5 PHASE QUILIBAIUM

AND CORROSION
PHASEEDIAGRAM OF SINGLE COMPONENT
PHASE DIAGRAM OF SYSTEM(WATER),
BINARY EUTECTIC SYSTEM
61 Write short
(Cu-Ag)
note on phase equilibrium.
Ans. Phase equilibriumiis the (R.GPV., Nov. 2019, June 2020)
study ofthe equilibrium which exists
or within different states of matter namely solid, liquid
and
between
ined as a stage when chemical potential of any gas. Equilibrium is
component present in the
SIStem stays steady with time Phase is a region where the
interaction is spatially uniform or in other words physical intermolecular
and
DrOperties of the system are same throughout the regionWithin chemical
the same
state, a component can exist in two different phases like allotropes of an
element. Also, two immiscible compounds in same liquid state can coexist in
twO phases.
Phase equilibrium has wide range of applications in industries including
production of different allotropes of carbof,lowering of freezing point of water
oy dissolving salt (brine), purification of components by distillation, usage of
emulsions in food production, pharmaceutical industry etc Solid-solid phase
equilibrium has asspecial placeiin metallurgy and is used to makealloysof different
physical and chemical properties FÍrinstance, melting point of alloys of copper
and silver is
lower than melting point of either silver.
copper or
2. Define the following terms
9 Component (i) Phase (ii) Degree of freedom.
(R. GPV, Dec. 2015)
Or
Define -
i) Phase (i) Component (ii) Degree offreedom.
(R.GPV, June 201)
Ans. (i) defined as the "The minimum
Component
umber of independently - Acomponent is
means of which the
variable constituents by
 136 Engineering Chemistry
composition of each phase can be expressed either (diRrect.GPV.ly orB.inTech
chemical equation'". The number ofchemical the
in dividual
cannot be the components of the system. For example, s present in fe
syle
In the water system
Ice Water Vapour
Solid liquid gas
The chemical composition of all three phases is H,0.
component system. Hence, it is
(ii) Phase- Any heterogeneous system consists of or
two men
homogeneous parts which are separated from each other by
bounding surface is known
surface. This boundy
as phase. The
two phases is called an interface. boundary betwn
For example -
CaCO, CaO + CO,
Phase I II
This system has three phases i.e., solid (CaCO;), solid (Ca0), (gas) C
(iüi) Degree of Freedom -The factors such as temperature, pressa
and composition of phases,which can affect the equilibrium of asystems a
known as degree of freedom. Hence, the minimum number of the variaie
factors must be arbitrarily fixed in order to define the condition of the sysen
completely, is known as degree of freedom of the system, For instance -
Ice Water Vapour
(solid) (liquid) (gas)
The ice-water-vapour system can be in equilibrium only at one certain pres
and temperature. Therefore the system will have no degree of variance or
varient.
(R.GPK,June N2
.3,/Write brief note on degree of freedom.
Ans. Refer to ).2 (iii).
e
0.4. What is Gibb's phase rule ? Explain the meaning ofthe
involved in it. What are the uses and limitations of phase rate
Ans. Phase Rule - Phase rule is an important generalization whichhelpa
wide applications in the study of systems. Withthepredice
phase rule the effect of pressure, heterogeneous
and composition may be
by#k
temperature
raheterogenous system in equilibrium. ItIt was first of all put
forward
Can

American Physicst Willard Gibbs and known as Gibb's phase rule,

written as mathematically,
or F=C-P+2
F+P= C+2
Shvan-2024) Phase Equilibrium and Corrosion 137

WhereFistthe number
of degree of freedom, Pis the number of phases
the
PCis number of components for the system. So, according to phase
a
heterogencous system in equilibrium, the number of components
mein
cqualthe number of degree of freedom plus the number of phases.
ns is
two

TermsInnvelved in it -
Refer to Q.2.
Usesof Phase Rule -7The uses of phase rule are as follows-
With the help of terms phases, components and degrees of
edomthe phaserule
givesa convenient basis for classification of equilibrium
elteofsystems.
The information about molecular structure is not necessary
ecauseitapplies to macroscopic systems.
(i) The phase rule shows that various systems having the same
fashion.
dearees of freedom behave in a similar
(iv) It helps in deciding whether the given number of substances
peether would exist in equilibrium under a given set of conditions.
Limitations The limitations of phase rule are given below -
() The phase rule is not applicable to the systems which are slow
nreachingto equilibrium state.
(ü) In phase rule, there is no consideration of magnetic and electric
intluence since system is defined only the variables like pressure temperature
and components force.
(i) It is necessary that all phases must be present under pressure
emperature and gravitational force.
(iv) Any liquid or solid present should not be in finally divided state
lerwise the value of their vapour pressure will differ from their normal value.
Q5.State and discuss phase rule and significance of triple point.
(R.GPL, June 2016)
Ans. Phase Rule - Refer to Q.4.
Significanceof Triple Point - Atriple point is invariant. The triple point
90Curs at a unique set of values of the temperature and pressure for a given
Bibstance. If either volume or temperature is altered, even slightly, one of the
ce phases disappears and the system changes from invariant to univariant.
0.6. Derive the expression F = C-P+2.
Or

