Science of corrosion

SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg.Chemistry

UNIT –II
INTRODUCTION

Metals have a natural tendency to revert back to combined states. During this process mostly, oxides are
formed though in some cases sulphides, carbonates, subparts etc. may result due to presence of impurities. Any
process of deterioration and loss of solid metallic material by chemical or electrochemical attack by its

SCIENCE OF environment is called corrosion. Corrosion is the reverse process of metallurgy.

Metal
Corrosion(Oxidation)

Metallic Compound + Energy
Metallurgy(Reduction)

CORROSION Eg: Rusting of iron when exposed to atmospheric conditions. Rust is hydrated oxide (Fe 2O3.xH2O).

CORROSION

It may be defined as “the process of gradual deterioration or disintegration or eating away of metal by a
chemical or electrochemical reaction in its environment ”.

P. V. NARAYANA REDDYM.Sc.,. Ph.D Rusting of Iron based on Electrochemical Theory of corrosion

Eg: Yellow rust, actually corresponds to Fe2O 3. 3H2O on the surface of iron.

Greenish layer formation on the surface of Copper

Theory of corrosion
There are three theories of corrosion: (i) Acid theory, (ii) dry or chemical corrosion and
(iii)Electrochemical or wet corrosion.

(i)Acid theory of corrosion

This theory suggests that corrosion of a metal (iron) is due to the presence of acids surrounding it.
According to this theory, iron is corroded by atmospheric carbon di- oxide, moisture and oxygen. The corrosion
UNIT-II
products are the mixture of Fe(HCO3)2, Fe(OH)CO3 and Fe(OH)3. The chemical reactions suggested are given
Science of Corrosion: Definition, Types of corrosion: Dry Corrosion, (Direct below
Chemical attack), Wet Corrosion, Theories of Corrosion and Mechanism, Electro
Chemical Theory of Corrosion. Galvanic Series, Galvanic Corrosion, Concentration
Cell Corrosion, Oxygen absorption type. Factors Influencing Corrosion. Control of
Corrosion – Cathodic Protection – Sacrificial anode and Impressed Current. Uses of
Inhibitors. Electro Plating, and Electro less plating (copper and nickel) This theory is supported by the analysis of rust that gives the test for CO32- ion. Further, the process of
rusting is reduced by the presence of lime and caustic soda (these two can absorb CO2, thus reducing corrosion).

1 2 Narayana Reddy

SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg.Chemistry SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg.Chemistry

Types Of Corrosion: Two types of Corrosion occur base on the environment e.g., Ag, Au and Pt do not undergo oxidation Corrosion.

1.Dry (or) Chemical Corrosion c) Volatile that is oxide layer volatilizes after formation and as such leaves the underlying metal
surface exposed further attack. This causes continuous which is excessive. e.g. Molybdenum oxide (MoO3)
This type of Corrosion occurs mainly through the direct chemical action of atmospheric gasses such as O2,
d) Porous that is the oxide layer formed having pores or cracks. In this case the atmospheric oxygen
halogens, H2S, SO2, N2 and anhydrous inorganic liquid with metal surfaces in its immediate proximity. Three types
passes through the pores or cracks of the underlying metal surface. This causes continuous corrosion till
of chemical Corrosion are as follows..
complete conversion of metal into its oxide.
Pilling Bedworth Rule: If volume of metal oxide on the surface of a metal is more than or equal to the volume of
(i)Oxidation Corrosion: This is carried out by the direct action of oxygen low or high temperatures on metals
metal, the oxide layer will be protective. For example, for Al2O3, Fe, Ni, ZnW, Cr. It will be non-protective if volume
in absence of moisture at ordinary temperature metals or very slightly attacked. The Exceptions are Alkali metals
of oxide is less than volume of metal. (The specific volume ratio of W is 3.6, Cr = 2.0, Ni = 1.6. Hence, the rate of
and Alkaline earth metals. At high temperature all metals are oxidized. The exception is Ag, Au and Pt.
corrosion is very less in tungsten.) It is called Pilling Bedworth rule.
2M 2Mn++2ne- (De-electronation)
(ii) Corrosion by other gases:Gases such as Cl2, SO2, H2S, NOx : In dry atmosphere, these gases react with metal
(Metal ion)
and form corrosion products which may be protective or non-protective. Dry Cl2 reacts with Ag and forms AgCl
nO2+2ne- 2nO2- (Electronation) which is a protective layer, while SnCl4 is volatile. In petroleum industries at high temperatures, H2S attacks steel
(Oxide ion) forming FeS scale which is porous and in- terferes with normal

