was more likely to proceed.
Therefore, changes in natural potential over time are considered to reflect changes in the condition of the metal surface.
Measurement Time and Potential Stabilization
Immediately after a metal is immersed in a solution, the natural potential may be unstable because the surface condition is changing.
Immediately after immersion, oxide-film formation, dissolution, adsorption, and interaction with the solution proceed, causing the potential to change over time.
Therefore, in natural-potential measurements, it is important to wait for a certain period and read the value after the potential has stabilized.
If the potential does not stabilize, possible causes include ongoing corrosion reactions, bubbles adhering to the surface, repeated formation and destruction of surface films, or instability of the reference electrode.
In a report, recording changes over time rather than only the value immediately after measurement begins makes the discussion easier.
Example Discussion:
The large change in natural potential immediately after measurement began was considered to result from the formation and dissolution of oxide films after the metal surface came into contact with the solution.
Because the potential approached a constant value over time, the balance between oxidation and reduction reactions at the metal surface was considered to have stabilized.
Therefore, natural potential must be read only after it has stabilized sufficiently.
Effect of pH
The pH of the solution greatly affects the natural potential and corrosion behavior of metals.
Under acidic conditions, a large amount of H+ is present, and the reaction in which H+ accepts electrons released by oxidation of the metal becomes more likely to proceed.
Therefore, metal dissolution and hydrogen evolution may proceed in acidic solutions.
Under neutral or basic conditions, oxygen reduction and the formation of hydroxides or oxide films affect the natural potential.
Changes in pH also change the solubility of metal ions and the types of corrosion products formed.
Therefore, comparing natural potentials under different pH conditions makes it possible to discuss how corrosion susceptibility changes with the environment.
Example Discussion:
If the natural potential shifted in the active direction in an acidic solution, reduction of H+ may have proceeded and metal dissolution may have been promoted.
On the other hand, under neutral or basic conditions, formation of oxide films or hydroxides may change the natural potential.
Therefore, the pH conditions of the solution must always be considered when comparing natural potentials.
Effect of Dissolved Oxygen
Oxygen in solution is an important reactant in reduction reactions associated with metal corrosion.
In neutral aqueous solutions, oxygen may accept electrons and produce OH-.
When the dissolved-oxygen concentration is high, the oxygen-reduction reaction proceeds more readily and the natural potential changes.
Under conditions with sufficient oxygen, the natural potential may shift in the noble direction.
However, because oxygen reduction proceeds, the oxidation reaction of the metal may also continue more readily and corrosion may progress.
Comparing a deaerated solution with an air-saturated solution makes it easier to discuss the effect of dissolved oxygen.
O2 + 2H2O + 4e- → 4OH-
Example Discussion:
Dissolved oxygen participates in reduction reactions associated with metal corrosion.
When oxygen is present, it accepts electrons released by the metal and the oxygen-reduction reaction proceeds.
Therefore, a high dissolved-oxygen concentration changes the natural potential and is also considered to affect the corrosion behavior of the metal.
Effect of Chloride Ions
Chloride ions, Cl-, can have a major effect on the natural potential and corrosion behavior of metals.
Cl- may destroy protective films on metal surfaces and promote localized corrosion.
Pitting corrosion caused by chloride ions is particularly problematic in metals such as stainless steel and aluminum that possess passive films.
If the natural potential shifts in the active direction in a solution containing chloride ions, the protective film may have become unstable and the underlying metal may have been exposed.
In addition, because salt water has high electrical conductivity, galvanic corrosion also proceeds more readily.
The tendency for corrosion to proceed readily in seawater environments can also be explained by these factors.
Example Discussion:
If the natural potential shifted in the active direction in a solution containing chloride ions, Cl- may have destroyed the protective film on the metal surface and produced a state in which corrosion was more likely to occur.
In addition, because NaCl aqueous solution has high conductivity, electrochemical reactions on the metal surface and between dissimilar metals proceed more readily.
