Corrosion

Have you ever seen old locks or keys turned reddish-brown? Or old nails, on a wooden bench have reddish-brown flakes chipping off of it? They were not always like that. When new, they were shiny and smooth. They have been corroded. 

Corrosion Corrosion

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Table of contents

    Corrosion is a dangerous phenomenon. If left unchecked, it can cause bathrooms to flood, pipelines to break, or even buildings to collapse. So what exactly is corrosion? Continue reding too find out.

    • This article is about corrosion.
    • We will start withthe definition of corrosion and the corrosion in metals.
    • Then, we will see the causes of corrosion and its prevention.
    • We are going to study the reaction of corrosion.
    • We will finish the article with the types of corrosion.

    Corrosion: definition

    Corrosion is the destruction of materials due to chemical reactions with substances in the environment.

    Corrosion in metals

    In metals, Corrosion is the natural conversion of refined metal into a more stable state such as metal oxide, sulphide, or hydroxide.

    So, corrosion is a chemical reaction, in which a material reacts with elements found in the atmosphere or water, and gets oxidized.

    An element is said to get oxidized when it loses electrons.

    Conversely, an element is said to get reduced when it gains electrons.

    Causes of Corrosion

    You may ask, how can corrosion happen naturally? The causes of corrosion can be many:

    • Corrosion of metal may occur due to exposure to humidity or condensation of water on metal surfaces. A metal can react with Oxygen and Hydrogen present in water to convert into metal oxides. You will learn more about this when you read about how Iron gets corroded due to water.
    • Exposure to hydrogen and oxygen, or corrosive gasses such as Chlorine, Ammonia, Sulphur Oxides, or Hydrogen Peroxide. In the process of corrosion, metals get oxidized i.e. they lose electrons. So, naturally, corrosive substances are good at oxidizing metals i.e. making them lose electrons and gaining electrons themselves.
    • Exposure of metal to electric currents for a long time can also cause them to corrode.
    • Metals such as steel can undergo stress corrosion when they are subjected to a corrosive environment and high stresses. High stresses can cause the metal to develop cracks in its micro-structure, which can then get corroded. This is extremely dangerous as it cripples buildings or bridges.

    A corrosive substance is something which is highly reactive and tends to cause corrosion to other materials.

    Corrosion Prevention

    A metal gets corroded when it is exposed to the environment. An exposed piece of metal is susceptible to corrosive agents and is dangerous for the entire structure which is not exposed. Corrosion can be prevented by placing a barrier between the material and the environment. There are many ways we can do this, like:

    • Painting.
    • Greasing.
    • Electroplating.
    • Galvanization.

    Electroplating is a process by which a layer of metal is deposited on another metal. The deposited layer of metal acts as a protective layer which prevents the corrosion of the actual object.

    Galvanization is a process in which a layer of Zinc is deposited on the surface of the metal object. Zinc is more reactive than Iron and therefore gets corroded to prevent corrosion of Iron. For this reason, Galvanization of Iron is also called sacrificial protection of Iron. The corroded Zinc can easily be replaced with another coat of Zinc to ensure the Iron underneath does not rust. Zinc can be coated on a metal by painting with zinc-rich paints, or electroplating.

    Some metals are more resistant to corrosion than others. Precious metals such as Gold, Silver, and Platinum are overall very less reactive to the environment, which is another reason why they are used in jewellery. Gold is also coated on some electronic components to prevent their corrosion.

    Stainless steel is the most corrosion-resistant alloy. Stainless steel is an alloy of Iron, Carbon, and Chromium. Some other metals such as Nickel, Titanium and others can be added to improve its corrosion resistance.

    Aluminium is considered safe to be used for packaging food items. The surface layer of Aluminium reacts with Oxygen in the air to form a thin layer of Aluminium Oxide. This protects the metal underneath from further getting corroded.

    Corrosion reaction

    Corrosion is an electrochemical reaction between the corroding agent (which can be oxygen and water, or any corrosive material), and the material being corroded.

    An electrochemical reaction is a chemical reaction in which there is a transfer of charges between 2 substances.

    Let us look at what happens when Iron gets corroded. Corrosion of iron is called rusting. Iron gets rusted when exposed to air and water, or steam. The chemical formula for rust is Fe2O3.H2O. By studying the detailed process of rusting, you will see how the flow of charges in this electrochemical reaction takes place.

    Corrosion Diagram showing the rusting process of iron StudySmarterFig. 1: Rusting Process of Iron | CorrosionPedia

    Anodic site is where the corrosion is occurring. At anode, electrons are being released and cations are being formed.

    Cathodic side where corrosion is not occurring. The cathode receives the free electrons released at anode.

    Let us see which chemical reactions lead to the formation of rust.

    Rusting is a complex process, but can be summarized in 1 chemical reaction -

    $$4Fe+3O_{2}+6H_{2}O\rightarrow 4Fe(OH)_{3}$$

    Ferric Hydroxide (Fe(OH)3) dehydrates to form Fe2O3.H2O.

    1. In the first step, Iron undergoes oxidation when it comes in contact with water, and Iron (II) ions are formed.

    $$Fe_{(s)}\rightarrow Fe^{2+}_{(aq)}+2e^{-}$$

    Iron loses electrons to become iron (II) ion.