2M+ nO 2 2Mn+ + 2nO2- (Metal oxide) operations.
Mechanism: At the surface of metal oxidation occurs and the resulting metal oxide scale forms a barrier which
(iii) Liquid metal corrosion: In several industries, molten metal passes through metallic pipes and causes
restricts further oxidation. For oxidation to continue either the metal must diffused outwards through the scale to
corrosion due to dissolution or due to internal penetration. For example, liquid metal mercury dissolves most
the surface or the oxygen must defuse inwards through the scale to the underlying metal. Both the cases are
metals by forming amalgams, thereby corroding them.
possible. But the outward diffusion of metal is generally more rapid than inward diffusion of oxygen since metal
ion is appreciably smaller than the oxygen ion and hence more mobile. 2.Wet or Electrochemical Corrosion:

Metal + Oxygen Metal oxide It is a common type of corrosion of metal in aqueous corrosive environment. This type of corrosion occurs when
the metal comes in contact with a conducting liquid or when two dissimilar metals are immersed or dipped partly
When oxidation starts, a thin layer of oxide is formed on the metal surface and the nature of this film
in a solution. According to this theory, there is the formation of a galvanic cell on the surface of metals. Some parts
decides further action.
of the metal surface act as anode and rest act as cathode.
If the film is,
The chemical in the environment and humidity acts as an electrolyte. Oxidation of anodic part takes place and it
a) stable, it behaves has a protective coating in nature e.g., the oxide films on Al, Pb, Cu, Pt etc., are stable results in corrosion at anode, while reduction takes place at cathode. The corrosion product is formed on the
and therefore further oxidation correction of prohibited. surface of the metal between anode and cathode.
b) Unstable that is the oxide layer formed decomposes back into metal and oxygen. So, oxidation Corrosion
To understand the wet theory, let us take the example of corrosion of iron. Oxidation of metal takes place at
is not possible.
anode while the reduction process takes place at cathode. By taking rusting of iron as an example, the reaction can
Metal oxide Metal + Oxygen
be explained as that it may occur in two ways:

3 Narayana Reddy 4 Narayana Reddy


(i) evolution of hydrogen type (ii) absorption of oxygen type This type of corrosion causes displacement of hydrogen ions from the solution by metal ions. All metals
above hydrogen in electrochemical series have a tendency to get dissolved in acidic solution with simultaneous
At anode: oxidation occurs. At cathodic part, reduction reaction (electro nation) occurs. It does not affect the
evolution of H2 gas. The anodes are large areas, whereas cathodes are small areas.
cathode, since most metals cannot be further reduced. At cathodic part, the dissolved constituents in the
conducting medium accepts the electrons forming ions (OH-, O2-). The metallic ions formed at anodic part and the Absorption of oxygen: - For example, rusting of iron in neutral aqueous solution of electrolytes in presence of
ions formed at cathodic part diffuse towards each other through conducting medium and form a corrosion product atmospheric oxygen. Usually the surface of iron is coated with a thin film of iron oxide. If the film develops cracks,
somewhere between anode and cathode. anodic areas are created on the surface. While the metal parts act as cathodes. It shows that anodes are small areas,
while the rest metallic part forms large cathodes.
Mechanism:

Electrochemical corrosion involves flow of electrons between anode and cathode. The anodic reaction involves
dissolution of metal liberating free electrons.

M Mn+ + ne-

The cathodic reaction consumes electrons with either evolution of hydrogen or absorption of oxygen which
depends on the nature of corrosive environment.

(i) Evolution of hydrogen: This type of corrosion occurs in acidic medium e.g., considering the metal Fe, anodic
reaction is dissolution of iron as ferrous ions with liberation of electrons.

