Unit 7 - Electrochemical Nature of Corrosion

FULL SCREEN

Aqueous Corrosion (corrosion in water media) is electrochemical in nature. This means that the corrosion reaction involves electrons (e^-).

All aqueous corrosion can be divided into Anodic and Cathodic reactions.

For example, if Zinc (Zn) is placed into Hydrochloric Acid (HCl) - Zinc Chloride (ZnCl₂) is formed, along with Hydrogen Gas (H₂). This is shown in the following reaction:

Zn + 2HCl → ZnCl₂ + H₂

The above reaction can also be written as:

Zn + 2H⁺ + 2Cl^- → Zn²⁺ + 2Cl^- + H₂

Please note that 2Cl^- is the same on both sides in the above reaction. As a result, it can be removed to give the following reaction:

Zn + 2H⁺ → Zn²⁺ + H₂

The reaction above can be divided into an Anodic Reaction and a Cathodic Reaction:

Zn → Zn²⁺ + 2e^-

2H⁺ + 2e^- → H₂

An Anodic Reaction is an oxidation process through which the valence of a metal increases from zero to a more positive value.

Aqueous Corrosion involves the loss of metal to the aqueous phase. When this occurs, the metal releases a number of electrons (e^-) into the Metal/Alloy.

There are many anodic reactions; however, we can identify two main types as follows:

M → Mⁿ⁺ + ne^- In this reaction, Mⁿ⁺ is a soluble corrosion product that does not protect the surface of the metal.

M + nH₂O → M(OH)_n + nH⁺ + ne^- In this reaction, M(OH)_n is a solid corrosion product that may - or may not - slow corrosion of the metal.

The Figure presented below is a schematic representation of a generic corrosion reaction.

If generic metal M corrodes, it goes into solution as Mⁿ⁺ and leaves a number (n) of electrons (e^-) in the metal.

M → Mⁿ⁺ + ne^-

The Figure presented below is a schematic representation of an anodic reaction for Iron (Fe).

In the case of iron, the metal (Fe) goes into solution as the ion Fe²⁺ and leaves two electrons (e^-) in the metal.

Fe → Fe²⁺ + 2e^-

A Cathodic Reaction is a reduction process during which the valence of the species is reduced.

The electrons (e^-) produced during the anodic corrosion reaction must be consumed at the same rate by a corresponding cathodic reaction.

While there are only two cathodic reactions in natural environments, there are many more cathodic reactions in various man-made environments.

Cathodic reactions consume electrons (e^-) and are reduction reactions. In a reduction reaction, the valence is "reduced" to a lower value.

The Figure presented below is a schematic representation of the Hydrogen Evolution Reaction (HER).

In this reaction, 2H⁺ accept electrons to become Hydrogen Gas (H₂).

2H⁺ + 2e^- → H₂

The Figure presented below is a schematic representation of the Oxygen Reduction Reaction (ORR).

In this reaction, Oxygen (O₂) in water is reduced to Hydroxyl Ions (OH^-).

O₂ + 2H₂O + 4e^- → 4OH^-

iPAD

Aqueous Corrosion (corrosion in water media) is electrochemical in nature. This means that the corrosion reaction involves electrons (e^-).

All aqueous corrosion can be divided into Anodic and Cathodic reactions.

For example, if Zinc (Zn) is placed into Hydrochloric Acid (HCl) - Zinc Chloride (ZnCl₂) is formed, along with Hydrogen Gas (H₂). This is shown in the following reaction:

Zn + 2HCl → ZnCl₂ + H₂

The above reaction can also be written as:

Zn + 2H⁺ + 2Cl^- → Zn²⁺ + 2Cl^- + H₂

Please note that 2Cl^- is the same on both sides in the above reaction. As a result, it can be removed to give the following reaction:

Zn + 2H⁺ → Zn²⁺ + H₂

The reaction above can be divided into an Anodic Reaction and a Cathodic Reaction:

Zn → Zn²⁺ + 2e^-

2H⁺ + 2e^- → H₂

An Anodic Reaction is an oxidation process through which the valence of a metal increases from zero to a more positive value.

Aqueous Corrosion involves the loss of metal to the aqueous phase. When this occurs, the metal releases a number of electrons (e^-) into the Metal/Alloy.

