What is Corrosion?
Corrosion refers to the process by which the metal is slowly damaged due to the constant reaction between its atoms and the atmospheric air and moisture. As time passes, the metals develop coloured corroded coating over them due to rusting. Thereby, particles of the metal object break down spontaneously from its exterior and start to fall off layer by layer. For this reason, that particular metal chunk loses all its shine and strength. Sometimes it becomes so weak and brittle that you can easily crush it with your hands.
Example of Corrosion
One of the very common examples is rusting of iron. You might have seen the iron objects developing a reddish-brown coating on them when they are constantly exposed to the atmosphere. The iron reacts with the atmosphere to form hydrated iron oxide, also known as rust. The rust formation occurs due to the chemical interaction between the atoms of iron, oxygen and water.
If we observe the rust formation at the molecular level, then we will see that the iron atoms are each losing 3 e- so as to become 3+ ions. Whereas the 2 oxygen atoms gain 2 e- each, with a total of 4 e-, and the oxygen becomes 2e-. Here the iron is getting oxidized (losing the e-) while the oxygen is getting reduced (gaining the e-). This makes the rusting process a redox reaction where oxidation and reduction take place simultaneously.
Content: Prevention of Corrosion
- Prevention of Corrosion
- Barrier Method
- Sacrificial Methods
Prevention of Corrosion
Metals around us are immensely useful to us in our day-to-day lives. But eventually, they get damaged by the corrosion after coming in contact with oxygen and moisture. Corrosion can compromise the external conformation of the metal and worsens its load-bearing ability. No metal is completely immune to corrosion as most metals are highly reactive to environmental factors.
Fortunately, we have effective corrosion prevention methods otherwise changing the metal part every time it rusts, would have become a real hectic. We can prevent the process of corrosion by monitoring the selection of metal, controlling the surrounding atmospheric condition as well as blocking the direct contact between the metal surface and environmental conditions. The preventive measures can extend the life of any metal object by up to 250%.
Generally, we can split the prevention methods into two main categories:
1. Barrier methods
1. Barrier Method
This method introduces a barrier film between the metal object and the environment. This film prevents the oxygen and water from ever touching the metal body.
One of the easiest and most cost-effective methods to avert corrosion is by using barriers like paint, oil, grease, waxes etc. Also, the powders like nylon, urethane and epoxy create a thin film around the metal surface. This film protects the internal metal body from coming in contact with water and oxygen.
Paints have the ability to prevent the metal from electrochemical charge coming from corrosive compounds. It is a feasible technique for smaller as well as bigger objects. Nowadays, the paint systems comprise various paint layers, each for performing different layers.
- The primer coat serves the role of inhibitor.
- The intermediate layering provides thickness to the paint film.
- The last (final) coat is for resistance to environmental factors.
Oiling or greasing also do the same job and generates a nonconductive layer for moisture. In addition to this, they also help in the lubrication of moving parts.
However, the only drawback of these methods is that they get easily scratched and need stripping and reapplying on a regular basis.
This process refers to the process of plating one metal over the other metal via hydrolysis. The metal to be coated should have resistance to corrosion, or its oxide should be strong enough to protect the inner material. While performing electroplating, the metal to be platted is at the cathode, while the metal on which electroplating has to be done is at the anode.
The metal to be coated should be negative and remains immersed in a positively charged metal salt (electrolyte) solution. This way, when the hydrolysis starts, the metal at the anode is platted at the cathode.
It is immensely useful in the industries like automobiles, jewellery, electronics, electrical etc.
- Nickel plating: Useful in protecting decorative items and household articles from corrosion. For example, door hinges, showers, taps, knives etc.
- Copper plating: It is quite common for plating electronic items such as circuit boards etc. It’s a highly feasible technique as copper is cheap and easily applicable.
- Gold plating: This is highly known for its high electrical conductivity and corrosion resistance. Mainly used for making jewellery and electronic gadget making.
- Chrome plating: It refers to the process of plating chromium on the surface of the iron. This plating gives a shiny appearance to the object.
- Rhodium plating: It is useful in jewellery making as it gives a shiny appearance. It is a kind of platinum that protects scratches.
2. Sacrificial Methods
This method involves adding a more reactive metal layer on the surface of the desired corrosive metal. For instance, iron is coated with molten zinc or aluminium, which are more reactive to moisture and oxygen compared to iron.
The principle behind this technique is that if the zinc/aluminium coated iron object is exposed to the atmosphere, the more reactive outer coating will oxidize first instead of iron. The upper oxidized layer serves the role of protector that blocks further contact with the inner layer of iron.
Galvanization is quite an ancient technique, around 250 years old. The process of galvanization involves dipping metal objects like iron or steel into molten zinc. It means coating zinc on the metal surface to form a tightly bonded coating for protection.
For instance, iron rusts very easily, so when it is coated with a zinc layer, the upper zinc coat is exposed to the atmosphere, and thus, it reacts with an atmosphere more than iron.
Process of Galvanization
When the iron reacts with oxygen and water, it forms rust of iron oxide. The rust is porous in nature, and it removes off gradually. As soon as the rust is removed, the inner layer of the iron gets exposed to the atmosphere and starts rusting. Thereby, the entire iron chunk is eaten up by the atmosphere with the passing of time.
In comparison, when the zinc corrodes, it forms a layer of zinc oxide on its surface. The corroded layer, instead of acting porous fragile film, acts like a protective coat for the lower zinc body. And thus, it doesn’t allow more of the zinc to interact with the atmosphere.
Hence, when we coat the metals like iron and steel with zinc, the zinc oxide layer protects the inner metals by blocking any kind of contact.
Disadvantages of Galvanization
- The process requires a specific setup which means that the companies must have to pull the equipment or any metal part out in order to be treated.
- Some of the objects are too large to be zinc coated.
- High exposure to atmospheric pressure can wear off the zinc coating very soon. For this reason, it demands continuous maintenance.
Anodizing is an electrochemical process which generates an oxide film (usually on aluminium) that offers corrosion resistance. The anode part has aluminium electrodes, and the current passes between them. Whereas cathode is a flat aluminium bar dipped in an electrolyte.
Anodizing leads to a change in the microscopic texture of the surface by developing a thick coating of oxide. This coating is porous; therefore, sometimes, a sealing process is required for complete corrosion resistance.
Even though aluminium doesn’t rust, it can deteriorate by oxidation due to atmospheric moisture and oxygen. This oxidized layer provides protection against further corrosion. Anodizing aluminium not only increases the corrosion resistance but also provides surface hardness, improved lubrication, improved adhesion and makes dyeing easier.
The alloy is the mixture of metals or metals or non-metals combined together in a fixed proportion. Alloying creates a new kind of substance that bears the properties of all the constituent elements. Therefore, the alloy has superior properties to that of individual constituents. For instance, alloys have better mailability, ductility, improved strength, hardness, toughness etc., than the original metal.
The metal that is predominant in the alloy is the base metal. While the other metal or non-metal added to it are alloying elements.
Examples of Alloy
- Steel: Made up of iron and carbon.
- Stainless steel: Formed of iron, nickel and chromium.
- Bronze: Made of copper and tin.
- Brass: Made of copper and zinc.
- Solder: Made of lead and tin
For alloying, the metals or the non-metallic are melted by heating. The molten mixtures are poured together into metal or sand moulds and left to solidify. The resulting solid will be an alloy having the characteristic properties of the combined element.
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