Module-2 Corrosion and Metal Finishing. Corrosion

Tuesday, May 05, 2015 1

This notes was prepared by, M.PRAVEEN, MSc., M.Tech.,(Ph.D)., according to VTU norms new syllabus -2015 For any doubts contact: [email protected]

Module-2

Corrosion and Metal Finishing.

Corrosion

Definition of corrosion:-

Corrosion can be defined as “The destruction (or) deterioration (or) loss of metals by the surrounding

environment through chemical (or) electrochemical reactions”.

Ex: - 1. Rusting of Iron: - A reddish brown scale formation on iron objects.

2. Green scales formed on copper vessels.

Corrosion may occur either in a dry environment (or) in an aqueous medium. The former is called dry corrosion

and the later is called wet corrosion.

Dry corrosion [chemical corrosion]

Dry corrosion involves the direct attack of metals by dry gases mainly through chemical reactions.

It occurs by direct attacks of atmospheoric gasses such as oxygen, hydrogen, sulphide, halogens and sulphur-

dioxide on the metal forming oxide layer.

Ex: - the attack of dry air (or) oxygen on a metal to form an oxide layer over the surface.

Wet corrosion [Electrochemical corrosion]

Wet corrosion involves reactions in aqueous solution medium. The conducting surface of the metal undergoes an

electrochemical reaction with the moisture and oxygen present in the atmosphere.

Ex: - Rusting of Iron.

Wet corrosion is explained on the basis of Electrochemical theory.

Electrochemical theory of corrosion:-

According to electrochemical theory, corrosion of metals takes place due to the formation of anodic and cathodic

regions on the same metal surface in the presence of a conducting medium.

OH- OH- Fe2+ Fe2+

Iron Metal

O2

H2O

Electrons

Cathodic region Anodic region

mailto:[email protected]



At the anodic region oxidation reaction takes place and the metal gets converted into its ions, liberating electrons.

Consequently, metal undergoes corrosion at the anodic region.

At the cathodic region, reduction reaction takes place. Since the metal cannot be reduced further, metal atoms at

the cathodic region are unaffected by the cathodic reaction.

At Anode:-

At anode ox ideation takes place in which the metal atoms are converted into their ions liberating electrons.

M Mn+ + e- (or) Fe Fe2+ + 2e-

At Cathode:-

The cathodic reaction is based on nature of the environment.

a). In the absence of oxygen.

i) In acidic medium, liberation of hydrogen takes place.

2H+ + 2e- H2

ii) In neutral (or) alkaline medium, Hydroxide ions are formed with simultaneous liberation of hydrogen.

2H2O + 2e- 2OH- + H2

b). In presence of oxygen:-

i). In acidic medium, absorption of oxygen takes place.

4H+ + O2 + 4e- 2H2O

ii). In neutral (or) alkaline medium, hydroxide ions are formed.

2H2O + O2 +4e- 4OH-

Then Iron ions (Fe2+) form anode combine with hydroxyl ions from cathode to form iron hydroxide on the

surface, b/w the anode and cathode areas.

Fe2+ + 2OH- Fe (OH) 2

Iron hydroxide further reacts with oxygen and water forming hydrated iron oxide which is the corrosion product

[Rust].

4Fe (OH) 2 + O2 + 2H2O 2[Fe2O3.3H2O]

Rust

Galvanic series:-

The Galvanic series has been prepared by studying the corrosion off metals and alloys in a given

environment.

In galvanic series chemically more active metals are placed at the top than less reactive ones.

The metal higher up the series corrodes faster than the metals in the lower of the series.

The greater the difference in their positions in the galvanic series faster will be the rate of corrosion.

Metals close to each other in galvanic series show lesser tendency to corrode in contact with each other.




Mg

Ba

se Meta

l

Mg alloys

ZN

Al

Cd

Al alloys

Mild steel

Cast steel

Pb

Sn

Brass

Cu

Ni

Stainless steel (18% Cr & 8% Ni)

Ag No

bel

Meta

l

Ti

Au

PT

Types of Corrosion:-

1. Differential Metal Corrosion:-

Differential metal corrosion arises when two dissimilar metals are in contact with one another and exposed to a

corrosive conducting medium, the metal higher up the electrochemical series behaves as anode and suffers from the

corrosion, where as the metal lower in the electrochemical series behaves as cathode and protected from corrosion. This

type of corrosion is known as galvanic corrosion (or) differential metal corrosion.