Ans,
Describe the derivation of Gibb's phase rule.
dieistnredibuteasd Consi
in Pdphases.
er a heterogeneous
The degree ofsystem
freedomconsisting of Ccomponents
of the system is completely
Sline the number of variables which must be arbitrarily fixed in order to
the system
completely.
 138 Engineering Chemistry
Hence, the total number of variable of the
variables defined by the system. When a system issystem
(RGPV, B1
mins Tech v
in he
only one temperature and one pressure, so these equilibriwillum heremothe ey
variables
ofthe system is defined by (C-
Thus the composition ofthe
Since the composition of the remaining
components
difference. Thus for the system of Pphases, the 1)composit en
can hbe vafi
or concentration variables will be P(C-11).Adding the numbers of defined
total
variables, we have the total number of variables. temperature and
- P(C- 1) +2
comppreetissm
On the basis of thermodynamic considerations,when
when asystem
equilibrium, the chemical potential () of the given component m
same in every phase. Suppose the system consists of three phases must be the
s(a),che(b) nt
(c) in equilibrium at a definite pressure and temperature, then the
potential of the given components is same in each phase. Ifthe
are designated by 1, 2, 3 then components
Hi(a) = (b)
H2(a) = z(b)
H3la) = 3(b)
and also, Ha) =(b) =(c)
In general for the system of P phases and C components the fact of the
system being in equilibrium may be given by
Ha) =(b) = (c) =...=Hp)
H2la) = H(b) = (c) = ....= 2p)

Hela) =(b) elc) = . . .*= HeP)

Which constitutes C (P - 1) independent equations. Since chemici
potential is a function of temperature, pressure and concentration, it meais
that cach equation represents one variable of the kind mentioned above.
Hence, the number of unknown variables or degree at freedom will
F- (P(C-) +2)-[C (P- ))
- PC-P+2-CP +c
F- C-P+2
Which is phase rule as stated by W. Gibbs.
0.7. Write the difference
between phase
Ans. Any homogeneous physically andand
distinct state.mechanically separablk
part of a heterogeneous system in equilibrium is known as phase,theseare
separated from each other by a boundary.
hian2024) Phase Equilibrium and Corosion 139
Physicallyappearance of a matter under ordinary conditions is its state
phases none the less
solds mayformtwo their state in one. Similarly two
mIscibleliquidsformtwo phases but their state is one.
08Whatdo you understand by one component three phase system ?
etamples.
Or
on
Arbrphaserule ice-water-vapour
Or
system.
Draw and explain phase diagram for water system.
(R.GPV., June 2016)
Or
Write short note on phase diagram of water.
(R. GPV., Dec. 2016, June 2017, Nov. 2018)
Or
Write brief note onphase diagram of single component systems.
(R. GP V, May 2019, Nov. 2022)
Or
Discribe phase diagram of one-component water system with suitable
Examples. (R. GPV, June 2023)
Or
What is triple point ? Explain water system with the help of neat phase
iagram. (R.GPV., June 2022)
Ans. Water System - This is the most common example of the one
mponent system. The water system consists of three phases i.e. ice, water
and water vapour.
lce Water Vapour
3 (solid) (liquid) (gas)
It is single or one Component system because H,0 is the only chemical
Sompound. When C=1, the phase rule is as follows -
F= C-P+2
F= 1-P+2
F= 3-P
t., the degree of freedom based on the number of phases present at
tuilibrium.0) TheP= vari1, Fous= cases are possible-
2 (bivarient system)
) P=2, F= 1(univarient
(i) P=3, F=0 system)
The maximum number (invarient system)of freedom is two for any one
of degrees
Cotponent system it is clear from above cases. Therefore, such a system can
140 Engineering Chemistry
be represented completely by atwo dimen-
variable are the pressure and temperature,
(R.GPV., B
sional diagram. The Tech ,
The water
diagrammatically.
Critieal
mostCovei
system
218 Pressure
Atms ice
Water
(Solid) (Liquid)
One
Atms|760mm!
Melting
Curve: Poir Critical
2 AS8mm?
Sublimatioir' Temperature
Curve
Vapour
R
(Gas)
0.0076°C
0°C T1 T; 374°C
Temperature
Fig. 5.1
The important features of the above phase diagram are listed below
)) The curve OA, OB, 0C
(i) The triple point 0
(ii) The areas AOC, AOB, B0C
These three curves meet at the point Oand divide the diagram into three regions
Vapour Pressure Curve of Water, OA It represents the vapout
pressure of liquid water at different temperature. The two phases, water and
water vapours co-exist in equilibrium along this curve. The curve OA terminats
at A, the critical temperature (218 Atm. temperature 374°C), when the lhqud
and vapour are indistinguishable from each other and there is left one phase
only.
When the vapour pressure is equal to one atmosphere, the corespondly
temperature is the boiling point (100°C) of water.
Sublimation Curve of lee, OB - It shows the vapour pressureofsolid
ice at differenttemperatures.
The two solid phases ice and vapour co-exist in equilibrium along the
curve. At the lower limit, the curve OB terminates at absolute zero(-2230)
where no vapour exists.
The Fusion Curve of Ice, pressure onthe
effect of
melting point of ice. Here ice andOC- It depicts
water co-exists in
the
equilibrium. The factthat
Wih
OC slopes to the left decreases
increase of pressure. indicates
Since thethat
ice the
meltsmelting point of ice
with decrease of Volumesothe
melting points is lowered by an
inerease in pressure.
 Phase Equilibrium and Corrosion 141
Sien20P4)

