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Corrosion

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Corrosion is a chemical phenomenon that we observe regularly in our daily life. Corrosion occurs in metal and in corrosion reaction metal react with air and moisture from the surrounding to form the more stable compounds of the atom that include oxides and hydroxides.

Generally, rusting of iron is also an example of corrosion as rusting iron reacts with oxygen and moisture from the air to form iron oxide (rust). Corrosion reactions of various metals can be easily achieved by allowing the metal to react freely with moist air. In this article, we will learn about, Corrosion, its types, examples and others in detail.

What is Corrosion

Corrosion is described as the chemical and electrochemical destruction of metals or alloys by the surrounding damp and humid environment. Corrosion is an oxidation process since it occurs in the presence of oxygen. Almost all metals corrode, however, the rate of corrosion varies depending on the metal.

Metals generate stable compounds such as oxides, hydroxides, and sulphides as a result of corrosion, which is a spontaneous and irreversible process.

Corrosion Definition

The degradation of pure material when they react with moisture, air or acid is called Corrosion. Corrosion is a redox reaction that causes the loss of pure metal by converting them into oxide and thus decreasing its strength and other physical properties.

Do All Metals Corrode?

A reactivity series is a series of elements based on their reactivity. The elements placed higher up in the reactivity series is more reactive as compared to the element placed below the element in the reactivity series. In the reactivity series, the metal that is placed up corrodes very easily than the metal placed below in the reactivity series. So all metal can be corroded but their rate of corrosion varies a lot. metal like iron and copper corrode very easily, whereas metal like gold and platinum does not corrode at all or corrode very little.

The most common exception to this is the metal Aluminium as even though it is placed up in the periodic table then also it does not corrode easily.

Factors Influencing Corrosion

Various factors influencing the corrosion of any material are,

Position of Metals in Electrochemical Series: In corrosion, the electrochemical series is extremely important. Reactive metals lose electrons more quickly and corrode more quickly. Iron, for example, is quickly oxidised, but gold is not because gold is less reactive than iron.

Impurities in Metals: In general, the presence of contaminants in metals accelerates corrosion because these impurities operate as the microscopic electrochemical cell that causes corrosion.

Presence of Electrolytes: Because they carry ions, dissolved salt in water acts as an electrolyte. Corrosion is accelerated when electrolytes are present in water.

Concentration of Oxygen: Corrosion is accelerated by an increase in oxygen content. An anode is a location with less oxygen concentration, whereas a cathode is a region with a higher oxygen concentration. Corrosion happens as a result.

Humidity in Weather: When the weather is humid, the presence of moisture with a high temperature accelerates corrosion because ions gain energy and begin moving quicker in a higher temperature, causing them to collide more frequently.

Temperature: The rate of Corrosion increases with temperature.

Rate of Corrosion

The formation of an Oxide layer on metal is explained by the Deal Grove model. With the help of this model, we can predict and control the formation of Oxide layers even in adverse situations. Since corrosion lead to the loss of metal, one can ask what is lost. The answer is the mass of the metal is lost. Hence, to measure how much corrosion occurred one should measure the mass lost from the metal. To measure the mass lost, weigh a clean pure piece of metal and then expose it to the environment for a definite period. After corrosion had occurred clean the corroded surface and again measure the mass of the metal block. 

We can easily calculate the rate of corrosion (R) using the following formula,

R = kW/ρAt

where,

  • k is the proportionality constant
  • W is the weight loss of the metal in time t
  • A is the surface area of the metal exposed
  • ρ is the density of the metal (in g/cm³)

Corrosion Types

There are various types of corrosion and all of them are explained below,

Uniform Corrosion

Corrosion of this sort is fairly prevalent. This kind of corrosion results in a homogeneous loss of metal throughout a whole surface. Rusting, tarnishing of silver, and so forth are examples.

Pitting Corrosion

Pitting corrosion is a type of localised corrosion that damages the metal surface, resulting in patches or pits. In neutral or acid solutions containing halides, such as saltwater containing NaCl, pitting corrosion can be observed in stainless steels.

Crevice Corrosion

Crevice corrosion is caused by a stagnant solution in crevices around the edges of metal surfaces, such as nuts and rivet heads. It can happen when two metals collide or when a metal collides with a non-metal. Corrosion on metal or non-metal gaskets, couplings, and joints, for example.

Galvanic Corrosion

When two dissimilar materials are connected and brought into electrical contact with a corrosive electrolyte, galvanic corrosion occurs. In galvanised iron, for example, a sheet of iron or steel is coated with zinc.

Stress Corrosion Cracking

It’s a form of corrosion in which fractures appear on the metal’s surface as a result of the combined effects of tensile strength and the corrosive environment. Stress Corrosion Cracking (SCS) is a kind of corrosion that occurs in aviation components.

Intergranular Corrosion

It’s a sort of corrosion that happens around the grain boundaries of a metal or alloy that contains impurities, rather than inside the metal or alloy. Die-cast zinc alloys containing aluminium exposed to steam in a maritime environment display intergranular corrosion.

Dealloying 

It’s a rare sort of corrosion that only affects particular metals, such as copper alloys and grey cast iron. The alloy metal loses its reactive element and creates a more stable corrosion-resistant element in a porous condition during the dealloying process. De-nickelification, de-zincification, and graphitic corrosion are all instances of dealloying.

Erosion Corrosion

Erosion Corrosion is a mechanically induced breakdown of a metal surface. In a corrosive environment, it is caused by the relative mobility of fluid particles and solid surfaces. Burrs left at the cut tube ends of pipelines, for example, might disrupt smooth water flow, causing localised turbulence and high flow velocities, resulting in erosion-corrosion.