The rate of corrosion depends mainly on the difference in potential b/w two metals. Higher the difference faster is

the rate of corrosion.

The reactions may be represented as,

At anode: - M Mn+ + e-

At cathode:- reduction reaction takes place, depending upon the nature of the environment.

2H+ + 2e- H2 (or)

2H2O + O2 +4e- 4OH-

Ex:-i). When iron is in contact with copper, iron becomes anodic and undergoes corrosion whereas copper becomes

cathodic and remains unaffected.

ii). Tin coating copper vessel.

iii). Steel pipe connected to copper plumbing.

Fe metal Cu Metal

Anode Cathode




2. Differential Aeration Corrosion:-

When a metal is exposed to different concentrations of air (or) oxygen, part of the metal exposed to lower

concentration of oxygen becomes anodic and undergoes corrosion. Whereas, other part of the metal exposed to higher

concentration of oxygen becomes cathodic and remains unaffected. This kind of corrosion is called as differential aeration

corrosion.

Ex:- Iron rod partially immersed in Nacl solution, part of the metal immersed in solution is exposed to lower concentration

of oxygen becomes anodic and undergoes corrosion. Whereas part of the metal

outside Nacl solution, is exposed to more oxygen becomes cathodic and remains

unaffected.

The anodic and cathodic reactions are

At Anode:- Fe Fe2+ + 2e-

At Cathode:- 2H2O + O2 +4e- 4OH-

Fe2+ + 2OH- Fe (OH) 2

Typical examples of differential aeration corrosion are the Water-line corrosion & Pitting corrosion.

a). Water-line Corrosion:- It is observed in steel (or) Iron water tanks partially filled with water.

Metal just below the water line which is exposed to lower concentration of oxygen becomes anodic and undergoes

corrosion, Part of the tank just above the water line which is exposed to higher concentration of oxygen, becomes cathodic

and remains unaffected.

The cell reactions are

At Anode: - M Mn+ +ne-

At Cathode: - 2H2O + O2 +4e- 4OH-

Finally, the ions combine to form corrosion product.

b). Pitting Corrosion:-

This type of corrosion is observed when dust particles (or) oil drops deposited over the metal surface.

Pitting corrosion is due to

i). Metal surface is not homogeneous. ii). External environment is not homogeneous.

iii). Crystallography directions are not equal in the reactivity.

Less O2, (Anode)

Nacl

solution

More O2,

(Cathode)

Iron

Rust

Water

More

oxygen,

(Cathode)

Less Oxygen

(Anode)




iv). Environments are not uniform with respect to concentration.

Pitting corrosion results when the portion of the metal covered by dust which is less aerated becomes anodic and

undergoes corrosion and form pit. The adjacent area of the metal which is exposed to higher concentration of oxygen

becomes cathodic and remains unaffected.

Once a small pit is formed the rate of corrosion increases because formation of small anodic area [pit] to corrode

faster because accepting electrons from the cathodic area increases.

The cell reactions are

At Anode: - Fe Fe2+ + 2e-

At Cathode: - 1

2O2 + H2O + 2e- 2OH-

Net reaction: - Fe2+ + 2OH- Fe (OH) 2 Fe (OH) 3

Stress Corrosion:-

Stress corrosion occurs only in the presence of specific corrosive environment and due to stress portion of the

metal.

It occurs when stressed region of the metal are expose to corrosive environment. The stress region act as anode

and undergoes corrosion, the unstressed region act as cathode and is unaffected.

Ex: - Stressed region of mild steel undergoes stress corrosion in the presence of NaOH solution.

Caustic embrittlement:-

Mild steel boilers undergo corrosion at the stressed portion when the operating pressures are b/w 10atm, and

20atm, at high temperature in the presence of alkaline medium called caustic embrittlement.




The boiler water usually contains a certain amount of sodium carbonate which is added to water during softening

process. At high temperature and pressure, the carbonate undergoes hydrolysis to give sodium hydroxide and carbon-di-

oxide and make the boiler water alkaline.

