Important Questions and Answers: Electro Chemistry and Corrosion

Electrochemistry & corrosion.

01. Define an Electrochemical cell. Give one example.

A cell which converts chemical energy into electrical energy is known as electrochemical cell. Example: Daniel cell , Batteries.

02. What do you mean by redox reaction?

Both reduction and oxidation takes place simultaneously in a cell reaction then it is known as redox reaction of an electrochemical cell.

03. What is electrode potential?

It is the measure of tendency of a metallic electrode to lose or gain electrons, when it is in contact with its own salt solution. It is denoted as “E”

04. Define an origin of electrode potential.

When a metallic electrode is placed in its own salt solution, two types of reaction takes place.

        +ve ions may pass into the solution. M → Mn+ + ne- (oxidation)

        +ve ions from the solution may deposit over the metal. Mn+ + ne-→M (reduction)

The above reaction takes place in an electrode then it is known as an origin of electrode potential.

05. Define oxidation potential and reduction potential.

Oxidation potential: The tendency of a metallic electrode to lose electrons, Reduction potential: The tendency of a metallic electrode to gain electrons

06. How an electrochemical is measured? Define EMF of an electrochemical cell.

It is measured by EMF.: “The difference of potential which causes flow of electrons from one electrode of higher potential to the other electrode of lower electrode potential”.EMF= ER-EL

07. What are the applications of electrochemical cell?

Determination of sparingly soluble salt.,Determination of the valency ion. Determination of standard free energy change and K.

Potentiometric titrations can be carried out.,Hydrolysis constant can be determined.

08. Define electrochemical series.

When various types of metallic electrodes are arranged in their increasing order of standard reduction potential on the basis of hydrogen scale is known as emf series.

09. Write the significance of electrochemical series.

To calculate the standard emf of the cell.,Relative ease of oxidation or reduction.

Displacement of one element by the other., Hydrogen displacement behavior. Determination of equilibrium constant (K) for the reaction.

10.Write the mathematical form of Nernst equation and give one application.

E = Eo + 0.0591/n log [Mn+]

Application: 1.It is used to calculate electrode potential of unknown metal.

        Corrosion tendency of metals can be predicted.3. Applications of emf series.

11.Define corrosion.

The gradual destruction of the metal or an alloy surface by the chemical or electrochemical reaction with its environment. (i.e) metal into metal oxide.

12. What are the types of corrosion?

(i). Chemical corrosion.  (ii). Electrochemical corrosion

13. What are the factors which affect corrosion?

(i).Air and moisture. (ii). Electrolytes in water.(iii). Presence of impurities in metal. (iv). Presence of gases like CO2 and SO2 (v). Differential aeration.

14. Define chemical corrosion. What are the types of chemical corrosion?

It is known as dry corrosion. It is due to the local attack of metal surfaces by the atmospheric gases like oxygen,H2S,SO2 , N2, etc.

3 types. (i). Oxidation Corrosion. (ii). Corrosion by other gases. (iii). Liquid-metal corrosion.

15. Define an Electrochemical corrosion. What are the types of Electrochemical corrosion?

It occurs due to the existence of separate “anodic” and “cathodic” areas between which current flows through the conducting solution. At anodic area oxidation occurs and anodic part of metal is destroyed. (i). Galvanic corrosion. (ii). Concentration cell / Differential aeration corrosion.

16. What are the differences between chemical and electrochemical corrosion?

Chemical corrosion

1)   It occurs dry state

2)   Local attack to metal by environment

3)   Chemical corrosion is self-controlled

4)   It follows adsorption mechanism

5)   Ex: formation of mild scale on iron surface

Electrochemical corrosion

1) Wet state (i.e) presence in moisture

2) Large number of cathodic and anodic areas

3) It is continuous process

4) It follows electrochemical reaction

5) Ex: rusting of iron in moisture

17.What isBedworth rule?

The ratio of the volume of the oxide formed to the volume of the metal consumed is pilling bedworth rule

18.Bolt and Nut made of the same metal is preferred in practice . Why?

Because such a combination will not permit galvanic corrosion to take place.