    The electrons released by Fe in this reaction react with the hydrogen ions in water and the dissolved oxygen to produce water.

    $$4e^{-}+4H^{+}_{(aq)}+O_{2(aq)}\rightarrow 2H_{2}O_{(l)}$$

    Hold on, rust has not formed yet. These 2 reactions produce water and Iron (II) ions. Now the second step takes place.

    2. In the second step, Hydrated Iron Oxide (rust) is formed. The consumption of H+ in the first step leaves Hydroxide ions (OH-) in the solution. Fe2+ ions react with these hydroxide ions to form Ferrous Hydroxide, or Hydrated Iron Oxide.

    $$Fe^{2+}_{(aq)}+2OH^{-}_{(aq)}\rightarrow Fe(OH)_{2(s)}$$

    Fe(OH)2 is also called green rust. Simultaneous to this reaction , Fe2+ also reacts with more H+ and oxygen to get oxidized further to Fe3+ .

    $$4Fe^{2+}_{(aq)}+4H^{+}_{(aq)}+O_{2(aq)}\rightarrow 4Fe^{3+}_{(aq)}+2H_{2}O_{(l)}$$

    These Fe3+ ions combine with OH- ions to form Ferric Hydroxide.

    $$Fe^{3+}_{(aq)}+3OH^{-}_{(aq)}\rightarrow Fe(OH)_{3}$$

    This Ferric Hydroxide dehydrates to form Fe2O3.H2O, which is what makes the reddish-brown rust we see.

    Rusting only refers to the corrosion of Iron, or alloys of Iron such as steel.

    Types of Corrosion

    Corrosion can be broadly categorized into three types:

    • General attack corrosion.
    • Localized Corrosion.
    • Galvanic corrosion.

    General Attack Corrosion

    General attack corrosion is also called Uniform Attack Corrosion. It occurs when the entire exposed surface of a metal is corroded evenly and at the same rate. It is the most common type of corrosion.

    Localized Corrosion

    In localized corrosion, a small area of an exposed metal surface is corroded. Localized corrosion is of three types:

    Pitting

    Pitting is when a small hole or cavity is formed on the metal surface. This can penetrate the metal and ultimately deteriorate it to failure.

    Crevice Corrosion

    Happens due to stagnant environment conditions like under a gaskets, washers, or clamps. Liquids or oxygen may get trapped in such crevices for a long time, and give way to corrosion for the exposed metal.

    Filiform Corrosion

    Filiform corrosion occurs when water, acids or other corroding substances enter cracks/gaps in the paint coating or metal coating. Filiform corrosion can spread inside the structure and cause structural weakness.

    Galvanic Corrosion

    This is interesting. Galvanic corrosion is referred to the corrosion that occurs when two dissimilar metals are in electrical contact with each other, and are exposed to a corrosive electrolyte. When such a condition is present, the two metals corrode at different rates. One of the metal corrodes faster than it would have corroded alone, and the other corrodes slower than it would have alone. The metal which corrodes faster is called the sacrificial metal.

    An electrolyte is any liquid containing ions. A solution of water and table salt is an electrolyte as it contains ions of Sodium (Na+) and Chlorine (Cl-) among other ions.

    Corrosion - Key takeaways

    • Corrosion is the destruction of materials due to chemical reactions with substances in the environment.
    • In metals, Corrosion is the natural conversion of refined metal into a more stable state such as metal oxide, sulphide, or hydroxide.
    • Corrosion is an electrochemical reaction between the corroding agent and the material being corroded.

    • Corrosion can be caused by water, oxygen, corrosive substances like Chlorine, Hydrogen Peroxide, or Sulphur Oxides. Corrosion can also be caused when subject to electric current for a long time.

    • Corrosion in metals can be prevented by coating them with paint, grease, or other metals.

    • The process of coating metals with other metals is called electroplating.

    • The process of coating metals with Zinc is called Galvanization. Galvanization can be done through electroplating, or painting with Zinc-rich paint.

    • Zinc is more reactive than Iron. Therefore, in galvanized Iron, Zinc gets corroded to prevent corrosion of Iron. This is called sacrificial protection of Iron.

    • Corrosion of Iron is called Rusting.

    • Chemical formula for rust is Fe2O3.H20. It is the dehydrated form of Fe(OH)3

    • Chemical reaction for rusting is:

      $$4Fe+3O_{2}+6H_{2}O\rightarrow 4Fe(OH)_{3}$$

    • Corrosion is broadly categorized into three types - General attack corrosion, Localized Corrosion, and Galvanic corrosion.
    Frequently Asked Questions about Corrosion

    What is corrosion?

    Corrosion is the destruction of materials due to chemical reactions with substances in the environment.

    What are the effects of corrosion?

    Due to corrosion, pipelines can break, historical monuments can get destroyed, buildings, bridges, and other physical structures can fall, electrical circuits can fail.

    How does corrosion work?

    In metals, Corrosion occurs when metals react with substances in the environment and for metal oxides. Refined metal when turned into its oxide can lose its properties and cause failure to the purpose for which it was used.

    What are the 3 types of corrosion?

    1. General attack corrosion

    2. Localized corrosion

    3. Galvanic corrosion

    What causes corrosion?

    Corrosion occurs due to reaction of a substance with corrosive substances in the environment. In metals, corrosion can occur due to reaction with Oxygen and water, acids, or corrosive substances like Hydrogen Peroxide, or Sulphur Oxide.

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