thecurve
OA, OB and 0Cthere are two phases in equilibrium and
Along
ecomponent, Therefore,
F C-P+2
-|-2+ 2 1
Point 0-In water system the point
where three phases (water/
Thple equilibrium and curves OA, OB and 0C meet from each
eiapour). are in fe. 5.1. This point at 273.16 K(or 0.0075°C) and 4.512
shown in fig. point. Since three phases are co-exist, the
is known as triple
as
rherpressure
mm variant.
ostemisin F= C-P+2
= |-3+2 =0
pressure be changed one of the three phases will
Ifeithertemperature or
disappear.
AOC, AOB, BOC - The areas or regions between the curves
Areas
conditions of temperature and pressure under which asingle AOC
phase
showthe stable existence. Thus, Area
water of water vapour is capable of water liquid. Area AOB represents
represents conditions for the
one phase
vapour. Area BOC represents
onditions for the one phase system water three areas, there being one
phase system ice. In all the
Conditions for the one
we have,
phase and one component,
F= C-P+2
= |-1+2 =2
water, water vapour or ice has two degrees of freedom
Thus each system,
ie. the system is bivarient.
Metastable System, Super Cooled Water Vapour- The vapour pressure
of water OA' can be continued past the triple point as shown by the
Curve cooled by carefully eliminating
dashed line OA. That is water can be super
vapour system is metastable. It at once
Solid particles. The super cooled water
stable system ice/vapour on the slightest disturbance or introducing
vers to the
2crystal of ice.
09. Explain the following -
a one component system
is two.
(0 Maximum value ofdegree offreedom in
system
) Atriple point in the phase diagram ofa one component
is an
invarient system. Component
Ans. (i) Maximum Value of Degree of Freedom in Onewhen only one
System is Two)-In a one system,
component system like water described only bytwo
phase is be
under certain conditions thenit could When temperature and
variable exifactors
sting pressure.
which are temperature andautomatically constant.
pressure are fixed then the volume become
 142 Engineering Chemistry
This also follows from thephase rule equation (R.GPV,BTech
F-C-P+2
-|-142 -2
(i) ATriple Point in the Phase
Sstem is an Ivarient System-In a one component Diagram of One
systerm the
are in equilibrium at triple point in phase diagram. If any of
temperature or pressure is changed then one of the phase the
Componen
three phelike
variahble
triple point in a one component system the degree of freedom is disappears. Ths
the system is termed as ainvarient. This also follows from Zero hehe
equation. the phase ne
F= C-P+2
F= |-3+2 =0
0.10.) What do you mean by eutectic system ? Write its applicatiom
Ans. Eutectic System - An eutectic is a binary system which
have h
substances, that are miscible in all proportions in the liquid state, but do s
react chemically. For example, mixture of lead and silver.
Eutectic Mixture - It is a solid solution of fixed proportions of two or
more substances which have the lowest freezing point. This is taken advantag
of in "alloys of low melting point", which are normally eutecticmixtures.
Eutectic Point - Point where the freezing point curves intersect each
other is known as eutectic point. This point shows a fixed composition of wo
constitutents. The temperature at this point is called eutectic temperature and
composition is called eutectic composition. In other words eutectic meas
lowest melting point.
Applications of Eutectic -
() Itis used as safety devices like fire sprinklers, plugs inautomobilks
(i) For freezing mixtures.
(i) Very low melting alloys are obtained by suitable choice of mealik
0.11 Write brief note on eutectic point. (R.GPK, June 2029
Ans. Refer to Q.10.
Q.12. What is eutectic ? Describe formation of simple eutectie w
general diagram. anotheri
Ans. The two components are completely miscible with onecomponens
the liquid state, on solidification, conly an intimate mixture ofthe pure
iscalled eutectic. two
of
Formation of Simple Eutectie Assume a general diagram
component condensed systems belonging to type I.
Sen-2024) Phase Equilibrium and
nhavecompletely miscible components Aand Bin the liquid phase 143
and Corrosion
heirsohitionsyield only pure Aor B. as solid states, Fig. 5.2 shows temperature-
Ompositioncurve of this system.
pointsA and Bare the melting points of pure A
howninthefig. 5.2. The freezing point of Ais lowered
The and B along AC when as respectively
quantitiesofBis added to A. Also, the
ereasing
ong BCwhen increasing quantities of Aare addedfreezing
to B. pointof Bis lowered
Liquid Melt
(Unsaturated Solution)
Temperature C
Solid B
+ Solution