Therefore, chloride ions are an important factor affecting both natural potential and corrosion behavior.
Effect of Pretreatment of the Metal Surface
Natural potential is highly sensitive to the condition of the metal surface.
If an oxide film, oil, dirt, polishing scratches, or corrosion products are present on the surface, the measured potential changes.
Therefore, when comparing several metals or conditions, the surface treatment must be standardized as much as possible.
Immediately after polishing, a fresh metal surface is exposed, so the natural potential tends to change over time.
In contrast, a sample on which an oxide film has formed may show a relatively noble potential.
If the surface treatment before the experiment is not recorded, it becomes difficult to correctly identify the cause of potential differences.
Example Discussion:
Natural potential is strongly affected by oxide films and contamination on the metal surface.
In a polished sample, the underlying metal is exposed, and the potential may change after measurement begins as an oxide film forms.
Therefore, when comparing natural potentials, it is important to standardize the pretreatment of the metal surfaces and record the polishing and cleaning conditions.
Effect of Temperature
Temperature affects the rates of electrode reactions, dissolved-oxygen concentration, ion mobility, and the formation of corrosion products.
Higher temperatures may increase reaction rates and make corrosion more likely to proceed.
Changes in temperature may also shift the natural potential.
In experiments comparing natural potentials, it is desirable to keep the temperature constant.
If the temperature differs among samples, it becomes difficult to determine whether the potential difference arises from the type of metal or solution conditions, or from the temperature difference.
In a report, temperature control can also be treated as a source of error.
Example Discussion:
Differences in temperature change the rates of oxidation and oxygen-reduction reactions at the metal surface and may therefore change the natural potential.
Under higher-temperature conditions, corrosion reactions may proceed more readily and changes in the condition of the metal surface may also occur more rapidly.
Therefore, the measurement temperature must be kept constant in order to compare natural potentials accurately.
When the Natural Potential Becomes More Noble Over Time
If the natural potential shifts in the noble direction during measurement, an oxide film or corrosion products may have formed on the metal surface and made the surface more protective.
For example, in aluminum and stainless steel, formation of a passive film may shift the potential in the noble direction.
However, a shift in the noble direction does not necessarily mean that corrosion has completely stopped.
Because the potential moves as a result of a change in the balance of surface reactions, it must be judged together with surface observations and mass changes.
It is important to relate the direction of potential change to the surface condition.
Example Discussion:
Because the natural potential shifted in the noble direction over time, an oxide film or corrosion products may have formed on the metal surface and created a more protective condition.
Such a change may reflect passivation or growth of a surface film.
However, corrosion should not be concluded to have stopped based on the potential change alone, and the result must be discussed together with surface observations.
When the Natural Potential Becomes More Active Over Time
If the natural potential shifts in the active direction over time, the surface film may have been destroyed or dissolution of the metal may have progressed.
In solutions containing chloride ions in particular, the protective film may be locally destroyed and a state in which corrosion readily proceeds may develop.
The natural potential may also become more active when an oxide film remaining immediately after measurement begins dissolves and a fresh metal surface is exposed.
A shift in the active direction may indicate that the metal has entered a more active state and should therefore be discussed in relation to the progress of corrosion.
Example Discussion:
If the natural potential shifted in the active direction over time, the protective film on the metal surface may have been destroyed and the underlying metal exposed.
In this state, the metal becomes more readily oxidized and corrosion more likely to progress.
In solutions containing chloride ions in particular, localized corrosion caused by film breakdown can be considered as a possible cause.
Causes of Error in Natural-Potential Measurements
Causes of error in natural-potential measurements include differences in pretreatment of the metal surface, oxide films and contamination, instability of the reference electrode, clogging of the liquid junction, errors in electrolyte concentration, differences in pH and temperature, insufficient measurement time, bubble adhesion, electrode spacing, noise, and poor electrical connections to the potentiometer.