19.What are the methods used to control the corrosion?

1. Sacrificial anodic method 2.Impressed current cathodic metho

20 What is paint?

Paint is a mechanical dispersion of one or more finely divided pigments in a medium (thinner + vehicle) .When paint is applied to a metal surface, the thinner evaporates, while the vehicle undergoes slow oxidation forming a pigmented film.

21.Differenciate Electroplating and Electro less plating

Electroplating

It is carried out by passing current

Separate anode is employed

Anodic reaction is M  → Mn+ + ne-

Thickness of the plating is 1-100μm

Electroless plating

Used reducing agent

Catalytic surface of the substrate acts as an anode

Anodic reaction is R → 0 + ne-

It has a thickness of 1-100μm

22. Define Electro plating and Electroless plating

Electro plating:

It‟s a process in which the coating metal is deposited on the base metal by passing a direct current through an electrolytic solution containing the soluble salt of the coating metal.

Electro less plating:

It is a technique of depositing of a noble metal from its salt solution on a catalytically active surface of a base metal by using a suitable reducing agent.

PART B

            1. What is electrochemical cell? Explain with example of Daniel cell.

            Electrochemical cell: Cells in which the e-s transferred due to redox reaction and converted into electrical energy.

            Example : Daniel cell.

Daniel cell consists of two electrodes like Zn and Cu.

Zinc electrode is dipped in 1M ZnSO4 solution and Cu electrode dipped in 1M CuSO4 solution.

The two solutions are inter connected by a salt bridge and two electrodes are connected by a wire through the volt meter.

Each electrode is known as half-cell.

The e^-s liberates from Zn electrode flow through the external wire and is consumed by copper ions at the cathode.

Salt Bridge: U- tube containing saturated solution of KCl in agar-agar gel. It connects two half cells.

Function of salt bridge: It eliminates liquid junction potential and electrical continuity between two half cells.

How electrochemical cell is measured by potentiometrically? Or What is emf? How emf is measured by pogendroff’s method?

03. What are electrochemical series? Give its applications.

Electrochemical series: An arrangement in which the standard electrode reduction potential of different metals are arranged in increasing order on the basis of

hydrogen scale is known as emf series.

Applications of EMF series: (i). To calculate the std. emf of a cell.

(ii). Relative ease of oxidation or reduction. (iii). Displacement of one element by other. (iv). To determine K of a reaction.

(v). Hydrogen displacement from acid solutions. 

(vi). Predicting spontaneity of redox reations.

(i). To calculate the std. emf of a cell.

The emf of a cell is calculated as follows, Ecell = ER-EL 

(ii). Relative ease of oxidation or reduction.

+ve value – higher reduction potential – high reduction. F^- = 2.87, higher reduction – get oxidized

Li⁺ = -3.07, lower reduction – high oxidation. 

(iii). Displacement of one element by other.

Metals which lie higher in the series can displace those elements which lie below them in the series. Cu can easily displaced by Zn

(iv). Determination of Equilibrium Constant K of a reaction.

From the value of E^o, we can be determined the equilibrium constant K (v) Hydrogen displacement Behavior

Metals with above H2 scale will displace the hydrogen from an acid solution (ie)-ve reduction potential. Zinc reacts with hydrogen but not in Ag. Why?

Because it has positive reduction potential

(vi) Predicting Spontaneity of Redox  Reactions:

E^o value is positive=Reaction is Spontaneous

E^o value is –ve =Non-Spontaneous Process.

04. Derive Nernst equation.

Applications:

               To calculate electrode potential of unknown metal

                        Application of emf series

                        Corrosion tendency of methods can be predicted

What is Chemical corrosion? Explain with its types.

It is classified into 3 types. i). Oxidation corrosion.

ii).Corrosion by the other gases – Hydrogen iii).Liquid – Metal corrosion

01. Oxidation corrosion

Oxygen present in atmosphere attacks metal surface resulting in the formation of metallic oxide which is a corrosion product and known as oxidation corrosion.