Solid A m

+ Solution

F
PrimaryA
A+B
PrimaryB
100% A D
100% B
0%B Composition 0% A
Fig. 5.2 Simple Eutectic System
The freezing point curve of the component A is the AC
ACis the composition of the solutions saturated with solid curve. The curve
A at temperatures
between A and AC. Thus the two phases along this curve are solid A and
solution of B in A.. The freezing point curve of the
curve. The curve BC is the composition of the solution component B is the BC
at temperatures between B and BC. Solid B is in saturated with solidB
in Balong this curve. equilibrium with solution ofA
These two curves showthe two-phase equilibria which are
can be seen by applying reduced phase rule univarient as
F= C-P+1
equation.
= 2-2 +1=1
Where both solids Aand B are in
two curves with the liquid phasethese
equilibrium
at this intersect at the point Since there are three phases in equilibrium
C.
Teducedpoint.
It is
invarient in character. This can be simply followed from the
phase rule equationF= C-P+1
-
=2-3 +1 =0
COnstHence, the temperature
ant as long
One of the as three phasesandco-exist
composition of the solution
in equilibrium must remain
at this point. At least
phases will of the two variables is changed.
Moreove,? as can be seen disappearwhen any
from the fig. 5.2, the point Cis the lowest temperature
144 Engineering Chemistry
(R.GPV, B. Tech.,Yaa
at which any liquid mixture can exist. The system is
below this point. Temperature at point Cis called eutectic completely solidif,
composition is called eutectic composition point Cis known as
As cooling continues till the eutectic temperature is
obtained. The temperature now remains constant untilaall the
treaached,
emeutpeeracttuiete
point
and
poiknt.
On complete solidification, further decrease in liquidswill
temperature
decrease intemperature of the mixed solids along the line kl. Cause fhe
is done in reverse, that is solid /is heated until it liquefies When the
the changes take place exactly in the reverse order. proce
completely abotis
0.13. Describe silver-copper eutectic phase diagram for
bingar
System.
Or
binary eutectie
Write brief note on phase diagram of Cu-Ag system.
Ans. Silver-copper alloys have eutectic phase (R.GP.V., Nov. 2018
system are as follows diagram for binary eutectie
Silver solder is an alloy of silver (Ag) and copper
a liquid that solidities to join two solid (Cu). Solder is applied as
pieces of metal of without melting the tNO
solidpieces Ag-Cu alloys have the eutectic phase
three single phase regions are a, B and liquid. Thediagram. In this diagram first
rich in copper, it has silver as the solute a-phase is a solid solution
The B-phase solid solution also has an component and an FCC crystal structure.
FCCstructure, but copper is the solute.
Pure copper and pure silver are also considered to be a
and Bphases.
Thus, the solubility in each of these solid phases is
temperature below line BEG only a limited concentration oflimited in that at any
in copper, and copper in silver similarly for a-phase and silver willdissolve
limit for the a-phase corresponds to the boundary line B-phase. The solubilty
the a/(a. + B) and oa/(a + i) phase regions. It labelled CBA, between
increases with
maximum at point B and decrease back to zero at the meltingtemperature t
temperature u
pure copper, point A. At temperature below 779°C the solid, solubility limit R
separating the a and a +Bphase regions is termeda solvus line, the bouu
AB between the a and a. + Bphase regions is termeda solvus line. The
boundary AB between the a and a + Lfields is the solidus line, asshowni
fig. 5.3. For the B-phase both solvus and andCE
solidus lines also exist Hg
respectively as shown in fig. 5.3.
Solubility in each of these solid phases is limited, in that at any temperatue
copper
below line BEG only alimited concentration at silver will dissolvein bounduy
(for the a-phase), and similarly for
for copper in silver (for B-phase).The regios
line CBA is the solubility limit between a/(a+ B) and l a/(a +L) phaseatpointB
it increases with temperatures to a maximum (8.0 wt% Ag Al t 779°C
 Phase Equlibrium and Corrosion 145
the meltingtemperature of purecopper, point A
zeroatthe
ereasesback to
RCAtlenperatureNbelow
below 779"C, the solid solubility limit line separating
regions is se
solvus line boundary AB between the a and
pphase
solidusline as shown in fig. 5,3. Forthe p-phase, both
solvus and
eklsis
alsoexist HG and GF respectively. At 779°C the maximumssolubility
Whlisline
B-phase, On tG
point (8,8 wt% Cu), also occurs. Horízontal
inthe
opper
parallelttothe composition axis andextends between these maximum
BGG also be considered a solidus line, it represents the
lubility positions may
liquid phase may exist for Cu-Ag alloy at
est temperature at which a
Auilibrium,
Composition (wt% Ag)
60 80 100
20 40 2200
I200
2000

