CAT 1

Water Technology: Hard and soft water, Units of Hardness (numerical problems). Disadvantages of hard water: Scale and sludge, caustic embrittlement, priming and foaming,corrosion. Estimation of hardness: EDTA, alkali titration method (numerical problems). Softening methods: Lime soda (numerical problems), zeolite, ion exchange, mixed bed deionizer, treatment of municipal water. Desalination:Desalination of sea water, brakish water, electrodialysis, reverse osmosis.

Corrosion: Dry Corrosion – Oxidation, corrosion by other gases, Liquid metal corrosionWet Corrosion: Hydrogen evolution corrosion, Oxygen absorption corrosionGalvanic Corrosion, Concentration Corrosion

CorrosionCorrosion is a process of deterioration (destructive) and unintentional degradation of a material (loss of solid material)caused by its environment (chemical or elctrochemical)

Corrosion is a process “reverse extraction of metals”

Reversion of base metal to its oxide formBase metal is unstable with respect to oxidation

Iron Ore

Rust

Iron MetalCorrosion

MetallurgialOperation

Theories of Corrosion and Mechanism of Corrosion

1. Chemical or Dry corrosionThis type of corrosion occurs mainly through the direct chemical action of environmental or atmospheric gases such as oxygen, halogen, H2S, SO2, N2 and anhydrous inorganic liquid with metal surfaces in immediate proximity

2. Electrochemical or Wet corrosionThis type corrosion occurs

i) where conducting liquid in contact with metal

ii) when two dissimillar metal or alloy are either immersed or

dipped partially in a solution

Chemical or Dry corrosion

1. Oxidation Corrosion2. Corrosion by other gases3. Liquid Metal corrosion

Oxidation Corrosion

Dry corrosion or oxidation occurs when oxygen in the air reacts

with metal without the presence of a liquid.

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

The basic reaction involved in dry corrosion is:

M M2+ + 2e-

1/2O2 + 2e- O2-

1/2O2 + M M2+ + O2- (MO)

Oxidation corrosion Mechanism

Oxidation first occurs at the surface of the metal resulting the formation of M2+.

Oxygen changes to its ionic form (O2-) and reacts with metal ion to form metallic oxide. This form as a film or scale on surface of the metal.Layer is called film when thickness is <300A and layer exceeds this thickness called scale

Nature of the film decides further action

Corrosion stops if the metallic oxide film produced is barrier for oxygen to penetratefor further oxidation

On the other hand, if the oxide film is porous or volatile, further corrosion occurs by the diffusion or direct contact of oxygen in its ionic form with metal ion to produceadditional oxide film

Metallic oxide filmFilm can be 1. Stable 2. Unsatble 3. Porous 4. Volatile

Stable: Film is fine-grain of oxide which forms a compact surface adhered tightly to the metal surface. Oxides of Al, Sn, Cu, are this type.Oxide layer produced on the surface of this metal are impervious in nature andStops oxygen to attack through diffusion

Unstable: This type oxide layer are mainly produced on the surface of the noble metals which decomposes reversibly to the metal and the oxide is liberated in the form of oxygenEg. Oxides of platinum, silverPorous : This type oxides have minute pores. Hence oxygen can diffuse towards fresh metal present under the oxide film to continue the corrosionMetal like Fe fall under this category

Volatile: Volatile oxides layer volatize as soon as they are formed theory by leading Further corrosion directlyOxides of Mo belongs to this type

Pilling-Bedworth Rule

This rule sates that:

Oxide layer is compact, non-porous as well as protective if the volume of metallic oxide is equal to or greater in volume to metal surface preventing corrosion.Metals like Al forms forms oxide whose volume is greater than the volume of the metal. Film : Non-porous: rate of oxidation rapidly decreases to zero

On the other hand volume of the metallic oxide is less than the volume of metal, then the oxide file is porous and hence film is non protective. Because it can not block oxygen diffuse towards fresh metal surface

Alkali metals like Li, K, Na, the alkaline earth metals like Mg, and metals like FeProduce oxide films whose volume is less than the volume of metal. As a result, Oxygen can diffuse through the pores of the film and corrosion is continuous process

Corrosion by other Gases

Corrosive effect depends on the chemical affinity between gases and metal and the gas involved

Degree of attack depends upon the formation protective or non protective film on the surface of the metal

If the film is non-porous and protective, attack by gas is lessEg. AgCl film is from Cl2 attack on Ag metal

If the film is porous and non protective, surface of the whole metal will be destroyed Eg. Dry Cl2 gas attack on Sn metal

Hydrogen Corrosion or Hydrogen Embrittlement

Dry corrosion attack by hydrogen gas on metal is known as Hydrogen Embrittlement.