Because natural potential is a highly sensitive value, even small differences in conditions may change the results.
In particular, if the potential is read immediately after measurement begins, it may not yet have stabilized.
In addition, if the reference electrode is in poor condition, all measured values may shift.
Causes of error are easier to discuss when divided into factors associated with the metal sample, the solution, and the measuring equipment.
Example Discussion:
Possible causes of error in natural-potential measurements include differences in polishing condition of the metal surface, presence or absence of oxide films, instability of the reference electrode, and changes in pH or temperature.
Immediately after measurement begins, film formation or dissolution may still be proceeding at the metal surface and the potential may not yet have stabilized.
Therefore, it is important to read the value after the potential approaches a constant value and to standardize the surface treatment and measurement conditions.
When the Results Can Be Considered Good
Natural-potential measurements can be considered to have produced good results when the measured values stabilize over time, reproducibility is obtained for the same metal, and differences in potential among the metals do not contradict their ionization tendencies or surface conditions.
In addition, if the measurement conditions are standardized and the reference electrode is stable, comparison among the metals becomes easier.
For example, if zinc and iron show more active natural potentials than copper, the result is broadly consistent with the ease of oxidation of the metals.
If aluminum or stainless steel shows a more noble potential than expected, passivation caused by a surface film can be discussed.
Even if the results do not completely agree with theory, a good discussion can be written if the differences can be explained by the surface condition or solution conditions.
Example Discussion:
In this experiment, the potential gradually stabilized after measurement began, and different natural potentials were obtained for each metal.
Zinc and iron showed more active potentials than copper, broadly agreeing with the ionization tendencies of the metals.
In addition, the potential shifted in the noble direction for metals that readily form surface films, suggesting that natural potential reflects not only the type of metal but also the surface condition.
Example Discussions When the Experiment Did Not Go Well
When a natural-potential measurement does not go well, possible causes should be considered from results such as the potential failing to stabilize, large variation for the same metal, an order opposite to that expected, sudden changes in measured values, or large noise.
Organizing the causes according to the metal surface, solution, reference electrode, measurement time, and connection condition makes the discussion easier.
Example Discussion:
In this experiment, variation in natural potential was observed even for the same metal.
One possible cause is that the polishing condition of the metal surface or the thickness of the oxide film differed among the samples.
In addition, if the value was read immediately after measurement began, the surface reaction may not yet have stabilized and the natural potential may not have reached a constant value.
Another Example Discussion:
Possible causes of large fluctuations in the measured value include clogging of the liquid junction of the reference electrode, poor electrical connections, adhesion of bubbles in the solution, and external noise.
Because natural-potential measurements involve measuring small potential differences, unstable measurement systems make the values difficult to reproduce.
To improve the measurement, the condition of the reference electrode should be checked, the electrodes should be installed securely, and the system should be allowed to stand for sufficient time.
How to Write Points for Improvement
In a discussion of natural-potential measurements, 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 metal sample, solution conditions, reference electrode, measurement operation, and recording method.
Improvements to the Metal Sample
- Polish the metal surfaces under the same conditions
- Remove oil and contamination
- Do not touch the samples with bare hands after polishing
- Keep the sample areas consistent
- Keep the standing time before measurement consistent
- Record the presence or absence of oxide films
Improvements to the Solution and Reference Electrode
- Prepare the electrolyte concentration accurately
- Measure the pH
- Keep the temperature constant
- Standardize dissolved-oxygen conditions
- Clearly state the type of reference electrode
- Check the internal solution and liquid junction of the reference electrode
Improvements to the Measurement Operation
- Fix the electrodes securely
- Prevent bubbles from adhering
- Read the potential after it has stabilized
- Record changes over time
- Perform multiple measurements under the same conditions
- Avoid external noise and poor electrical contact
Example of How to Write Points for Improvement:
To improve the reproducibility of natural-potential measurements, the metal surfaces must be polished under the same conditions and oil and contamination removed before measurement.