Liquidus 1800
1000 Liquid F
Solidus
1600
atL B+L
E
n 800 779°C(T) 91.2 1400
B 71.9
8.0
(C) (C_E)
f(CoE) 1200

600
Solvus 1000

800
400
600
C H
400
200 60 80 100
20 40
Composition (wt% Ag) (Ag)
(Cu)
Diagramn
Fig. 5.3 Ag-Cu Eutectic Phase
In this way three-two phase regions found for the copper-silver systemn in
B-phase solid solutions
Ove fig. 5.3 like a+ L.B+L and a+B. The a and
C0-exist for all compositions and temperatures within the a + Bphase field. a+
liquid andß+ liquid phases also co-exist in their respective phase regions.
As silver is addedto copper, the temperature at which the alloys become
totally liquid decreases along the liquidus line, line AE; thus, the melting
The same may be said
lemperature of copper is lowered by silver additions.
for silver, the introduction of copper reduces the temperature of complete
nelting along the other liquidus line, FE. These liquidus lines meet at the point
on the phase diiagram, through which also passes the horizontal isotherm
146 Engineering Chemistry
line BEG Point Eis called an invariant point, which is (RGPV, B. Tech, Y
composition C andtemperature Tp: for the the
d esign
copper-silver system,
of Cg and Tp are 71,9 wt% Ag and 779°C (1434°F),
An important reaction occurssfor an alloy of respecti v ely
ate
.
d by
the vlves e
composition
temperature in passing through TE; this reaction may be Cgasit chang
below
cooling
writ en as gves
L(CE) a(CaE) +BCpE)
heating
On cooling, aliquid phase is transformed into the two
phases at the temperature T; the opposite reaction occurs uponsolid a and
is called a eutectic reaction (eutectic means easily method), andheating T
C and T.
represent the eutectic composition and temperature, respectively; Cas an
CBE are the respective composition of the oa and Bphases at Tg. Thus, for the
copper-silver system, the eutecticreaction. Equation()as written in this wa
L(71.9 wt% Ag) cooling a(8.0 wt% Ag) + B(91.2 wt% Ag)
heating
Often, the horizontal solidus line at T is called the eutectic isothem.
The eutectic reaction, upon cooling, is similar to solidification for pure
components in that the reaction proceeds to completion at a constant
temperature, or isothermally, at Tg. However, the solid product of eútecte
solidification is always two solid phases, whereas for a pure component only
a single phase forms. Because of this eutectic reaction, phase diagrams similar
to that in fig. 5.3 are termed eutectic phase diagrams; components exhibiting
this behaviour comprise a eutectic system.
0.14. Draw a phase diagram of copper-silver system.
(R.GPV, Dec. 2015)
Ans. Refer to Q.13.

