corossion1

05/01/2023CORROSION AND ITS CONTROL

1

CORROSIONAyushi Sanjay Sharma

Impossible to Eliminate


2 Outline

Importance Definition Chemistry behind corrosion Types/Forms of corrosion Factors Influencing Corrosion Prevention and control methods


3 Importance of Corrosion control

India’s cost of Corrosion 5% of total GDPA ~ 2 lakh Crore

Direct Cost Indirect Cost Expensive Materials Over Design Cost of

Repair/Replacement

Cost of Labour Loss of productivity Cost of equipments

used for related activity

ETC…


4 Definition

Any process of deterioration (or destruction) and consequent loss of a solid metallic material, through an unwanted (or unintentional) chemical or electrochemical attack by its environment, starting at its surface is called corrosion.(Engineering Chemistry by Jain-Jain)

Electrochemical degradation of metal as a result of reaction with environment

Tendency of a metal to revert back to its native state


5 Native State of Metal

For Example : Fe (Iron) Ores : Hematite, Magnetite

Courtesy : YouTube

Blast FurnaceMetal Ore CaCO3 Coke

Hot air

C + O2 CO2

CaCO3 CaO + CO2

CO2 + C 2CO Fe2O3+ 3CO Fe + 3CO2

2C+O2+CaCO3 4CO2 + CaOCO + Fe2O3 +Fe


6 Basic Chemistry behind Corrosion

What is Cathode? What is Anode? What is Electrolyte? What is RedOx?

Reduction Reaction Oxidation reaction


7 Basic Chemistry behind Corrosion

Reaction at Zn Surface: Zn Zn2+ + 2e-

Reaction at Cu Surface: Cu2+ + 2e- Cu

Reaction in ZnSO4: Zn2+ + SO4

2- ZnSO4

Reaction in CuSO4: CuSO4 Cu2+ + SO4

2-

Anodic Reaction/Oxidation Reaction

Cathodic Reaction/Reduction Reaction

Reactions in Electrolyte


8 Understanding Corrosion

HCl

Zn

Zn

2e-

Zn2+

HCl

Cl-

H+

ZnCl2

H2

Anodic Reaction:Zn Zn2+ + 2e-

Cathodic Reaction:2H+ + 2e- H2


9 Types of Corrosion

Dry or Chemical Corrosion Oxidation Corrosion Corrosion by other gases

Wet or Electrochemical Corrosion Galvanic (Bimetallic) Corrosion Concentration Cell Corrosion Pitting corrosion Stress Corrosion


10 Dry Chemical CorrosionOxidation Corrosion

Direct action of Oxygen in absence/negligible moisture. Reaction:

2M 2Mn+ +2ne- (Loss of Electrons) O2 + 2ne- nO2- (Gain of electrons) 2M + O2 2Mn+ + nO2-

Metal OxideFor n=12M 2M+ +2e- (Loss of Electrons) O2 + 2e- O2- (Gain of electrons)M + O2 MO


11 Mechanism of Oxidation CorrosionEngineering Chem

istry by Jain-Jain

The MO film can be :1. Stable2. Unstable3. Porous


12 Dry Chemical CorrosionCorrosion by Other Gases

Corrosive effect depends on “chemical affinity between metal and gas” The Film Formed may be:

Porous (Non-Protective) Non-Porous (Protective)


13 Wet or Electrochemical Corrosion

CathodeAnode

Ionic Conduction

Electron Conduction Electrochemical corrosion involves Anodic and Cathodic areas or parts

in contact with each other Presence of conducting medium Corrosion of Anodic areas Formation of corrosion product

between anodic and Cathodic areas


14 Mechanism of Electrochemical Corrosion At Anode:

Metal dissolution resulting in Metallic ions and free electron liberation At Cathode:

Electron Consumption Occurs by: Evolution of Hydrogen

OR Absorption of oxygen


15 Corrosion by Evolution of Hydrogen(Small Cathode-Large Anode)

Usually occurs in acidic environment

Anodic Reaction : Fe

Fe2+ + 2e-

Cathodic Reaction: 2H+ +2e-

H2 (H+ from acidic solution)

Engineering Chemistry by Jain-

Jain


16 Corrosion by Absorption of Oxygen(Small Anode- Large Cathode)

Anodic reaction :Fe Fe2+ + 2e-

Cathodic Reaction: O2+H2O + 2e- 2OH-

Over all reaction:Fe2+ + 2OH- Fe(OH)2

If enough oxygen is present:4Fe(OH)2 + O2 + 2H2O4Fe(OH)3

If Limited Oxygen Present, Fe3O4 (anhy. Magnetite) is formed

Engineering Chemistry by Jain-

Jain


17 Difference Chemical Corrosion Electrochemical corrosion

Occurs in dry condition by attack of gas or liquid

Occurs in presence of moisture or electrolyte

Corrosion product accumulate at the site

Corrosion occur at anode and product formed elsewhere

Self-controlled corrosion Continuous processNo path of electron flow required Path of Electron flow requiredSame sites for Oxidation and reduction

Oxidation and reduction occurs at different place i.e. Separate Anode and Cathode

Product may be Unstable, volatile or porous

Products are normally the native compounds


18 Galvanic (Or Bimetallic Corrosion)

Metal with higher Electrochemical series undergoes corrosion


19 Electrochemical Series Potassium Sodium Lithium Calcium Magnesium Aluminum Zinc Iron Tin Lead Copper Mercury Gold

React with water

React with Acid React with

Oxygen

Very Reactive

Very UnReactive


20 Concentration Cell Corrosion

HCl

Zn

Zn

2e-

Zn2+

HCl

Cl-

H+

ZnCl2

H2

Anodic Reaction:Zn Zn2+ + 2e-

Cathodic Reaction:2H+ + 2e- H2 O2+H2O + 2e- 2OH-

Differential aeration corrosion


21 Pitting Corrosion

Localized acceleration attach, resulting in the formation of cavities Can result by breakdown or cracking of protective film on metal Localized acceleration attach, resulting

in the formation of cavities


22 Stress Corrosion

This type of corrosion has two requisites: Presence of Tensile stress Specific Corrosive environment


23 Factors influencing corrosion

Nature Of Metal: Position in Galvanic Series Relative area of Anode and

Cathode Purity of metal Physical state of metal Nature of Surface film Solubility of corrosion

products Volatility of corrosion products

Nature Of Corroding Environment: Temperature Humidity of air Impurities in Environment Suspended particulates in

Environment Influence of pH


24 Prevention and Control Methods

Cathodic Protection Sacrificial anodic protection method Impressed current Cathodic protection

Application of Protective coatings Anodic coatings Cathodic coatings

Use of Inhibitors Anodic inhibitor Cathodic Inhibitor


25 Cathodic Protection

Forces the metal to be protected by behaving as cathode. Sacrificial anodic method:

Metallic structure connected to wire of more anodic metal “Sacrificial anode” Impressed current Cathodic protection:

Corrosion current nullified by impressing current in opposite direction. Corroding metal turns from anode to cathode.


26 Application of Protective coatings

Surface can be protected by applying coatings of behavior as follows: Chemically inert under particular conditions of temperature and pressure Prevent penetration of Environment to material

Anodic Coating: Coating which are anodic to the base metal are applied (Galvanic cell)

Cathodic Coating: Coating more noble metal than the base metal


27 Methods of applying metal coating

Galvanizing Tinning


28 Use of inhibitors

To decrease the rate of corrosion Anodic Inhibitors

They Adsorb on metal surface forming a thin protective film reducing corrosion Depletion of film may leas to high rate corrosion

Cathodic Inhibitors By slowing down diffusion of Hydrated H+ ion By eliminating Oxygen from Cathodic area and from the corroding medium


29 Thank you…

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