In addition, it is important to keep the electrolyte concentration, pH, and temperature constant and confirm the condition of the reference electrode.
During measurement, reliable comparisons can be made by reading the potential only after it has stabilized sufficiently rather than using the unstable value immediately after immersion.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of natural-potential measurements of metals, simply writing that “the potential was low” or “the metal corrodes easily” results in a superficial discussion.
A good discussion relates natural potential, redox reactions, ionization tendency, surface films, solution conditions, and corrosion caused by potential differences.
| Superficial Discussion | Good Discussion |
|---|---|
| The potential was low. | Because the natural potential was active, the metal was considered to be in a state in which it readily loses electrons and is easily oxidized under the experimental conditions. |
| It corrodes easily. | A metal with an active natural potential tends to act as an anode and may be preferentially oxidized when in contact with a dissimilar metal, so the corrosion risk is considered high. |
| The potential changed. | The change in potential during measurement may reflect the formation of an oxide film on the metal surface, film breakdown, formation of corrosion products, or progression of reactions with the solution. |
| The order of the metals differed from what was expected. | Because natural potential is affected not only by ionization tendency but also by surface films, pH, dissolved oxygen, chloride ions, and pretreatment conditions, results may differ from the standard order. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of natural-potential measurements of metals.
Adjust the necessary parts according to your own experimental results.
- Natural potential is the electrode potential exhibited by a metal when no external current is applied.
- Natural potential is determined by the balance between the oxidation reaction of the metal and reduction reactions in the solution.
- Metals with more active natural potentials tend to be more easily oxidized.
- Metals with more noble natural potentials tend to be more difficult to oxidize.
- Unlike standard electrode potential, natural potential reflects actual solution conditions and surface condition.
- The greater the natural-potential difference between dissimilar metals, the greater the driving force for galvanic corrosion.
- Formation of a surface film may shift the natural potential in the noble direction.
- Chloride ions may destroy protective films and shift the natural potential in the active direction.
- Because the potential is unstable immediately after measurement begins, values must be compared after stabilization.
- The measured natural potential must be stated together with the type of reference electrode used.
Points to Check When Discussing Natural-Potential Measurements of Metals
Checking the following points before writing the report makes the discussion easier to write.
- Is the definition of natural potential explained?
- Is it stated that the potential is measured relative to a reference electrode?
- Is the difference from standard electrode potential explained?
- Is natural potential related to corrosion susceptibility?
- Are the meanings of active and noble potentials explained?
- Is the relationship with the ionization tendency of metals considered?
- Is dissimilar-metal contact corrosion explained in terms of potential difference?
- Is the effect of surface films considered?
- Are the effects of pH, oxygen, and chloride ions described?
- Are changes in potential over time discussed?
- Are errors caused by the reference electrode and measurement conditions considered?
- Do the points for improvement correspond to the causes of error?
Summary
The natural potential of a metal is the electrode potential that the metal naturally exhibits when immersed in a solution without external current being applied.
This potential is determined as a result of the balance between oxidation of the metal and reduction reactions such as oxygen reduction.
Metals with more active natural potentials tend to be more easily oxidized and more susceptible to corrosion.
However, natural potential differs from standard electrode potential and reflects the actual solution conditions and the condition of the metal surface.
The value changes depending on pH, dissolved oxygen, chloride ions, temperature, oxide films, and pretreatment conditions.
Therefore, corrosion rate should not be determined from natural potential alone, and it is important to discuss the result together with surface observations and other electrochemical measurements.
In a report, rather than simply writing that the potential is low or high, organize and discuss the meaning of natural potential, the reference electrode, the difference from standard electrode potential, corrosion susceptibility, potential difference, galvanic corrosion, surface films, solution conditions, causes of error, and points for improvement.
Natural-potential measurement is an important experiment for electrochemically understanding the corrosion behavior of metallic materials.
