What Is Galvanic Corrosion Its Mechanism and How to Prevent It
Galvanic corrosion also is also known as bimetallic corrosion or dissimilar corrosion. In the process of galvanic corrosion, metals are placed within an electrolyte with another metal. Due to the involvement of two metals, it is called as bimetallic corrosion. Corrosion usually refers to a metal degrading in the presence of oxygen and moisture whereas galvanic corrosion involves degradation of one metal and protection of another metal placed with the precious metal. Here there is one thing that is to be noted. The two metals are needed to be electrochemically different from each other that is why it is denoted as dissimilar corrosion.
CAUSES OF GALVANIC CORROSION
Every galvanic cell functions due to potential difference. This potential difference causes the flow of electrons within the cell. The ability of the metal electrode to lose electrons and to become oxidized is known as oxidation potential. The more is the oxidation potential, then more easily metal will give up its electrons. Opposite to oxidation potential is the reduction potential. It is the ability of a metal ion in solution to pick up electrons and become reduced. On the basis of the oxidation potential of metals, an electrochemical series is arranged.
The elements at the top of the series are more easily oxidized thus there are major chances of losing the electrons. The entire mechanism depends upon the oxidation potentials of the two metals in the galvanic corrosion. The difference in the oxidation potential of the two metals is the driving force of galvanic corrosion.
When two metal electrodes are placed in an electrolyte and the circuit is completed by joining the metal through a wire then this is the point where the process of corrosion takes place. Metals with the higher oxidation potential will start acting as anode and will keep losing electrons whereas the metals with lower oxidation potential will start acting as cathode respectively. The activity of the metals is based on the electrochemical series. Thus the flow of current is from more active metal (anode) to less active metal (cathode).
GALVANIC CORROSION EXAMPLES
Few metal pairs that undergo galvanic corrosion are given below-
1. ZINC AS ANODE AND COPPER AS CATHODE
When zinc and copper metal are placed in an electrolyte which can be of water/aqueous solution containing any salt like copper sulfate it then the circuit gets completed. Now Zinc starts to oxidize due to its high oxidation potential compared to the cathode. Thus zinc slowly starts to corrode and form zinc ions. Copper ions, on the other hand, start to gain electrons and are reduced so it is protected. The process continues until the zinc electrode entirely gets dissolved. The hydrogen ions present in the electrolyte also gets reduced to hydrogen gas and is seen as bubbles at the cathode.
Reaction at Anode
Zn-------> Zn+2 + 2e-
Reaction at cathode
2H++2e----------> H₂↑
Other examples-
2. STEEL AND ALUMINUM
When steel and aluminum are placed in the electrolyte then aluminum will act as anode and will start to corrode whereas steel will be protected.
PREVENTION OF GALVANIC CORROSION
Galvanic corrosion is not required in many places thus has to be prevented. There are various methods of preventing galvanic corrosion.
Some of them are given below.
1. AVOID GALVANIC COUPLING
This means that two metals that can undergo galvanic corrosion must not be placed together otherwise one out of the two metals will corrode. We have already seen a few examples of such metals. Avoid using metals that are electrochemically different from each other.
2. USE OF INSULATING MATERIAL
If two different metals are kept together then insulating materials must be used between them which will prevent the circuit formation and flow of electrons.
3. PAINT
Paint creates a barrier by covering the metal so it is also a way to prevent galvanic corrosion.
4. AVOID LARGE CATHODE AND SMALL ANODE.
This also reduces dissimilar corrosion.
Galvanic corrosion starts when metals are dissimilar, if there is an electrolyte present or if the circuit gets completed. All these conditions must be avoided to prevent galvanic corrosion.
FAQs on Galvanic Corrosion in Electrochemistry
1. What is galvanic corrosion in chemistry?
Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially when it is electrically connected to a different metal in the presence of an electrolyte. It occurs because the two metals have different electrode potentials, creating a galvanic cell.