Na2CO3 + H2O 2NaOH +CO2

Fine hair line cracks may develop at the stressed portion of the boiler. Boiler water containing alkaline impurities

pass into hair-line cracks and crevices by capillary action. This water evaporates and leaves behind caustic soda (NaOH)

in the cracks, whose concentration increase to 10%, thus a galvanic cell is set up b/w the iron under stress and the iron in

the main body.

Iron under stress acts as the anode and gets corroded resulting in boiler failure. The NaOH reacts with iron and

forms Na2FeO2 [sodium ferrate] which decomposes to give Fe3O4 and NaOH.

2NaOH + Fe Na2FeO2 +H2

3Na2FeO2 + 4H2O Fe3O4 +4H2 +6NaOH

NaOH thus formed, further reacts with iron to cause corrosion this leads to the explosion of corrosion.

Factors affecting the rate of corrosion:-

Several factors affect the rate of corrosion of metals. These are broadly classified as primary factors which are due

to the metal and secondary factors which are due to the environment.

Primary Factors:-

1. Nature of Metal:-

The position of the metal in the galvanic series decides the rate of corrosion and extent of corrosion.

The metals with lower e-de potential values are more reactive and more susceptible for corrosion than the metal

with higher e-de potential values.

The rate of the corrosion depends upon the difference in the position of the metals in the galvanic series, higher

the difference, faster id the corrosion at anode.

Ex: - Li corrodes faster than mg.

Zn corrodes faster than Fe, Cu corrodes faster than Ag.

2.Nature of the Corrosion Product:-

The product of the corrosion is usually the oxides of the metals. The nature of oxide layer determines the

corrosion rate.

If the corrosion product formed is insoluble, stable, uniform, non-porous and non-volatile, it acts as a protective

film and prevents further corrosion of metal because it acts as barrier b/w the fresh metal surface and the corrosive

environment.

Ex: - Meals like Al, Ti, Pt and Cr the oxide film formed acts as protective film hence the corrosion is controlled.

If the corrosion product formed is soluble, unstable, non-uniform, porous and volatile in nature continues the

corrosion process because it acts as non-protective film.

Ex: - In case of metals like Fe, Zn and Mg the oxide film formed acts as non-protective film and corrosion is

continued till the metal is completely destroyed.




3. Anodic and Cathodic Areas: - The rate of corrosion is highly influenced by the relative area of anode and cathode.

If the metal has small anodic area and large cathodic area, all the electrons liberated at anode are consumed at the

cathodic region; therefore the rate of anodic reaction is greater and increase the rate of corrosion.

If the metal has, large anodic area and small cathodic area, demand for electrons will be less and this results in

decrease rate of dissolution of metal at anodic region and decrease rate of corrosion.

The rate of corrosion is directly proportional to the ratio of area of cathode to the area of anode.

Rate of corrosion = Area of cathode

Area of anode

Higher the value of rate of corrosion greater is the rate of corrosion.

Ex: - A broken coating of tin on iron surface, a small

exposed part of iron acts as anode and rest of large

tin coated area act as cathode. Because of small anodic area to

cathodic area the rate of corrosion is high.

4.Polarization of Anodic and Cathodic regions:-

Polarization is defined as” The deviation of the potential of an electrode from the open circuit potential

due to irreversible reactions around the electrode”.

During the corrosion process polarization of anode (or) cathode decreases corrosion rate.

If the anode alone undergoes polarization, the rate of corrosion is said to be controlled by anodic polarization.

It occurs due to accumulation of hydroxyl ions in the vicinity of anodic region, hence rate of corrosion decreases.

If the cathode alone undergoes polarization, the rate of corrosion is said to be controlled by cathodic polarization, it

occurs due to the accumulation of hydroxyl ions in the vicinity of cathodic region, hence rate of corrosion

decreases.

Secondary Factors:-

1.PH:-

Acidic media are generally more corrosive than alkaline (or) neutral media. The PH of the solutions decides the

type of cathodic reaction.

The rate of corrosion increases with decreases in PH

Ex: - At PH greater 10, corrosion of iron practically decreases due to the formation of a protective coating of

hydrous oxides of iron. b/w PH 3, iron undergo rapid corrosion due to continuous evolution of hydrogen at the

cathodic region.

The rate of corrosion decreases with increase in PH

Ex: - Zn suffers from corrosion even in the presence of mild acidic medium, where as corrosion is minimum at

PH=11.