This is due to the the formation of atomic hydrogen as a result of chemical or electrochemical reaction occuring at the metal surface under a specific environment

For ex. Aqueous solution of H2S in the diffused caused evolution of atomic hydrogen at the Iron surfaces

Evolved atomic hydrogen readily in to the metal and collects in the voids where it recombines to give molecular hydrogen. This increases the pressure of hydrogen in VOIDS and produces blisters and fissures.

This causes lowering of strength and ductility of the metal . The corrosion attack by atomic hydrogen is also possible at high temperature under specific environment

H 2S + Fe FeS + 2 H

H 2 + H H 2

H 2H eat

H + H

Liquid Metal Corrosion

Liquid metal corrosion is due to chemical action of flowing liquid metal at hightemperature on solid metal or alloy

This type corrosion occurs in devices used in nuclear power

This type corrosion involves either i) Dissolution of solid metal in liquid metal ii) or internal penetration of liquid metal into the solid metal

Both of these are weakening the solid metal

Theories of Corrosion and Mechanism of Corrosion

2. Electrochemical or Wet corrosionThis type corrosion occurs

i) where conducting liquid in contact with metal

ii) when two dissimillar metal or alloy are either immersed or

dipped partially in a solution

Electrochemical Corrosion

This occurs due do the existence of separate anode and cathode area or parts, between which current flows through the conducting solution

At anode oxidation takes place (loosing electron), Hence anode metal destroyed by either dissolving in conducting solution or forming combined oxide film

Electrochemical CorrosionAt anode oxidation takes place (loosing electron), Hence anode metal destroyed by either dissolving in conducting solution or forming combined oxide film

Metal Mn+ + ne-

Mn+ Dissolves in solution

MO (Metal oxide)

Electrochemical CorrosionAt Cathode reduction takes place (gaining electron). Cathode metal is already reduced form. It can not be further reduced,Hence dissolved constituents in the conducting medium accepts the electron at cathodic site to form some ions like OH- and O2-

Metalic Ions an anodic part and non metalic ions at cathodic part diffuse towards each other through conducting medium to forma corrosion product some where between anode and cathode

Metal Mn+ + ne-Anode

Cathode

ne-

Flows towards cathode

Medium½ O2 + H2O+ ne- n OH-

Electrochemical CorrosionThis involves

1. Formation of anodic and cathodic part and in contacwith each other

2. The presence of conducting medium

3. Corrosion of anodic area only

4. Corrosion product somewhere between anodicand cathodic areas

Electrochemical CorrosionAt anode oxidation takes place (loosing electron), Hence anode

metal destroyed by either dissolving in conducting solution or forming combined oxide film

At Cathode site electrons are consumed by dissolved constituents in the conducting medium.

Electron consumption at cathodic site takes place either by

a) Evolution of hydrogen or

a) Absorption of Oxygen depending on the nature of the corrosion environment

Evolution of hydrogen type corrosionThis type corrosion occurs usually in acidic(H+) environment

Consider metal like Iron (Fe), at anodic reaction is dissolution of iron as ferrous ions with liberation of two electrons

Fe Fe2+ + 2e-

These electrons flow through the metal from anode to cathode where H+ ions (of acidic solution) are eliminated as hydrogen gas.