- The more reactive metal acts as the anode and undergoes oxidation.
- The less reactive metal acts as the cathode and is protected.
- An electrolyte (such as salt water) completes the circuit and allows ion movement.
2. How does galvanic corrosion occur?
Galvanic corrosion occurs when two dissimilar metals are electrically connected in the presence of an electrolyte, causing electron flow from the more reactive metal to the less reactive metal.
- The more active metal oxidizes at the anode: for example, Fe(s) → Fe2+(aq) + 2e-.
- Electrons travel through the metal connection to the cathode.
- A reduction reaction occurs at the cathode, such as oxygen reduction in water: O2(g) + 2H2O(l) + 4e- → 4OH-(aq).
3. What is the difference between galvanic corrosion and electrolysis?
The key difference is that galvanic corrosion is a spontaneous electrochemical process, while electrolysis is a non-spontaneous process driven by an external power source.
- Galvanic corrosion: Forms a spontaneous galvanic cell; chemical energy produces electrical energy.
- Electrolysis: Requires an external battery; electrical energy drives a chemical reaction.
- In galvanic corrosion, the more reactive metal corrodes naturally.
- In electrolysis, the reaction direction is controlled by applied voltage.
4. Which metal corrodes in galvanic corrosion?
The more reactive metal, with the more negative standard reduction potential, corrodes in galvanic corrosion.
- This metal acts as the anode and undergoes oxidation.
- For example, in a zinc–copper pair, zinc corrodes: Zn(s) → Zn2+(aq) + 2e-.
- Copper acts as the cathode and remains protected.
5. What is an example of galvanic corrosion?
A common example of galvanic corrosion is iron in contact with copper in salt water, where iron corrodes faster.
- Iron acts as the anode: Fe(s) → Fe2+(aq) + 2e-.
- Copper acts as the cathode and facilitates oxygen reduction.
- The presence of salt water provides the electrolyte.
6. How can galvanic corrosion be prevented?
Galvanic corrosion can be prevented by isolating dissimilar metals, removing the electrolyte, or using sacrificial protection.
- Electrical insulation: Use plastic washers or coatings to prevent metal-to-metal contact.
- Protective coatings: Paint or plate the surface to block moisture.
- Sacrificial anode: Attach a more reactive metal like zinc to protect iron (cathodic protection).
- Material selection: Choose metals close together in the electrochemical series.
7. What is the role of the electrolyte in galvanic corrosion?
The electrolyte allows ionic conduction between the two metals, completing the electrical circuit required for galvanic corrosion.
- It enables movement of ions such as Fe2+, OH-, or Na+.
- Common electrolytes include salt water, acidic solutions, or moist soil.
- Without an electrolyte, electron flow cannot be balanced by ion movement.
8. What is the galvanic series and why is it important?
The galvanic series is a list of metals arranged according to their corrosion potentials in a specific environment, usually seawater.
- Metals higher in the series are more anodic and corrode more easily.
- Metals lower in the series are more cathodic and are protected.
- The greater the separation between two metals in the series, the higher the risk of galvanic corrosion.
9. How does the electrochemical series relate to galvanic corrosion?
The electrochemical series predicts galvanic corrosion by ranking metals based on their standard reduction potentials (E° values).
- Metals with more negative E° values oxidize more easily.
- For example, E°(Zn2+/Zn) = −0.76 V and E°(Cu2+/Cu) = +0.34 V.
- Zinc will act as the anode when paired with copper.
10. Is galvanic corrosion the same as rusting?
No, galvanic corrosion is a general electrochemical process between dissimilar metals, while rusting specifically refers to the corrosion of iron.
- Rusting involves iron reacting with oxygen and water to form hydrated iron(III) oxide, often simplified as 4Fe(s) + 3O2(g) → 2Fe2O3(s).
- Galvanic corrosion can involve many different metal combinations.
- Rusting can occur without a second metal, but galvanic corrosion requires two dissimilar metals.