At anode: Fe Fe2+ + 2e- (Oxidation)
Fe Fe2+ + 2e- ( Oxidation)
At cathode:

The released electrons flow from anode to cathode through iron metal.

½ O2 + H2O + 2e- 2OH-(Reduction)

Fe2+ + 2OH- Fe(OH)2

(a) If oxygen is in excess, ferrous hydroxide is easily oxidized to ferric hydroxide.
4Fe(OH)2 + O 2 + 2H2O 4Fe (OH)3

The product called yellow rust corresponds to Fe2O3. xH2O.
The electrons released flow through the metal from anode to cathode, whereas H+ions of acidic solution are
eliminated as hydrogen gas.
Concentration Cell Corrosion :
2H++2e- H2
This type of corrosion is due to electrochemical attack on the metal surface exposed to an
The overall reaction is
electrolyte of varying concentrations or of varying aeration. The most common type of concentration cell corrosion
Fe+2 H+ Fe2+ + H2 is the differential aeration corrosion which occurs when one part of metal is exposed to different air concentration
from other part. This causes a difference in potential between the differently aerated areas. Experimentally it has


been observed that poor oxygenated parts are anodic. Differential aeration of metal causes a flow of current called particular atmosphere, i.e. sea water. In galvanic series, oxidation potential of metals is arranged in the decreasing
the differential current. order of activity of a series of metals. The series is towards the increasing noble nature.

Differential Aeration Corrosion: It occurs when a metallic surface is partially immersed in an electrolyte and More anodic: Mg, Mg alloys, Zn, Al, Cd, Fe, Pb, Sn, Ni–Mo–Fe alloys) Brasses, Cu, Ni, Cr–steel alloy, Ag,
partially exposed to air as shown in the figure below. Poorly oxygenated metallic part becomes anodic and Ti, Au, Pt towards noble nature.
undergoes oxidation. Well oxygenated part becomes cathodic. At the cathode, O 2 takes up electrons to form OH-
ions. Electrochemical series Galvanic series
If a metal e.g., Zn is partially immersed in a dilute solution of a neutral salt e.g., NaCl and the
solution is not agitated properly, then the parts above and adjacent to the waterline are strongly aerated and hence a. This series consists of metals and non- a. This series consist of metals and alloys.
metals b. Position of pure metal and when present in
become cathodic. Whereas parts immersed show a smaller oxygen concentration and become anodic. so there is a b. The position of a metal in this series is the form of alloy is different.
difference of potential which causes flow of current between two differentially aerated areas of same metal. Zinc permanently fixed. c. It predicts the relative corrosion
c. It predicts the relative displacement tendencies.
will dissolve at anodic areas and oxygen will take up electrons at the cathodic areas forming hydroxyl ions. tendencies. d. Corrosion of metals and alloys is studied in
d. Electrode potentials are measured by unpolluted sea water.
Zn Zn2+ + 2e- (Oxidation) dipping pure metals in their salt
solution of 1M concentration.
½ O2 + H2O + 2e- 2OH- (Reduction)

Following are the facts about differential aeration corrosion:

(a) Less oxygenated part is the anode. Therefore cracks serve as foci for corrosion.
(b) Corrosion is accelerated under accumulation of dirt, scale or other contaminations. This restricts the access
of oxygen resulting an anode to promise greater accumulation. The result is localized corrosion.
(c) Metals exposed to aqueous media corrode under blocks of wood or glass which restricts the access of
oxygen.
GALVANIC SERIES

Electrochemical reactions are predicted by electrochemical series. A metal having higher position can
replace (reduce) other metals that have lower position in the series. For example,

Zn + CuSO 4 ZnSO 4 + Cu

that is, Zn + Cu++ Zn ++ + Cu

Or in other words, zinc will corrode faster than copper.

Some exceptions have been observed in this generalization. For example, Ti is less reactive than Ag.

