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Corrosion and rusting are the two chemical processes that eventually result in the degradation of materials such as metals. It is a common phenomenon that occurs in many abandoned vehicles, shipyards, wrecked buildings, and factories. Both rusting and corrosion are oxidation reactions. Corrosion can occur over a wide range of objects whereas rusting mainly occurs on iron. Corrosion can be understood as a kind of oxidation while rusting can be understood as a kind of corrosion. The main difference between rusting and corrosion is that corrosion can take place due to various chemicals while rusting is not caused by chemicals though it can be accelerated by them.
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Key Terms: Rust, Corrosion, Oxidation, Chemical Processes, Iron, Chemical reaction, Humidity, Moisture
What is Rust?
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Rusting refers to the oxidation of the objects formed of iron. Rust can be understood as the red or orangish coating that is formed when the surface of an iron object is exposed to moisture and air. This takes place due to the chemical reaction that occurs between the surface of the metal and the oxygen and moisture in the air. Rusting is most commonly seen in steel and iron objects. This does not happen as a result of the addition of some chemicals, however, the addition of chemicals can certainly accelerate the process by increasing the electrical activity between oxygen and iron.
For instance, in steel Fe+2 can get oxidized to Fe+3 during the corrosion of iron. The rate of the process of rusting depends upon various factors such as the surface area of the metal being exposed to air, the humidity of the air, etc.
Prevention of Rusting
Various methods can be used to protect metals from rusting. Some of these methods are:
- Modification of the environment
- Electroplating, where a thin layer of metal, like chromium or nickel, is deposited on the surface of the steel.
- A layer or a coating of paint can be applied to the surface to avoid rusting.
- Corrosion inhibitors can be used which refer to the chemicals that can avoid the formation of rust by interfering with the oxidation reaction on the surface of the metal.
- Galvanization can be performed wherein a coating of zinc can prevent iron from rusting as it acts as a sacrificial anode.
What is Corrosion?
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Corrosion refers to the deterioration process of a substance as a result of chemical, electrochemical, or other kinds of reactions that takes place on the surface of that substance. Corrosion can take place on both metal and non-metal surfaces. It affects the structure of the surface of the material. A common example of corrosion is the process of rusting. The quality and the colour of the steel is changed in this process.
When some corrosive chemicals are dropped on various surfaces, the surface can undergo a deterioration. The chemicals include strong acids as well as strong bases such as acids like H2SO4, HCl, and bases such as KOH, NaOH, etc. The chemicals that cause corrosion can lead to significant visible damage on surfaces such as wood, skin, metals, and eyes, which can even cause permanent destruction.
Prevention of Corrosion
The prevention of corrosion can take place through the following methods:
- Painting the surface of the object.
- Handling chemicals with extreme care.
- Galvanization
Difference between Rust and Corrosion
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The differences between rust and corrosion are as tabulated below:
| Rusting | Corrosion |
|---|---|
| Rusting is a part of the corrosion process and results in the formation of an orange or a red coating on the metal surfaces. | Corrosion refers to the deterioration process of the materials that occur as a result of electrochemical, chemical, or other reactions. |
| It usually takes place on iron surfaces or its alloys. | It can occur on various surfaces such as wood, skin, metals, etc. |
| The process of rusting mainly occurs when a metal is exposed to moisture and air. | The process of corrosion can take place when the surface of the substance is exposed to corrosive chemicals. |
| Iron oxide is formed when rusting takes place. | Oxides of salts or metals can be formed due to corrosion. |
| Rusting affects only iron and its alloys. | Corrosion can occur in various materials such as ceramics and polymers. |
Despite these differences, rusting and corrosion are interconnected as both of them are oxidation reactions. Both of them leads to the degradation of the substance. All kinds of rusting can be referred to as corrosion whereas all forms of corrosion is not rusting. Both of these processes can be prevented by applying a coat of paint, galvanization, corrosion inhibitors, etc.
Things to Remember
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- Rusting can only take place in metals which includes iron and its alloys such as steel.
- Corrosion can affect both metals and non metals. It can even affect the skin.
- Corrosion can be most effectively prevented by coating the surface of the materials.
- Rusting can be effectively prevented by galvanization.
- Rust is formed as a result of air and moisture whereas corrosion can take place by air, moisture, or both.
- Corrosion involves the reaction of some corrosive substances or chemicals whereas chemicals are not required for rusting though they can accelerate the process.
Sample Questions
Ques. Explain the process of galvanization. (3 marks)
Ans. Due to the reaction with the environment, the iron and steel articles get corroded easily. Galvanization is a method used to prevent the corrosion of a substance. Galvanization basically refers to the application of a protective zinc layer on a metal. The same can be done by dipping the metal that is to be protected in molten hot zinc or through the process of electroplating. The coating of zinc can protect the surface from reacting with the environment which avoids weathering and corrosion.
Ques. Why does stainless steel not rust? (2 marks)
Ans. Stainless steel shows no rusting as it has a presence of about 10 to 20 percent of chromium. Chromium works by forming a thin layer as it reacts with water and air. This thin film or layer prevents the stainless steel from the process of rusting.
Ques. What are the factors that affect the rusting of iron? (5 marks)
Ans. The factors affecting the rusting of iron includes:
- Moisture: The corrosion of iron depends on the availability of water in the environment. Exposure to rain is the most common cause of rusting of surfaces.
- Salt: As various salts are present in the sea, iron tends to rust faster in it. Saltwater has many ions that speed up the process of rusting through various electrochemical reactions.
- Impurity: Pure ion tends to rust slower than iron which contains a mixture of elements.
- Acid: If the environmental pH around the iron is low, the speed of the rusting process increases. This is why when the iron is exposed to acid rain, it starts to quickly rust. A higher pH inhibits the process of corrosion.
Ques. What is Cathodic Protection? (5 marks)
Ans. When metals are exposed to an electric charge, it can inhibit the electrochemical reactions that eventually result in rusting.
- The process can be performed by making the steel or iron as a cathode by attaching a sacrificial anode to it.
- The sacrificial anode that is attached must have a more negative electrode potential than the iron.
- Some of the metals that are most probably used as a sacrificial anode are zinc, magnesium, and aluminium.
- Once these metals get corroded away, they should be replaced quickly to protect the iron or steel from rusting.
Ques. Explain the various types of corrosion. (5 marks)
Ans. The various types of corrosion are:
- Crevice Corrosion: The corrosion that occurs in confined spaces known as crevices, where the access of the working fluid to the atmosphere is limited, is known as crevice corrosion.
- Galvanic Corrosion: The corrosion that is most often noticed in galvanized iron, a sheet of iron or steel coated with zinc.
- Pitting Corrosion: It is localized corrosion that leads to the formation of small holes in the metal.
- Uniform Corrosion: The corrosion that occurs at a consistent rate on an exposed metal surface is known as uniform corrosion.
Ques. What is dry and wet corrosion? (2 marks)
Ans. The corrosion that takes place in the absence of moisture or water is known as dry corrosion. The metal in this corrosion gets oxidised on its own.
The corrosion that occurs in the presence of moisture is known as wet corrosion.
Ques. Do all metals get corroded? (3 marks)
Ans. Metals that are placed higher in the reactivity series such as zinc and iron are corroded very easily whereas metals that are placed lower such as palladium, platinum, and gold do not corrode. This is due to the oxidation state of metals. As we go down the series, the oxidation potential which refers to the tendency of metals to get oxidised gets very low.
Though aluminium is very reactive, it does not corrode as it is already covered by a layer of aluminium oxide which further protects it from corrosion.
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