2H+ + 2e- H2

Fe + 2H+ Fe2+ +H2

Evolution of hydrogen type corrosion

This type corrosion causes displacement of hydrogen ions from the acid solution by metal ionsAll metals above hydrogen electrochemical series have a tendency to get dissolved in acid solution with evolution of hydrogen gasAnodes are very large areas where as cathodes are small area

Absorption of Oxygen type corrosionRusting if Iron in neutral aqueous solution of electrolytes like NaClsolution in the presence of atmospheric Oxygen is a common example of this type of corrosion

Surface of Iron is usually coated with at thin film of Iron Oxide. However if this thin film develops some cracks, Anode areas are created on the surface, while rest of the metal part acts as cathode. Small area of anode and large area of cathode are created

At anode Iron dissolves as Ferrous ion with liberation of electrons

Fe

Fe2+ + 2e-

Liberated electron flows from anode to cathode through iron metal , whereElectrons are intercepted by dissolved oxygen as½ O2 + H2O+ 2e- 2 OH-

Absorption of Oxygen type corrosion

Fe2+ ions at anode and OH- ions at cathode diffuse and when they meet Ferrous hydroxide is precipitated

Fe2+ + 2OH- Fe(OH)2

If enough O2 is present, Fe(OH)2 is easily oxidized to give Fe(OH)3

4 Fe(OH)2 + 2H2O + O2 4 Fe(OH)3

This product is called yellow Rust Fe2O3.H2O

If the supply of oxygen is limited, corrosion product may be even black anhydrous magnetite Fe3O4

Fe2+ forms at anode and OH- forms at cathode; Combination takes place neaCathode: Smaller Fe2+ diffuse faster than larger OH- .

Corrosion occurs at anode but rust deposited at or near the cathode

Absorption of Oxygen type corrosion

Increase in oxygen content

Forces that cathodic reactions to produce more OH-

It removes more electron at anode and accelerates the corrosion at the anode

Both of the above effect produce more reactants for rust formation

Hence Presence of Oxygen in more,Accelerates corrosion and rust formation

Galvanic (or Bi-metallic) corrosionWhen two dissimilar metals (Zn and Cu) are electrically connected and exposed to an electrolyte, the metal higher electrochemical series (more negative electrode potential) undergoes corrosionThis type is Galvanic corrosion.

Zn is in higher electrochemical series where as Copper is on lower in electrochemical series

Hence Zn forms anode and is attacked and gets dissolvedCu acts as cathode

In acidic solutions, corrosion occurs by the hydrogen evolution typeWhile neutral or slightly alkaline solution, oxygen absorption occurs

Electron current flows from the anodic metal Zinc to the cathodic metal CopperCorrosion occurs at anode while cathodic part is protected from the attack

Zn Zn2+ + 2 e-

Copper fitting on steel pipe carrying water:Steel pipe acts as anode, Copper acts as cathode.Pipe is direct contact with fittings andwater is conducting electrolyte for ion migration

Copper fitting on steel pipe carrying water:

Galvanic Corrosion

Galvanic Corrosion

1. Steel crews in a brass marine hardware2. Lead-antimony solder around copper wire3. A steel propeller shaft in bronze bearing4. Steel pipe connected to copper plumbing

This type corrosion can be avoided by

1. coupling metal close to the electrochemical or Galvanic series2. Insulating material between two metals can be fixed3. Making cathode metal smaller and anode metal larger in area

Galvanic Series vs Electrochemical series

Galvanic Series Electrochemical seriesLi Li+ + e- - 3.05

Lithium

Titanium

Galvanic Series vs Electrochemical series

In the electrochemical series (reduction electrode potentialarranged down in an decreasing order), a metal low in theseries (more negative value) is more anodic and undergoes faster corrosion

Li corrodes faster than MgZn corrodes faster than FeFe corrodes faster than SnCu corrodes faster than AgHowever there are exceptions to thisTi is less reactive than AgZn-Al couple, Zn is corrodedThis is because, Ti and Al form strong adhering oxide layer on their surfaceIt is clear that electrochemical series does not account for all metals and alloys

Galvanic Series vs Electrochemical series

Hence more practical series called Galvanic series have been prepared by studying the corrosion of metals and alloys in a given environment like sea-water

Galvanic series give real and useful information for studying the corrosion of metals and alloys

Concentration Cell Corrosion