Various Examples of Corrosion

Metals with a higher reactivity series, such as iron, copper, and zinc, corrode more quickly. Corrosion can manifest itself in a variety of ways.

The image added below shows the corrosion in various materials.

Examples of Corrosion

 

Rusting of Iron

Rusting is the process related to iron metal, the corrosion of iron in the presence of air and water is called Rusting. It results in the formation of a Reddish-Brown surface. When iron oxidises is formed in the presence of oxygen and water we say that rusting has occurred. The chemical formula of the rust is Fe2O3.xH2O

Though rusting of iron occurs naturally, we can represent the formation of rust as an electrochemical reaction as,

At Cathode: 

H+ + e→ H

4H + O2 → 2H2O

4H+ + O2 + 4e→ 2H2O

At Anode:

2Fe → 2Fe2+ + 4e

Oxidation Half-reaction

Fe(s) → 1/2Fe2+(aq) + 2e

Reduction Half-reaction

4H+ + O2 + 4e→ 2H2O

Overall Cell reaction

2Fe(s) + 4H+ + O2 → 2Fe2+(aq) + 2H2O

2Fe3+ + 3O2 → 2Fe2O3

Fe2O3 + xH2O → Fe2O3.xH2O(rust)

Corrosion of Copper

When exposed to moist air, copper metal or alloys of copper corrode, i.e. they form a pale-looking copper oxidises layer, and the shiny copper surface tarnishes, i.e. it turns dark brown or black, then green.

The blue-green colour of the “Statue of Liberty” is an example of corrosion of copper metal. It was not painted the same colour as it is now when it was built. Rather, it was brown in colour. The copper oxidised over time and reacted with sulphur trioxide, carbon dioxide, and water in the atmosphere to generate a new chemical that gave the Statue of Liberty its distinctive blue-green patina.

The following is the chemical equation involving the corrosion of metal is,

Cu + H2O + CO2 + O2 → Cu(OH)2 + CuCO3

Tarnishing of Silver

Tarnishing is the process of the silver metal becoming black due to a covering of silver sulphide. Chemical interaction between the silver and sulphur-containing compounds in the air causes the silver to tarnish. Silver sulphide is formed when silver is mixed with sulphur. 

The following is the chemical equation showing the tarnishing of silver,

2Ag + H2S → Ag2S + H2

Prevention of Corrosion

Corrosion is an unwelcome phenomenon that must be avoided since it causes significant damage to metallic constructions such as bridges, sculptures, monuments, metal utensils, and so on.

All corrosion prevention methods are based on the same basic concept: cutting off the metals’ water and air supply. The following are some of the corrosion prevention measures:

Painting: Painting iron items, such as gates and rails, prevents metal from being exposed to air and water. Painting metallic items protect them from corrosion.

Oiling and Greasing: Oiling and greasing, like painting, provide a protective coating on the metal surface that protects it from corrosion.

Galvanisation: Galvanisation is the process of coating iron and steel things with a thin layer of zinc to protect them. The galvanised products are protected against corrosion or rusting even after the zinc coating has been broken.

Alloying: Alloys are made up of a homogeneous combination of metals and non-metals. We may achieve the required characteristics of metals by alloying them. Iron, for example, is a very useful metal, yet it rusts fast. As a result, when this iron is combined with Nickel and Chromium, stainless steel is created. Stainless steel is now widely utilised in the manufacture of kitchenware.

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FAQs on Corrosion

Q1: What is Corrosion? 

Answer

The reaction of any metal with the oxygen and water from the surrounding making more stable oxides and the hydroxides of the metal is called corrosion. The rusting of iron is an example of corrosion.

Q2: What are Ways to Prevent Corrosion?

Answer:

Various ways to prevent corrosion are,

  • Painting, oiling, and greasing iron products prevent the metal from being exposed to air and water. Corrosion is therefore avoided.
  • Galvanisation of the metal, it is the process of coating iron and steel things with a thin layer of zinc to protect them as zinc does not corrode easily.
  • Making alloy of the metals.

Q3: What are Factors Affecting Corrosion?

Answer:

Various factors affecting the corrosion are,

  • Position of metals in Electrochemical Series
  • Impurities in Metals
  • Presence of Electrolytes
  • Concentration of Oxygen
  • Humidity of Air, etc.

Q4: What are Types of Corrosion?

Answer

There are various types of corrosion that include,

  • Uniform Corrosion
  • Pitting Corrosion
  • Crevice Corrosion
  • Galvanic Corrosion
  • Stress Corrosion Cracking
  • Intergranular Corrosion
  • Dealloying
  • Erosion Corrosion

Q5: What is Tarnishing of Silver?

Answer

Tarnishing of silver is a chemical process between the silver and sulphur-containing chemicals in the air. When silver is combined with sulphur, silver sulphide is created. Tarnishing occurs when a layer of silver sulphide coats the silver metal, turning it black. The reaction of trashing of silver is represented as,

2Ag + H2S →  Ag2S + H2

Q6: What is Rusting of Iron?

Answer

The corrosion of iron is called the rusting of iron in this reaction the iron reacts with water and oxygen from the air to form the iron oxide. This reaction is represented as,

Fe2O3 + xH2O → Fe2O3.xH2O

Q7: What is the Effect of Temperature on Corrosion?

Answer:

The effect of the temperature on the corrosion is that the corrosion increases with the increase in temperature.



Last Updated : 19 Jun, 2023
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