Real-World Examples of Galvanic Corrosion in Everyday Life
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 What Is Galvanic Corrosion? Causes, Effects & Solutions
1. What is galvanic corrosion in simple terms?
Galvanic corrosion, also known as bimetallic corrosion, is an electrochemical process that occurs when two different metals are in electrical contact with each other in the presence of an electrolyte, like saltwater. In this setup, one metal (the more reactive one, called the anode) corrodes at an accelerated rate, while the other metal (the less reactive one, called the cathode) is protected from corrosion.
2. What three conditions are essential for galvanic corrosion to occur?
For galvanic corrosion to happen, three specific conditions must be met simultaneously:
Dissimilar Metals: There must be two metals with different electrode potentials.
Electrical Contact: The two metals must be in direct physical or electrical contact.
Presence of an Electrolyte: Both metals must be immersed in a shared conductive liquid (an electrolyte) that can transport ions between them.
3. Can you provide some real-world examples of galvanic corrosion?
Galvanic corrosion is commonly observed in various applications. Some key examples include:
A steel screw rusting rapidly when used in a brass fitting, especially in a marine environment.
The degradation of aluminium roofing sheets where they are fastened with steel nails, particularly after rainfall which acts as an electrolyte.
In plumbing, when copper pipes are directly connected to steel or galvanised iron pipes, the steel pipe corrodes near the junction.
A ship's steel hull corroding preferentially where it is connected to a bronze propeller in seawater.
4. What are the most effective ways to prevent galvanic corrosion?
Several effective strategies can be employed to prevent or mitigate galvanic corrosion:
Material Selection: Choose metals that are close to each other in the galvanic series to minimise the potential difference.
Insulation: Use non-conductive materials like plastic or rubber gaskets to electrically isolate the two dissimilar metals.
Cathodic Protection: Attach a more reactive metal (a sacrificial anode like zinc or magnesium) to the structure. This sacrificial anode corrodes instead of the more important metal.
Apply Coatings: Use paint, epoxy, or plating to create a barrier that prevents the electrolyte from coming into contact with the metal surfaces.
5. What is the galvanic series and how does it help predict corrosion?
The galvanic series is a ranked list of metals and alloys based on their electrochemical potential in a specific electrolyte (typically seawater). It is a practical tool used to predict which metal will corrode. When two metals from the series are connected, the one that is higher up the list (more active or "less noble") will act as the anode and corrode. The one lower down (less active or "more noble") will be the cathode and remain protected. The greater the separation between the two metals in the series, the faster the rate of galvanic corrosion.
6. Why is it dangerous to have a small anode connected to a large cathode?
This scenario is highly dangerous because the rate of corrosion is dictated by the current density at the anode. When a small anode is connected to a large cathode, all the electrochemical current is concentrated on the small anodic area. This creates a very high current density, causing the small anode to dissolve or corrode extremely quickly, which can lead to rapid and catastrophic failure. Conversely, a large anode with a small cathode results in a low current density and a very slow, manageable corrosion rate.
7. How is galvanic corrosion different from general rusting?
The key difference lies in the mechanism. Galvanic corrosion specifically requires two different metals in electrical contact to create an electrochemical cell. In contrast, general rusting is the oxidation of a single metal, iron, which forms iron oxide in the presence of oxygen and water. While both are corrosion processes, rusting can occur on a uniform surface of a single metal, whereas galvanic corrosion is an accelerated process driven by the potential difference between two distinct metals.
8. What is the role of the electrolyte in galvanic corrosion and can corrosion happen without it?
The electrolyte's role is crucial; it acts as a conductive medium that allows for the flow of ions between the anode and the cathode, thereby completing the electrical circuit. Without an electrolyte, there is no path for these ions to travel, and the electrochemical cell cannot function. Therefore, galvanic corrosion cannot occur in its absence. For example, two dissimilar metals in completely dry air will not undergo galvanic corrosion.
9. How do standard electrode potentials from electrochemistry relate to galvanic corrosion?
Standard electrode potentials (E°) quantitatively determine which metal will act as the anode and which will be the cathode. The metal with the more negative (or less positive) standard reduction potential has a higher tendency to be oxidised and will therefore be the anode (the one that corrodes). The metal with the more positive reduction potential will be the cathode (the one that is protected). The difference between these E° values indicates the voltage of the galvanic cell and the thermodynamic driving force behind the corrosion.
