Corrosion

Elements are primarily divided into two groups based on physical and chemical properties. These two groups of elements are metals and nonmetals. Chemically active metals get eroded in the presence of air and moisture in the atmosphere. Metals placed at higher reactivity series (such as Magnesium, Iron, Zinc, and Tin) are much susceptible to corrosion. Corrosion is a natural phenomenon that transforms a refined material into a more chemically stable form, such as its oxide, hydroxide, or sulfide. It is the immediate destruction of material (generally metals) by chemical and/or electrochemical reactions with their environment. Corrosion decays the useful properties of metal and structures including strength, appearances, and permeability to liquid and gases. Corrosion is the decaying of metals on the grounds of the chemical reaction in the presence of the immediate environment. In this article, we will discuss what is corrosion, corrosion definition, types of corrosion, prevention of corrosion, causes of corrosion, etc.

Corrosion Definition

Corrosion definition states that it is a natural process that causes the transformation of pure metals into undesirable substances when they react with substances such as water or air. This reaction causes damage or disintegration of metal as it interfaces with the environment and gradually spreads to the entire bulk of the metal.

Causes of Corrosion

Metals begin to corrode when it reacts with other substances such as oxygen, hydrogen, an electric current, or even bacterias or dirt. Corrosion can also take place when metals such as steel are occupied under so much stress compelling materials to crack.

Corrosion Example

• When an iron ore comes in contact with oxygen present in moist air, a brown coat is deposited over it because of the formation of carbon dioxide. This is known as the rusting of iron.

• When a silver ore comes in contact with oxygen present in the air, a black coat is deposited over it because of the formation of sulphur dioxide. This is known as corrosion or tarnishing of silver.

• When a copper metal comes in contact with the carbon dioxide present in the air, a green coat is deposited over it because of the formation of copper carbonate. This is known as corrosion or tarnishing of copper.

How quickly metal rusts or corrodes can be described using something called the Deal-Grove model. This model helps us predict and control the formation of a layer called oxide in different situations. Another way to measure corrosion is by using the weight loss method. In this method, we take a clean, weighed piece of metal, expose it to a rusting environment for a while, clean it to remove rust, and then weigh it again to see how much weight it lost.

The rate of corrosion (how fast it rusts) is calculated using a formula:

$R = \dfrac{kW}{\rho At}$

Where:

R is the rate of corrosion,

k is a constant,

W is the weight loss of the metal over time,

A is the surface area of the metal exposed, and

ρ is the density of the metal (in grams per cubic centimeter).

Corrosion and Its Types

The corrosion and its types are given below:

1. Uniform Corrosion

In the Uniform Corrosion, a uniform layer of rust is formed on the surface of the metal and spread over the entire surface of the metal. This type of corrosion is widely seen on metals that are not protected by surface coating. Aluminum, zinc, and lead are some metals commonly affected by uniform corrosion.

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2. Erosive Corrosion

Erosive corrosion is caused by mechanical abrasion due to the relative movement between the metal surface and corrosive liquids. In this, the surface of metals gets deteriorated gradually by the abrasion of fast-moving liquid, and cavities are also formed. This type of corrosion is commonly seen in metals carrying moving fluids in them.

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3. Fretting Corrosion

Fretting corrosion occurs at the contact area of two materials that are joined together. This usually appears when the contact area is subjected to slips and variations. This type of corrosion can be seen generally in bolted and riveted joints, clamped surfaces, etc.

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4. Corrosion Fatigue

Fatigue material is defined as the failure of material due to repeated use of stress. If the fatigue of metal is developed in the corrosive environment, then it is known as corrosion fatigue. This can be improved by preventing the fatigue resistance of the material.

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5. Intergranular Corrosion

Intragranular corrosion is the corrosion that appears on the granular region and the grains are not affected in this case. It appears when there is a noticeable difference in reactivity against impurities between grains and grain boundaries. the difference in reactivity appears due to defective welding, heat treatment, stainless steel, copper, etc.

Prevention of Corrosion 

The below prevention of corrosion points shows how corrosion can be prevented in different ways.

1. Electroplating

Electroplating is a process that uses electric current to cover the thin layer of metal over the top of another less expensive metal. It is usually done to provide finishing to the cheaper metals and include certain properties such as anti-corrosion and anti-rust.