There are many anodic reactions; however, we can identify two main types as follows:

M → Mⁿ⁺ + ne^- In this reaction, Mⁿ⁺ is a soluble corrosion product that does not protect the surface of the metal.

M + nH₂O → M(OH)_n + nH⁺ + ne^- In this reaction, M(OH)_n is a solid corrosion product that may - or may not - slow corrosion.

The Figure presented below is a schematic representation of a generic corrosion reaction.

If generic metal M corrodes, it goes into solution as Mⁿ⁺ and leaves a number (n) of electrons (e^-) in the metal.

M → Mⁿ⁺ + ne^-

The Figure presented below is a schematic representation of an anodic reaction for Iron (Fe).

In the case of iron, the metal (Fe) goes into solution as the ion Fe²⁺ and leaves two electrons (e^-) in the metal.

Fe → Fe²⁺ + 2e^-

A Cathodic Reaction is a reduction process during which the valence of the species is reduced.

The electrons (e^-) produced during the anodic corrosion reaction must be consumed at the same rate by a corresponding cathodic reaction.

While there are only two cathodic reactions in natural environments, there are many more cathodic reactions in various man-made environments.

Cathodic reactions consume electrons (e^-) and are reduction reactions. In a reduction reaction, the valence is "reduced" to a lower value.

The Figure presented below is a schematic representation of the Hydrogen Evolution Reaction (HER).

In this reaction, 2H⁺ accept electrons to become Hydrogen Gas (H₂).

2H⁺ + 2e^- → H₂

The Figure presented below is a schematic representation of the Oxygen Reduction Reaction (ORR).

In this reaction, Oxygen (O₂) in water is reduced to Hydroxyl Ions (OH^-).

O₂ + 2H₂O + 4e^- → 4OH^-

iPHONE

Aqueous Corrosion (corrosion in water media) is electrochemical in nature. This means that the corrosion reaction involves electrons (e^-).

All aqueous corrosion can be divided into Anodic and Cathodic reactions.

For example, if Zinc (Zn) is placed into Hydrochloric Acid (HCl) - Zinc Chloride (ZnCl₂) is formed, along with Hydrogen Gas (H₂). This is shown in the following reaction:

Zn + 2HCl → ZnCl₂ + H₂

The above reaction can also be written as:

Zn + 2H⁺ + 2Cl^- → Zn²⁺ + 2Cl^- + H₂

Please note that 2Cl^- is the same on both sides in the above reaction. As a result, it can be removed to give the following reaction:

Zn + 2H⁺ → Zn²⁺ + H₂

The reaction above can be divided into an Anodic Reaction and a Cathodic Reaction:

Zn → Zn²⁺ + 2e^-

2H⁺ + 2e^- → H₂

An Anodic Reaction is an oxidation process through which the valence of a metal increases from zero to a more positive value.

Aqueous Corrosion involves the loss of metal to the aqueous phase. When this occurs, the metal releases a number of electrons (e^-) into the Metal/Alloy.

There are many anodic reactions; however, we can identify two main types as follows:

M → Mⁿ⁺ + ne^- In this reaction, Mⁿ⁺ is a soluble corrosion product that does not protect the surface of the metal.

M + nH₂O → M(OH)_n + nH⁺ + ne^- In this reaction, M(OH)_n is a solid corrosion product that may - or may not - slow corrosion of the metal.

A Cathodic Reaction is a reduction process during which the valence of the species is reduced.

The electrons (e^-) produced during the anodic corrosion reaction must be consumed at the same rate by a corresponding cathodic reaction.

While there are only two cathodic reactions in natural environments, there are many more cathodic reactions in various man-made environments.

Cathodic reactions consume electrons (e^-) and are reduction reactions. In a reduction reaction, the valence is "reduced" to a lower value.

One Cathodic reaction is the Hydrogen Evolution Reaction (HER). In this reaction, 2H⁺ accept electrons (e^-) to become Hydrogen Gas (H₂).

2H⁺ + 2e^- → H₂

Another Cathodic reaction is the Oxygen Reduction Reaction (ORR).

In this reaction, Oxygen (O₂) in water is reduced to Hydroxyl Ions (OH^-).

O₂ + 2H₂O + 4e^- → 4OH^-

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