CORROSION - TYPES, MECHANISMS &PREVENTION

0.15. Define corrosion. Write is causes and eflects.

Or
What are the consequences of corrosion ? (R.GPE Dec. 2015)
Or
Write brief note on corrosion. (R.GPK, Non 2019
Ans, Corrosion - Any process of deterioration and consequent lossof
nunwanted chemical or
solid metallic materials through ansurface,
attack
at its is called electrochemical
corrosion
by its environment starting
extraction of corrosion. Thus,
is a process
"reverse of metalsor example, rusting of iron.
 Phase Equilibrium and Corrosion 147
Couses Metals exist in nature in the form of carbonates, sulphides and
Thesechemically
jointed states of metals known as ore has a
hates.
low
hence thermodynamically stable state of metal. The metals are
Nfiomthese ores after supplying aenough amount of energy. Metals
uCOmbincd l condition have a higher energy and are in an unstable state.
e thisinteracting chemically/or electro-chemically with its
do s
nkcompoundat the surface and thus environment
metal undergo corrosion.
m
Effects-Thefollowing are the effects of corrosion
o Loss of useful properties of metal.
() Replacement of corroded equipment is time consuming.
) Increase in maintenance and production cost.
(iv) Loss of efficiency.
(v) Contamination of product.
QJ6.Define corrosion of metals. Write factors influencing corrosion
iaplainany four.
Ams. Corrosion Refer to Q.15.
Factors Influencing Corrosion The rate and extent of corrosion,
nends on the following factors -
) Temperature
) Humidity
(i) Effect of pH
(iv) Presence of impurities
(v) Nature of ions
present
(vi) Purity of metal
(vii) Physical state of metal
(vii) Nature of surface film.

cical Temperature - Corrosion increases with temperature because

a reactions rate and diffusion of ions rate
bigher temperature increase with temperature.
passive metal may become active.
(i)
Boisture. Humidity - Atmospheric coorrosion increases in the presence
gCn in theThis due to the fact that moisture works as the solvent for the
is
air to
furnish the
electrolyte
Svase. relative essential for setting up acorrosion
humidity reaches from 60 to 80%, rusting of iron
of air

sive(ii)
than Effect of and
pH- Generally, acidic media (i.e., pH <7) are more
alkaline
dissolve in neutral
alkalinemedia. However,
solution the pH(likeof
ions. If metals
atmospheric
as complex
148 Engineering Chemistry
(R.GPV, B. Tech.,
oxygen free water is below 5then corrosion rate of iron is
corresponding corrosion rate in presence of is slow. The
Consequently, corrosion of metals, readily attackedoxygen
by acids, canmuch
by increasing the pH of the attacking environment e.g. Zn. be irghex.
(iv) Presence of Impurities Atmosphere, innthe
reduca
areas, contains corrosive gases like COz, H,S,SO, and fumes vicisofnity off industra
etc. In presence of these gases, the acidity off the liquid, adjacent HCI ,
to theH-SO,
surfaces, increases and its electrical conductivity also rmetal
increases.the Iocal
consequently, results in an increase of corrosions current flowing in t Thís
electrochemical cells on the exposed metal surfaces.
0.17. What is oxidation corrosion and how does it take place ?Descrik.
the mechanism of oxidation corrosion. (R.GP.V., June 202)
Ans. Oxidation corrosion is one of the most common ways in which
metals are attacked by direct action with oxygen, usually, in the absence of
moistureAtordinary temperatures, most of the metals are very slightly attackele
but alkali and alkaline earth metals are, however, rapidly oxidized at loN
temperature,,whereas at high temperatures, practically all metals except Ag
Auand Pt are oxidized.
Practically, all metals on exposure to air get covered with a film of oxide
which is a few Angstrom unit thick. The thickness of the oxide film varies
with the metal and the temperature.
The reactions in the oxidation corrosion are -

2 M’ 2M+2ne (Loss of electrons or oxidation)

Metal Metal lons

nO, + 2ne’ 2n O (Gain of electrons or reduction)

Oxygen Oxide lons

2M + nO, ’ 2M++ 2n 0
Metal Oxygen Metal Oxide
lons lons
Metal Oxide
firstatthe
Mechanism of Oxidation Corrosion -The oxidation occurbarrier,that
surface of the metal and the resulting metal oxide scale formseither
a
the metal
tends to restrict further oxidation: For oxidation to continue, Oxygenmust
must diffuse outwards through the scale to the surface or the transtèrs
Both much
diffuse inwards through the scale to the underlying metal.generally,
occur (see fig. 5.4), but the outward diffusion of metal is, metalion8
more rapid than the inward diffusion of oxygen, since the higher
appreciably smaller than the oxygen ion and consequently, of much
mobility.
 Phase Equilibrium and Corrosion 149
ShiaN2024)

Metal-metalOxide Interface
Reectionat M’ Mnt + ne (Oxidation)
IewardDifusion(Slow)
ThronghScale
arOxidelons
Atmospherie
Oxygen (Air) Metal (M)