Galvanic series is the series of metals that is made keeping in view the process of corrosion of a metal in a



Active (Anodic) 1. Mg a) Metal surface are not homogeneous.
2. Mg alloys
3. Zn b) External environment is not homogeneous.
4. Al c) Films are not perfectly uniform.
5. Cd
6. Al alloys d) Crystallography directions are not equal in the reactivity.
7. Mild steel
8. Cast Iron e) Environment is not uniform with respect to concentration.
9. high Ni cast Iron Pitting is usually the result of the breakdown or cracking of the protective film on a metal at specific points. This
10. Pb-Sn solder
11. Pb gives rise to the formation of small anodic and large cathodic areas. In process of correct environment this
12. Sn
13. Iconel produces corrosion current.
14. Ni-Mo-Fe alloys
15. Brass e.g., Stainless steel and aluminum show characteristic pitting on chloride solution. Pitting is caused by the presence
16. Monel
17. Silver solder of sand, dust scale and other extraneous impurities present on the metal surfaces. Because of differential amount of
18. Cu oxygen in contact with the metal, the small part (underneath the impurity) becomes the anodic areas and the
19. Ni
20. Cr stainless steel surrounding large parts become the cathodic areas. Intense corrosion takes place in the anodic areas underneath
21. 18-8 stainless steel
22. 18-8 Mo stainless steel the impurity. Once a small pit is generated, the rate of corrosion will be increased.
23. Ag
24. Ti
25. Graphite
26. Au
Noble(Cathodic) 27. Pt

Table: Galvanic series

GALVANIC CORROSION :

When two dissimilar metals are electrically connected and exposed to an electrolyte, the metal higher in
electrochemical series undergoes corrosion. This type of corrosion is called Galvanic corrosion .e.g., Zinc (higher in
electrochemical series) forms the anode and is attacked and gets dissolved; whereas copper (lower in Factors Influencing Corrosion:
electrochemical series) acts as cathode.
The rate and extent of corrosion depends on
Mechanism: If the solution is acidic then corrosion occurs by hydrogen evolution process and if the solution is
(i) Nature of the metal
neutral or slightly alkaline in nature then corrosion occurs by oxygen absorption process. The electrons flow from
(ii) Nature of corroding environment.
the anodic metal to the cathodic metal.
Nature of metal:
Zn Zn2+ + 2e- (Oxidation)
(a)Position in galvanic series: when 2 metals or alloys are in electrical contact in presence of an electrolyte the more
Thus the corrosion is a localized accelerated attack resulting in the formation of pits, holes or active metal having higher position in the galvanic series undergoes corrosion. The greater is the difference in
cavities. Pitting corrosion therefore results in the formation of pinholes, pits and cavities in the metal. The position, the faster is the corrosion.
pitting corrosion may be due to following reasons:


(b)Over voltage: reduction in overvoltage of the corroding metal accelerates the corrosion rate. E.g. Zn in 1N H2SO4 (c) Impurity of atmosphere: Pollutants like H2S, SO2, CO2 and acid vapours cause more pollution where they
undergoes corrosion slowly because of high overvoltage of zinc metal (0.7 V) which reduces the effective potential dissolve. In sea water (salty in nature which acts as an electrolyte) corrosion rate increases. Some suspended
to a small value. In presence of CuSO4 the corrosion rate of zinc is accelerated.
particles are dissolved in humidity and form electrolyte which helps in corrosion.
(c) Relative areas of anodic and cathodic parts: When 2 dissimilar metals are in contact, the corrosion of the anodic (d) pH Value: pH value means concentration of H+ (acidic nature). In acidic medium (pH less than 7),
part is directly proportional to the ratio of areas of cathodic part and the anodic part. Corrosion is rapid and corrosion is faster. Also, in basic medium pH > 7, some metals such as Pb, Zn, Al, etc. form complexes and hence
localized if anodic area is small, because the current density at a smaller anodic area is greater. they corrode. Pourbiax relation between pH of medium and potential of metal deals with the corrosion process and
(d) Purity of Metal: Impurities in a metal generally cause heterogeneous state forming minute electrochemical cells it gives idea how to reduce corrosion.
resulting corrosion of anodic part. E.g. Zinc metal with impurities Pb or Fe. Corrosion resistance of a metal may be Example: Zn corrodes minimum at pH 11, but at higher pH (more than 11) it corrodes faster. At pH 5.5, Al
improved by increasing its purity. corrodes minimum.
(e)Physical state of Metal: The rate of corrosion is influenced by physical state of the metal such as grain size, (e) Nature of ions present: Cu++ ions present in the vicinity of Fe, accelerate corrosion, while silicates present in the
stress; orientation of crystals etc., The smaller the grain size of even in pure metal becomes the anode undergoing vicinity resist corrosion.
corrosion. (f) Conductance effect: Due to presence of salts and water in earth, it is of conducting nature. More conductance
(f) Nature of surface film: In aerated atmosphere, all metals get covered with a thin surface film of metal oxides. The leads to more stray current and hence fast corrosion. Dry sandy soil is less conducting and hence less corrosion,
ratio of the volumes of metal oxides to the metal is known as specific volume ratio. Greater is this value lesser is the while mineralised clay soil is more conducting hence more corrosion occurs.
oxidation corrosion rate. (g) Oxygen concentration cell: Oxygen is one of the important element responsible for corrosion. It forms oxides
(g) Passivity of Metal: Passive metals are resistant to corrosion due to the formation of highly protective but very and hydroxides (in presence of H2O) on the surface of metal as corrosion product. Oxygen concentration cell
thin film on the metal or alloy surface E.g. Corrosion resistance of stainless steel is due to passivity character of is formed on the surface of metal due to difference in oxygen concentration (iron rod half dipped in water corrodes
Chromium present in it. due to this effect). Dipped portion will be anode and outer portion will be cathode.
(h) Solubility of corrosion products: In electrochemical corrosion if the corrosion product is soluble in the corroding
medium then corrosion is rapid. If the corrosion product is insoluble, then acts as barrier thereby suppressing
further corrosion.
(i) Volatility of corrosion products: If the corrosion product is volatile, then the underlying surface is exposed for Cathodic Protection :
further attack. This causes rapid and continuous corrosion. E.g. MoO3 is volatile.
The cathodic protection of metals is used to control corrosion metals where it is impractible to alter the
(ii) Nature of corroding environment:
nature of the corrosion medium. The principle involved in this method is to protect metals and alloys from
(a)Temperature: As the temperature of environment is increased the reaction rate is increased thereby accelerating
corrosion by making them completely cathodic. Since there will not be any anodic area on the metal, therefore
corrosion.
corrosion does not occurs.
(b)Humidity of air: critical humidity is defined as the relative humidity above which the atmosphere corrosion rate
The following are 2 types of cathodic protections.
of metal increases sharply. The value of critical humidity depends on nature of metal and corrosion products.
1. Sacrificial anodic protection
Corrosion of a metal is furnish in humid atmosphere because gases (CO2, O 2) and vapours present in atmosphere
2. Impressed current cathodic protection
furnish water to the electrolyte essential to establish an electrochemical corrosion cell. The oxide film on the metal
Sacrificial anodic protection:
surface has the property to absorb moisture. In presence of this absorbed moisture, corrosion rate is enhanced.
In this method, the metal structure can be protected from corrosion by connecting it with wire to a more
Rain water may also wash away the oxide film from the metal surface. This leads to enhanced atmospheric attack.
anodic metal. As this more active metal is sacrificed in the process of saving metal from corrosion, it is known as
The exceptions are Cr, Al.