In order to electroplate a metal, two different metals, an electrolyte solution, two electrodes, a battery or other source will be required that will create the electric current.

Once the power is turned on, one metal gets positively charged and the other metal gets negatively charged. Over a certain period, the molecules of the positively charged metal shift to the surface of the negatively charged metal, which creates a very thin layer.

A common example of electroplating is brass and copper. In such a case, brass and copper would be placed in an electrolyte solution.

The electrolyte would be attached to each piece of metal and battery. Once the power is turned on, the copper metal will slowly attach itself to the brass, creating a thin copper coating on the surface of the brass. For such a case, you would probably require a solution that contains copper sulfide.

2. Galvanization

This process includes a coating of iron with a thin layer of zinc. It is usually done by dipping iron in molten zinc. This zinc layer coating protects the inner part of the iron from corrosion.

3. Painting and Greasing

Providing a layer of grease or paint on the metal can prevent the exposure of metal to the external environment thereby preventing corrosion.

4. Selection of Optimum Material

Choosing an optimum material can also help in preventing corrosion. Aluminum and stainless steel are highly corrosion resistant.

5. Using Corrosion Inhibitors

Corrosion inhibitors are the element that has the ability to diminish the rate of corrosion (when added to the corroded environment).

Corrosion Examples, Reactions and Effects

Corrosion happens when metals react with the air or other substances around them. Let's look at an example with copper.

1. Copper Corrosion:

When copper is exposed to the air, it reacts with oxygen to create red copper (I) oxide:

2Cu(s)+½O2(g)→Cu2O(s)

Then, Cu2O changes to black CuO:

Cu2O(s)+½O2(g)→2CuO(s)

CuO reacts with substances like CO2, SO3, and H2O in the air, forming blue Cu2(OH)2 (Malachite) and green Cu4SO4(OH)6 (Brochantite). This is why copper turns bluish-green.

For example, the Statue of Liberty, coated with copper, turns blue-green due to this process.

2. Silver Tarnishing

When silver comes into contact with sulfur and sulfur compounds in the air, it turns black due to the formation of silver sulfide (Ag2S). This reaction occurs as silver reacts with hydrogen sulfide (H2S) in the atmosphere, which is present because of certain industrial processes.

2Ag(s) + H2S(g) → Ag2S(s) + H2(g)

3. Corrosion of Iron (Rusting)

Rusting of iron, a common occurrence, takes place when iron is exposed to air or water. This process can be understood as a typical electrochemical cell reaction. The metal iron loses electrons and transforms into Fe2+ (considered the anode). The electrons then move to the other side, where they combine with H+ ions. These H+ ions are released either by water (H2O) or carbonic acid (H2CO3) in the atmosphere (considered the cathode).

${H}_2 {O} \rightleftharpoons {H}^{+}+{OH}^{-}$

${H}_2 {CO}_3 \rightleftharpoons 2 {H}^{+}+{CO}_3^2$

Anode reaction

$2 {Fe}({s}) \rightarrow 2 {Fe}^{2+}+4 {e}^{-}$

$E_{{Fe}^{2+/ {Fe}}}^0=-0.44 {~V}$

Cathode reaction

$O_{2(g)}+4 H^{+}{ }_{(a q)}+4 e^{-} \stackrel{2}{\longrightarrow} H_2 O_{(l)} E_{H^{+}}^o / O_2 / H_2 / O=1.23 {~V}$

Overall reaction

$2{Fe}_{({s})}+{O}_{2({~g})}+4 {H}^{+}({aq}) \rightarrow 2 {Fe}^{2+}{ }_{({aq})}+2 {H}_2 {O}_{({l})} {E}_{\text {cell }}^{\circ}=1.67 {~V}$

The Fe2+ ions formed at the anode react with oxygen in the atmosphere, oxidizing to Fe3+ and forming Fe2O3, which appears as rust in the hydrated form as Fe2O3.xH2O.

${Fe}_2 {O}_3 \cdot {xH}_2 {O}$

${Fe}^{2+}+3 {O}_2 \rightarrow 2 {Fe}_2 {O}_3$

${Fe}_2 {O}_3+{xH}_2 {O} \rightarrow {Fe}_2 {O}_3 . {xH}_2 {O} \text { (rust) }$

Factor Affecting Corrosion

The factors affecting corrosion are discussed below:

• Presence of impurities such as salt (eg, NaCL).