MetalOxide
Formationof of
M;0)at the Point 02
and
MeetingofMnt
-Direction of Electrons Conduction
Part -Metal Oxide Seale Formed by Direct
Reaction at Exposed Reaction of Metal and Oxygen Ions
2e02
120, +
(Reduction) -Outward Diffusion (Fast) of Metal
Ions Through Scale
Metals
Fig. 5.4 Oxidation Mechanism of
important part in oxidation corrosion
Nature of the oxide formed plays an
process.
(corrosion product)
Metal+ Oxygen ’ Metal oxide on the metal
layer of oxide is formed
When the oxidation starts, a thin the further action. If the film is -
nature of this film decides
Surface and the
is fine-grained in structure and can get adhered tightly
()Stable - It impervious nature. Such a
surface. Such a film can be of
to the parent metal thereby shielding the metal surface.
fim behaves as protective coating in nature,
oxide films on Al, Sn, Pb, Cu, Pt etc., are stable, tightly-adhering and
he oxidation corrosion is prevented.
ipervious in nature. Consequently, further
(ii) Unstable - In this case, the oxide layer formed, decomposes
the metal and oxygen. So oxidation corrosion is not possible in such
CK into corrosion.
ses, For example, Ag and Au do not undergo oxidation
Unstable
Exposed Metal Oxide
Surface
Metal Oxide
+02 Metal +O2
Metal (0f Air)
Metal
Decomposes

Fig. 5.5 Unstable Oxide Layer

(ii) Volatile- In such cases, the oxide layer volatilizes as soon as it
fomed, thereby leaving the underlying metal surface exposed for further
Allack. This causes rapid and continuous corrosion, leading to excessive
SiTOSion, e.g, molybdenum oxide(MoO,) is volatile.
150 Engineering Chemistry
Exposed Unstable
(R.GPV, B.Tech.
Surface Metal Oxide Fresh SurfaceAttaek
for Further Exposed
Metal
+0) Metal Oxide
Metal
(OfAir) Volatilizes Metal
Fig. 5.6 Volatile Oxide Layer
(iv) Porous - In such cases, the oxide layer
and the atmospheric oxygen has an access to the have pores or cracks
through the pores or cracks of the layers. Therefore,
the
underlying
surface of metal
unobstructed, till the entire metal is completely converted corrosion
into its Oxide continues
Exposed Porous
Surface Metal Oxide