sacrificial anode. The metals which are commonly used as sacrificial anodes are Mg, Zn, Al and their alloys. The Inhibitors are mainly classified into types
important applications of this method are Anodic inhibitors:
1. Protection of underground cables and pipelines from soil corrosion. Anodic inhibitors are those which prevent the corrosive reaction occurring at anode by reacting with the ions of
2. Protection of ships and boat hulls from marine corrosion. the anode and forming insoluble precipitates. The precipitate thus absorbed on the surface of the metal and forms
3. Prevention of rusty water by inserting Mg sheets or rods into domestic water boilers or tanks. a protective coating resulting in reducing the corrosion rate. If insufficient inhibitors are used, it results in certain
area unprotected leading to severe local attack. The anodic inhibitors used are phosphates, chromates, molybdates,
alkalis, tungstates etc.
Cathodic Inhibitors:
The cathodic reaction occurs in acidic solution is the evolution of hydrogen given as
2 H+ + 2e-  H2
The diffusion of hydrogen ions in the acidic solution can be slowed by using organic inhibitors like a mines,
Impressed current cathodic protection: mercaptanes, and heterocyclic nitrogen compounds etc. This organic inhibitor is absorbed at the metal surface and
As the name implies, an impressed current is applied to convert the corroding metal from anode to cathode. reduces the corrosion rate.
The applied current is in opposite direction since to nullify the corrosion current. This can be accomplished by The cathodic reaction occurs at the neutral solution is
applying sufficient amount of direct current source like battery or rectifier to an anode like graphite, high silica H2O + ½ O 2 + 2e-  2OH-
iron, stainless steel or platinum buried in the soil or immersed in the corrosion medium. And connected to the In this, the corrosion reaction rate can be controlled by reducing the diffusion of oxygen to the cathodic
corroding metal structure which is to be protected as shown in the diagram below.. area or by removing oxygen from the corroding medium. For this, Na2SO3 is used to eliminate O2 and Mg, Zn or Ni
salts are used to reduce the diffusion of O2 to the corroding area.
These inhibitors react with OH- ions at the cathode forming a layer of insoluble hydroxides which are
impermeable and hence reduce the diffusion of oxygen to the cathode area.
ELECTROPLATING:

Principle of Electroplating:

This process involves coating of a thin layer of one metal over another metal by passing direct current
through an electrolytic solution. The base metal to be plated is made of cathode whereas the anode is made of
either coating metal itself or an inert material in the electrolytic cell.

In impressed current cathodic protection, electrons are supplied from an external cell, so that the object Procedure:
itself becomes cathodic and not oxidized. This type of cathodic protect ion has been applied to buried structures
The article to be electroplated is first treated with organic solvent to remove oils, greases etc. then, it is made free
such as tanks and pipelines, transmission line-towers, marine piers, laid-up ships etc. since, their operating and
from surface scales, oxides, etc., by treating with dil. HCl or H2SO 4. The cleaned article is then made cathode of an
maintenance costs are less, they are well suited for large structures and long term operations.
electrolytic cell. The anode is either the coating metal itself or an inert material of good electrical conductivity.
INHIBITORS: The electrolyte is a solution of a soluble salt of the coating metal. The electrolytic solution is kept in an
A corrosion inhibitor is “a substance which when added in small quantities to the aqueous corrosive electroplating tank. The anode and cathode are dipped in the electrolytic solution. When direct current is passed,
environment effectively decreases the corrosion of a metal”. coating-metal ions migrate to the cathode and get deposited there. Thus, a thin layer of coating-metal is obtained



on the article, made as the cathode. For brighter and smooth deposits, favourable conditions such as low
temperature, medium current-density and low metal-ion concentration are used.

“Coming together is a beginning; keeping together is progress; working together is
success.”

ELECTROLESS PLATING:
Definition: The process of producing a thin, uniform and hard deposit of metal on an activated substrate (Metal or
non-metal) by using suitable soluble reducing agents without any electrical energy, and the driving force for the
deposition is auto catalytic redox reactions.

The reducing agent reduces the metallic ions to metal, which gets plated over the catalytically activated
surface giving a uniform thin coating.

Metal ions + Reducing agent Metal + Oxidised products

Process:

The process involves

1. Pretreatment or activation of work piece to be plated.
2. Preparation of bath composition.
1. Pretreatment or activation of work piece to be plate

(i) Metals like Cu, Ag etc. are known as non-catalytic metals. Surface of such metals need activation. They are
activated by using steel or iron pieces for initiating the reactions.
“Time is the coin of your life. It is the only coin you have, and only you can determine how it will
(ii) Non-metals like glasses, ceramic, plastics are activated by dipping in SnCl2, PdCl2 in HCl. This process be spent. Be careful lest you let other people spend it for you.”
produces a thin film of palladium coating on non-metal surfaces which in turn causes the work piece to get
activate for electroless plating.

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Science of corrosion

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