Oxidation occurs first at the surface of the metal and the resulting metal oxide scale forms a barrier that tends to restrict either further oxidation. For oxidation to continue either the metal must diffuse outwards through the scale to the surface or the oxygen must diffuse inwards through the scale, to the underlying metal.

Both transfer occurs, but the outward metal diffusion is generally, much more rapid than the inward diffusion of oxygen. Since the metal ion is appreciably smaller than the oxygen ion and consequently of much higher mobility.

02. PILLING – BEDWORTH RULE:

The ratio of the volume of the oxide formed to the volume of the metal consumed is pilling bed worth rule.

03. Corrosion by other gases – Hydrogen.

i) At ordinary Temperature : Hydrogen Embrittlement.

The process of formation of cracks and blisters on the metal surface, due to high pressure of hydrogen gas is called hydrogen embrittlement

Metal direct contact with H2s  and forms atomic hydrogen

l direct contact with H2s  and forms atomic hydrogen

                Atomic hydrogen diffuses readily into metal to form molecular hydrogen

         At high temperature – Decarburization

The process of decrease in carbon content in steel is termed as decarburization of

steel.

04) Liquid – metal corrosion

This is due to the chemical action of following liquid metal at high temperature. The corrosion reaction involves

Either dissolution of a solid metal by a liquid metal.

                        Liquid metal may penetrate into the solid metal.

            Explain Electrochemical corrosion with its types.

01. Galvanic corrosion:

The corrosion occurs when two different metals are in contact with each other in presence of an aqueous electrolyte solution or moisture is known as Galvanic corrosion.

The more active metal acts an anode the less active and metal acts as a cathode

In Zn-Fe bimetallic couple, Zn undergoes to corrosion, because compare with Fe, it has higher –Ve emf Value. So it acts as an anode. Iron acts as cathode and it is protected.

But in the case of Fe – copper bimetallic couple Fe dissolves and copper is protected. (ie) Fe acts an anode and copper as a cathode.

Prevention of Galvanic corrosion:

To prevent Galvanic corrosion, the bolt and but are made by same metal. If they are n‟t corrosion will take place easily.

It may be minimized by putting an insulating material between the two metals.

02. Differential aeration corrosion:-  Otherwise it is known as concentration cell corrosion.

It is due to the formation of concentration cell formed by the variation of concentration mainly of oxygen or any electrolyte on the surface of the base metal.

i).When a metal is partially immersed in a solution, the metal inside the solution has very poor aeration compared with the metal that is outside the solution.

 ii).The difference in the air concentration of the base metal can produce the anode with less aerated area and cathode.

iii).Corrosion will take place at the anode area where the metal is converted into metal ion. Example: A zinc metal partially dipped in a brackish solution.

Example: Pitting Corrosion - The holes are formed at corrosion in concentrated places. Low oxygen part= anode , More oxygen part=cathode.

It is a localized attack, resulting in the formation of a hole around which the metal is relatively unattached. Ex: metal area covered by a drop of water, sand, dust, etc.

07. Distinguish between Chemical corrosion and Electrochemical corrosion.

Chemical corrosion

1)      It occurs dry state

2)      Local attack to metal by environment

3)      Homogeneous metal surface gets corroded

4)      Corrosion products accumulate in the same place, where corrosion occurs

5)      Chemical corrosion is self-controlled

6)      It follows adsorption mechanism

7)      Ex: formation of mild scale on iron surface

8)      3 types. i). Oxidation corrosion

          ii). Corrosion by other gases- H2

          iii). Liquid – Metal corrosion.

Electrochemical corrosion

1) Wet state (i.e) presence in moisture

2) Large number of cathodic and anodic areas

3) Heterogeneous surface (i.e) Bimetallic contact.

4) Corrosion occurs at the anode, while the products formed else where

5) It is continuous process

6) It follows electrochemical reaction

7) Ex: rusting of iron in moisture

8) 2 types.

i). Galvanic corrosion.

ii). Differential aeration corrosion.

08. What are the factors influencing corrosion?

The rate of corrosion is mainly depends on i)

Nature of the metal

                        Nature of the environment

i). Nature of the metal

a). Position in emf series: Metals above hydrogen in emf series, corroded easily.