Fe-metal

[anode]

Tin-metal

[[[cathode]

Small anodic area




2. Temperature: - the rate of chemical reaction increases with increase in temperature.

The rate of corrosion increases with increase in temperature.

If the medium is acidic in nature, cathodic reaction involves the evolution of hydrogen gas, as temperature

increases, the diffusion of hydrogen ions towards cathode increases, hence the rate of corrosion increases.

If the medium is alkaline (or) neutral, cathodic reaction involves the absorption of oxygen. As the temperature

increases, the solubility of dissolved gases decreases, hence the rate of corrosion increases.

A Corrosion resistant passive metal becomes active at high temperature and increase the rate of corrosion with

increasing temperature. Ex: - Caustic embrittlement in high pressure boilers.

3.Conductance:-

The rate of corrosion is directly proportional to the conductance medium.

The rate of corrosion increases in the presence of conducting species in the atmosphere because corrosion is in

electrochemical phenomenon.

Ex: - i). The corrosion of metal structures is faster within the clay and mineralized soils than in dry sandy soils.

ii). The rate of corrosion is more in ocean water and less in river water.

Corrosion Control Methods:- Corrosion can be controlled by preventing the formation of galvanic cells on the surface of metals.

The methods to control corrosion can br classified as,

1. Design & Selection of materials.

2. Protective coatings. i). Inorganic coatings. ii). Metal coatings.

3. Cathodic Protection.

4. Anodic protection.

5. Corrosion inhibitors.

1. Design and Selection of Materials:-

Corrosion can be controlled by selecting proper materials which resist the corrosive environment and

designing them suitably during fabrication.

By avoiding contact b/w two dissimilar metals in a corrosive environment and there by different metal corrosion can

be controlled.

The rate of the corrosion can be minimized by maintaining large anodic area of the metal.

The two dissimilar metal selected should be as close as possible in the galvanic series; this will reduce the

differential metal corrosion.

The rate of corrosion can be controlled by avoiding crevices in joints and minimizing sharp edges.




The most common method of preventing corrosion is the selection of proper metal (or) alloy for a particular

application.

Ex: - a). Stainless steels-Nitric acid. Ii). Lead-H2SO4

Selecting pure metals to minimize corrosion.

The component should be free from residual stress.

2. Protecting Coating:-

Corrosion is prevented by the application of protective coating on the surface of metal, there by the

metal surface is isolated from the corrosive environment.

Important types of protective coating are,

a. Metal coating. b. Inorganic coating.

a. Metal coating:-

“The process of coating base metal with a layer of protective metal is known as metal coating”.

There are two types of metal coating,

1. Anodic coating. 2. Cathodic coating.

1. Anodic coating;-

“The coating metal is anodic to base metal is called anodic coating”.

Ex: - Galvanization.

Galvanization:- “The process of coating zinc on the base metal [Fe, Steel] surface by hot dipping is known as galvanization”.

The galvanization process is carried out as follows

The metal surface is washed with organic solvent to remove organic matters.

The metal surface is treated with dil.H2SO4 to remove rust and other deposits.

Then it is well washed with water & it is air dried by passing with hot air.

The metal is treated with a mixture of zinc chloride and ammonium chloride solution which prevents the oxidation

of the coated metal.

Then the metal is dipped in molten Zinc at 4500c

Excess of zinc on surface is removed by passing a pair of hot rollers, which removes excess of zinc and produced

thin coating.




Application:- Galvanized materials are used in fencing wire, buckets, bolts, nuts, nails, screw etc.

Note:- Galvanized sheets cannot be used for preparing (or) storing food because zinc dissolves in acidic medium and

forms toxic compounds.

2. Cathodic coating:-

“The coating metal is cathodic to base metal is called cathodic coating”.

Ex: - Tinning.

Tinning:-

“The process of coating a thin film of molten tin on the base metal surface by hot dipping “is called tinning.

The Tinning process is carried out as follows,

The Iron sheet is washed with organic solvent to remove oil (or) grease.

The Iron sheet is passed through dil.H2SO4 to remove rust (or) scale and other deposits.

Then it is well washed with water.

The Iron sheet is treated with a mixture of ZnCl2 + NH4Cl for dried.

It is then passed through a tank contains molten Tin [2190-3100c].

Then the excess of tin is removed by rolling.