• An increase in temperature increases corrosion.

• Exposure of metal to air conditioning gases such as CO₂, SO₂, So₃, etc.

• Exposure of metal to moisture especially salt water ( which escalates the rate of corrosion).

• An increase in temperature increases corrosion.

• Some oxides such as AL₂0₃ form an insoluble protective layer that can further stop corrosion. Others like rust easily disintegrate and exhibit the remaining part of the metal.

Extra Points to Remember

• The formation of the oxide layer is often determined by the Deal-Grove model. It helps in predicting the formation of the oxide layer. In the meanwhile, the weight loss method is used for measuring corrosion. 

• A clean weighed piece of alloy or metal is exposed to the corrosive environment for a certain duration of time in the weight loss method and then the cleaning process follows, where the corrosion is removed and the piece then is weighed to determine the loss of weight of the metal or alloy.

• Corrosion has several effects on different materials. Below are listed some of them:

• Natural resources are wasted due to this process

• It can also cause hazardous situations such as weakening the structure built of things. 

• Accidents can be caused by corroded materials. 

• It can cause pipelines to break apart or crash. Therefore it is essential to prevent corrosion. 

• Examples of corrosion may include– Copper corrosion, Corrosion of iron, as well as, silver tarnishing. 

Quick Tips to Understand Corrosion

Chemistry is a hard subject, or so it is considered by several students but it is not impossible to understand Corrosion as one of the chapters. JEE aspirants must bear in mind the following things while understanding and practicing this topic: 

• Read the Chapter Thoroughly

A JEE aspirant must be willing to read this chapter first that is present in the book in order to understand the topic in depth. It can be vast and confusing if taken lightly. 

• Read the Notes 

After the student is finished reading the chapter, the notes are the next best thing to sharpen the knowledge. A student should be well versed with the notes for a clearer and simpler understanding of corrosion. 

• Note the Equations 

A student must note the chemical equations for better understanding and truly mastering the concept of corrosion on several metals. Chemistry is all about the in-depth understanding of fundamental aspects of the chapters so a regular touch with this topic will surely help a student or aspirant for honing skills. 

• Make Notes 

Every student or aspirant must make their own notes for quick study to refresh the concepts. Relying completely on the notes or the textbook will never be enough for the exams. 

• Solve Questions From Practice Papers and Sample Papers

A student or aspirant must solve the questions present in the sample papers and practice papers to memorize this topic accurately and test their knowledge. Practicing sample questions will help a student or an aspirant gain a better grip on this topic and verse well with the equations. 

• Clear the Doubts 

A student or aspirant should avoid piling up doubts and get them cleared as soon as possible in order to proceed further with the chapter. It will be better during the exams if the concepts are understood properly. 

• Check the Solved Examples

The solved examples are a great way to understand the chemical equations from this chapter. For example, the equations of effects of corrosion are provided with solved examples for a better understanding of students and aspirants. The rate of corrosion also can be memorized and mastered in the same way.

• Practice Mock Tests

A JEE aspirant must always keep on testing their knowledge to enhance their skills. It is a great way to expect and prepare for the possible questions that might also involve the chemical equations from this topic which will be helpful for the main exams. 

Incorporating Vedantu for Easier Understanding of Corrosion

1. The notes of the chapter on Corrosion are available on Vedantu’s official website that a student can refer to in order to study and understand the topic. It can be downloaded free of cost at any time and at any place. 

2. Vedantu offers online doubt clearing sessions on this topic in case a student or an aspirant feels stuck at any equation or basic concept of corrosion then they may reach out and sign up to get their doubts cleared up. 

3. The sample questions along with other materials necessary for JEE exams on this topic are also available on Vedantu’s website and mobile application which will surely help the students to get a better grip on this topic. 

4. Vedantu offers previous year question papers which can help an aspirant get a better idea about the possible questions on Corrosion for JEE main and advanced level exams. 

5. The practice test and mock tests on Corrosion will also make sure the aspirant is well prepared for the exams. It is a great way to stay in touch with the equations on this topic.