Metals have high –ve reduction potential undergoes corrosion.

            Relative areas of the anode and cathode: Rate of the corrosion has higher % in anodic area, the rate of corrosion will be more, when the cathodic area is larger.

        Purity of the metal: The 100% pure will not undergo any type of corrosion. If

impurity present in higher percentage, corrosion takes places at anode.

d) Over voltage: Corrosive environment is inversely proportional to corrosion rate. e) Nature of the surface film: It is known as pilling- bed worth rule.

f) Nature of the corrosion product: Corrosion is faster when the corrosion product is soluble in corroding medium. If the corrosion product is volatile, the corrosion rate will be faster.

ii). Nature of the environment:

i). Temperature: Rate of corrosion α temperature.

The rate of corrosion increase with temperature

ii). Humidity: If the humidity is high in the environment, corrosion will be more.

iii). Presence of corrosive gases: The acidic gases like CO2,SO2,H2S are produce electrolytes, which are acidic and increases the electrochemical corrosion.

iv). Presence of suspended particles: Particles like NaCl present in the moisture and acts a

powerful electrolyte and then enhance the electrochemical corrosion.

v). Effect of pH; the rate of corrosion will be maximum when the corrosive environment is acidic (ie) PH<7.

Explain the Sacrificial anode and impressed current techniques for the preventions of corrosion.

i). The application of sacrificial anodes in cathodic protection is based on the differences in electrochemical reactivity of metals.

ii). In this method, the metal to be protected from corrosion is connected to more active metal which acts as an anode.

iii). In a redox reaction involving iron and zinc, the zinc will serve as the anode, and iron the cathode.

iv). The zinc anode will oxidize and provide electrons for the reduction of Fe²⁺ to elemental iron. v). This is called cathodic protection. The zinc anode is termed a sacrificial anode.

vi). Iron pipes buried in the ground, and designed to carry water, would normally be expected to rust pretty quickly.

vii).If they are buried along with a piece of zinc, and connected by a wire the zinc will provide cathodic protection.

2. Corrosion control through Impressed current method:

An alternative method of providing the current to protect a system is to use some sort of external power supply. As with the sacrificial system , the structure to be protected is made the cathode, the difference being that the driving force. Behind the current is not the difference in potential between the anode and cathode of the system but from the power supply.

Both these types of cathodic protection may be applied to buried pipelines and to steel hulled ships. For oil drilling platforms, however the method employed is sacrificial in the vast majority of cases in the north sea.

This technique is useful for protecting large structures like water tanks, underground oil collers, laid up ships,etc.

10. What is paint? What are the constituents and their functions in paint?

Paint: paint is a mechanical dispersion of one or more finely divided pigments in a medium (thinner + vehicle) . When paint is applied to a metal surface, the thinner evaporates, while the vehicle undergoes slow oxidation forming a pigmented film.

Constituents:

1. Pigments. 2. Vehicle or drying oil. 3. Thinner. 4. Drier. 5. Filler or extender. 6. Plasticizer 7. Antiskinnig agent.

i)  Pigments: They are colour producing substances in paint.

Ex: white colour- white lead, black=- carben black red-indian red, green – chromium oxide.

Function: i). It gives colour and opacity to the film. ii). It also provides strength to the film.

iii). It provides weather resistance of the film.

ii) Vehicle (or) drying oil: This is a non-volatile portion of a medium and film forming constituent of the paint. These are high molecular weight fatty acids present in vegetable and animal oils. Ex: Linseed oil, castor oil.

Function:

               They form a protective film by the oxidation and polymerization of the oil ii) They hold the pigment particles together on the metal surface

               They impart water repellency , toughness and durability to the film.

            Thinners or solvents: Thinner is a volatile substance present in the medium.

Ex:               turpentine, toluol, xylol etc.

Function:

               It increases the elasticity of the film

II)             It helps easy drying of the paint film

                It increases the penetrating power of the vehicle.

            Extenders or fillers: These are colourless (white) pigments which improve the quality of the paint. Ex: Gypsum, Jal ,china clay, silica, etc.