The coated tin is passed through palm oil which prevents the oxidation of the coated tin.

Advantages:-

1. Tin is non-toxic in nature and more nobel than the base metal.

2. Tinning gives a film which completely covers the surface and provides a corrosion resistant coating.

b. Inorganic coating:-




These coatings are produced at the surface of the metal by chemical (or) electrochemical reactions.

Ex: - Anodizing and Phosphating.

Anodizing:-

Anodizing coatings are generally produced on non-ferrous metals like Al, Zn, Mg and their alloys by anodic

oxidation processes in which the base metal is made as anode.

Anodizing of Aluminium:-

When aluminium metal is made anodic in an electrolytic bath with H2SO4 (or) chromic acid as the electrolyte, a

thin layer of Al2O3 is formed on the surface. This process is called Anodizing (or) Anodic oxidation of Aluminium.

It is carried out to produce a porous (or) non-porous coating.

The porous coating is obtained by anodic oxidation; it is carried out by making it anode in an electrolytic bath

containing a suitable acid like chromic acid (or) H2SO4 at 350-400c. A plate of lead (or) stainless steel is made the cathode.

When current of moderate density is passed, the O 2 liberated at the anode combines with it to form oxide which takes the

form of thick film Al2O3 deposits on the surface of the object.

Over all reaction:- 2Al + 3H2O Al2O3 + 3H2

The coating is slightly porous and it is sealed by

dipping in hot water. Al2O3 gets hydrated to Al2O3.H2O

which occupies more volume and

hence protects the metal from corrosion.

The non-porous coatings are produced by using non-corrosive electrolytes like boric acid and borax. These

coatings are applied on electrolytic condensers.

Phosphating:-

Phosphating is the process of depositing a coating of metal phosphate on metal surface by chemical (or)

electrochemical reactions.

The Phosphating bath contains three essential components,

1. Free phosphoric acid.

2. A primary metal phosphate like Fe, Mn, and Zn-phosphates etc.

3. An accelerator such as nitrates, chlorates, H2O2 etc..

The PH of bath is maintained in the range of 1.8-3.2, the temperature is maintained at 350c. The reaction involves

dissolution of metal as ions, resulting in the formation of surface film consisting of crystalline Zn-Fe (or) Mn-Fe

phosphates.




Over all reaction: - 3M + 2H3PO43- M3 (PO4)2 + 3H2

Ex:- coating of mixture of iron & zinc phosphate on steel fabrications such as refrigerators and car bodies.

Cathodic Protection:-

Cathodic protection is the technique of offering protection to a specimen against corrosion by providing electrons

from an external source.

There are two important methods of cathodic protection.

1. Sacrificial Anode Method.

2. Impressed Current /Voltage Method.

1. Sacrificial Anode Method:- Sacrificial anode method involves the use of a metal which is anodic to the specimen.

In this method the metallic structure to be protected is connected to a more anodic metal using a metallic wire.

More active metal gets corroded, while the parent metallic structure is protected from corrosion. Since the more active

metal (or) anodic metal are sacrificed and hence it is known as Sacrificial Anode method.

The commonly used sacrificial anodes are Mg, Al, Zn, and their alloys.

Ex: - 1. Protection of ship’s hull by fixing Zinc plates.

2. Protection of an underground pipeline with a magnesium anode.




Impressed Current Method:-

In Impressed current method the electrons are supplied from a source of direct current [Battery].

In this method, the article to be protected is made as cathode by connecting it to a –ve terminal of the DC

supply and an auxiliary e-de made of nobel metal (or) graphite is connected to the +ve terminal of the Dc supply, acts as

anode. The anode is buried in the soil, the anode is usually surrounded by backfill consisting of gypsum (or) coke breeze,

which improves electric contact b/w anode and surrounding soil. Current passes to the article & corrosion is suppressed.

Advantages:-

1. Requires low installation cost and minimum maintenance cost after installation.

2. Method protects large area of the base metal.

Tank

[Cathode]

Metallic Wire

Iron Pipe (Cathode)

Magnesium

(Anode)




It is used for protecting water storage tanks and oil pipe lines.


Date post:	12-Feb-2017
Category:	Documents
Upload:	ledang
View:	226 times
Download:	0 times

Module-2 Corrosion and Metal Finishing. Corrosion

Documents