Functions:

            It reduces the cost of paint

            It prevents the cracking and shrinkage of the film iii) It modifies the shades of the pigments

v).Driers: these are the substances, used to accelerate the process of crying. Ex: metallic soaps.

Functions:  i).They are oxygen –carriers (or) catalists.

ii). They provide oxygen , which is essential for oxidation, polymerization of drying oil. vi).Plasticisers: these are chemicals added to the paint to provide elasticity to the film and to prevent cracking of the film. Ex: triphenyl phosphate, tricresyl phosphate, etc.

vii). Antiskinning agents: These are chemicals added to the paint to prevent gelling and skinning of the paint. Ex: polyhydhroxy phenol.

11. Explain the electroplating of Gold.

Electroplating: It‟s a process in which the coating metal is deposited on the base metal by passing a direct current through an electrolytic solution containing the soluble salt of the coating metal.

Ex: Electroplating of Au (gold) over copper object.

The copper object to be plated is first treated with dilute acid.

Then this copper object and gold rod are immersed in an electrolyte. Copper object acts cathode and gold rod is an anode.

These electrodes are connected with direct power supply.

When current is passed from battery through the solution, gold dissolves in electrolyte and deposits uniformly in copper object.

12. Explain the Electroless plating of Nickel.

Metal ions + Reducing Agent  Metal (Deposited) + Oxidised Product

Definition: it is a technique of depositing of a noble metal from its salt solution on a catalytically active surface of a base metal by using a suitable reducing agent.

Electroless Nickel plating:

Step 1: Pretreatment and activation of the surface:

The surface to be plated is first degreased by using organic solvents or alkali followed by acid treatment.

Step 2: preparation of plating bath composition:

The pretreated object is immersed in the plating bath for the required time during which the following reduction reaction will occur and the Ni gets coated over the object.

GLOSSARY

CELL: Cell _is an assembly of two electrodes and an electrolyte. It consists of two half cells. Each half cell contains an electrode material in touch with electrolyte.

CURRENT: Current is flow of electricity through a conductor. It is measured in ampere.

ELECTRODE: Electrode is a material or a metallic rod/bar/strip which conducts electrons.

ANODE: Anode is an electrode at which oxidation occurs.

CATHODE: Cathode is an electrode at which reduction occurs.

ELECTROLYTIC CELLS: Electrolytic cells are cells in which electrical energy is converted into chemical energy.

ELECTOCHEMICAL CELLS: Electrochemical cells are cells in which chemical energy is converted into electrical energy.

SALT BRIDGE: Salt bridge consists of a U – tube containing a gel saturated with KCL or NH₄ NO3 in agar – agar. It consists the two half cells of the galvanic cells by the electron movement through it.

OXIDATION POTENTIAL: Electrode lose electrons REDUCTION POTENTIAL: Electrode gain electrons.

SINGLE ELECTRODE POTENTIAL: Measure of the tendency of a metallic electrode to lose or gain electrons, when it is in contact with a solution of its own salt.

STANDARD ELECTODE POTENTIAL: Measure of the tendency of a metallic electrode to lose or gain electrons, when it is in contact with a solution of its own salt of a 1 molar concentration at 25⁰C.

ION – SELECTIVE ELECTRODES: These are electrodes which have the ability to respond only to a particular ion and develop potential, ignoring the other ions in a mixture. The potential developed by an ion – selective electrodes depends only on the concentration of particular ions.

ELECTROCHEMICAL SERIES: An increasing order of the standard reduction potentials CORROSION: Corrosion is defined as the gradual destruction of metals by the chemical or electrochemical reaction with its environment.

PILLING BEDWORTH RATIO; The ratio of the volume of the oxide formed to the volume of the metal consumed is called ”Pilling – Bed worth ratio” or “Pilling – Bed worth rule”.

PAINT: Paint is a uniform dispersion of finely dived pigments, filters and driers in a liquid called medium (thinner + vehicle ).When a paint applied to a metal surface , the thinner evaporates, while the vehicle undergoes slow oxidation forming a pigmented film.