Metal +02 Metal

(Of Air) Further Attack Through
Pores/Cracks Continues
Fig. 5.7 Porous Oxide Layer
According to
if the volume of thePilling-Bedworth, "An oxide is
oxide is at-least as great as the protective or non-porous,
volume of the metal from
which it is formed". On the other hand If the
the volume of metal, the oxide layer is volume of the oxide is less than
porous (or non-continuous) and hence,
non-protective, because it cannot prevent the access of oxygen to the fresh
metal surface below". Thus alkali and
Mg) form oxides of volume less than the alkaline-earth metals (such as Li, K, Na,
volume of metals. Consequently, the
oxide layer faces stress and strains, thereby developing cracks and pores in ts
structure. Porous oxide scale permits free access of oxygen to the underlymg
metal surface (through cracks and pores) for fresh action and thus,
continues non-stop. corroston
On the other hand metals like Al forms oxide, whose volume is greaiet
than the volume of metal (Al). Consequently, an extremely tightly-adhering
non-porous layer is formed. Due to the absence of any pores or cracksinthe
Oxide film, the rate of oxidation rapidly
decreases to zero.
Q.18 Explain electrochemical theory of corrosion with reference W
iron. Discuss process of
galvanization to protect iron. 2019, June2020
(R.GPV., May corosion,
Ans. Electrochemical Theory of Electrochemical
also known as Corrosion-
immersed corrosion, occurs under the following
two
conditions
(i) When a conducting liquid is in contact with the metal.
Shien-2024) Phase Equilibrium and Corrosion 151
When two different metals or alloys are partially or completely
solution.
mersedin a
Vious theories have been put forward to explain wet or immersed
sion, but themost widely accepted theory is the electrochemical theory.
ne iron metal is the most common which undergoes wet corrosion, the
rosion is explained by taking iron as an example.
According to electrochemical theory, the chemically non-uniform surfaces
metals behave like smallelectric cells in presence of water containing
ssolved oxygen and carbon dioxide. Thus corrosion of a metal in aqueous
electrochemical phenomenon which involves flowof electron
current between the anodic and cathodic areas. The anodic reaction involves
dissolution of metal as corresponding metal ions with the liberation of free
lectrons, while the cathodic reaction consumes the electrons, evolved in the
anodic reaction, with the evolution of hydrogen or absorption of
depending upon the nature of corrosive environment. Thus, corrosion oxygen,
place as a result of takes
local action.
(a) Evolution of Hydrogen - This type of
occurs in acidic environments. Consider the case of corrosion generally
iron
Teaction involves dissolution of Fe as Fe ion with the metal, the anodic
Fe ’ Fe +2e
liberation of
electrons.
These electrons flow from anode to (Oxidation)
ons from the acid solution are cathode through the metal and H
reduced to hydrogen gas at the cathode.
2H* +2e ’ H2
Thus the overall reaction is (Reducton)
Fe+ 2H’ Fe+ H,‘
Thus this type of corrosion
e acidic
solution by metal ions. causes displacement of hydrogen ions from
(b) Absorption of
EXanple this type of corrosionOxygen-
of
is
Themost important and
the rusting of iron. In comnmon
ulonof clectrolytes such as neutral
NaCl solution in present of
surtace is coated with a thin aqueous
ide
devclops some cracks,
film of oxide. atmospheric
HHowever, if this filmoxygen,
BApOs cd ron metal anodic
AOdic s while the rest metal surface areas are of iron
developed the surface of the
on
small, while cathodie area is forms cathode. Thus here,
Ae anodic area, iron dissolves as large.
Aoxidaio)
The ferrous ions with liberation of electrons
oeallibwher
eron).ateed clelectectronsronsareflowakenfromup anodic to cathodic areas
O
jens (reduci
by the
dissolved oxygen through the
152 Engineering Chemistry
(R.GPV., B.Tech.,
Atanode Fe -’ Fe?t 42e
At cathode o, +H;0+ 26
2 ’ 20H
The Fe2+ ions at anode and OH ions at cathode diffuse
and
fom ferrous hydroxide, Fe(OH),:
Fe2+
combine
+20H’ Fe(OH);
In case sufficient amount ofoxygen is present, ferrous
oxidised to ferric hydroxide.
4Fe(OH)2+ 2H,0 + O, ’ 4Fe(OH),
shydroxidesis easily
This product, called yellow rust,
In case, limited amount of
actually corresponds to Fe,0, Ha
oxygen is present, the corrosion
FeO4,the black anhydrous magnetite. product
Process of Galvanization Galvanizing is the
coating of zinc or process of
iron or steel to prevent them from rusting.
carried out as follows The processi
1 imparting
The iron or sheet article (e.g. sheet,
sulphuric acid for about 15-20 minutes atpipe, wire) is first cleaned with dilue
60-90°C. Technically, this proces
known as pickling and it also helps in removing any scale,
other impurities present on the base metal. The rust (oxide layer) and
then dipped in a bath of molten zinc, article is then washed, dnied ani
bath is covered with a flux to preventmaintained
at 425-430°C. The surface ofte
oxide
zink layer is then taken out of the bath andformation. The article coated with te
passed through a pair
which removes any superfluous zinc and thus only a thin layer of zincof hot roles
is adher
on the article. Finally, it is annealed at a temperature 650°C and
then cooled siowy
Amm. Chloride Pair of Hot
Flux Rollers Galvaale
Iron She

Anca
Chane

Drying Hot
Chamber Air
Water BxcessZie
Dil. H2S04 Washing Molten Zine Reamoved
at 60-90°C Bath at425-430°C
Fig. 5.8 Galvanizing of SteelSheet sheets, showl
Galvanizing is most widely used to protect roofing It
corrosion.anndcoot
bolts, nails, screws, buckets ete. from atmospherieusedforstoring
remembered that zinc coated utensils shouldnot be
Shivan-2024) Phase Equilibrium and Corrosion
153
pods,especially acidic ones, because zinc dissolves in dilute acids to form
tooxicand even poisonous compounds.
hehly
electrochemical theory of corrosion.
Q.18. (R.GPV, Dec. 2016)
Ans. Refer to
Discuss various theories to explain corrosion.
Ans.The different theories of corrosion are as follows -
Direct Chemical Attack - This theory is also known as
chemicalIor dry corrosion. Chemical corrosion occurs mainly through the
directchemical action of environment or atmospheric gases like oxygen,
ibgeh, bydrogen sulphide, sulphur dioxide, nitrogen or anhydrous inorganic
oud with metalsurfaces in immediate proximity. Chemical corrosion follows
korption mechanism. Chemical corrosion occurs in the dry state.
For example, chlorine and iodine attack silver generating aprotective film
of silver halide. Similarly, during de-tinning of tinned low-carbon steel cans
BSing chlorine gas at high temperatures, volatile SnCl, is formed and so all the
tin is readily removed from the metal surface. However, the base metal iron is
very little affected or attached by dry chlorine because if reacts with iron to
fom solid ferric chloride (non-volatile) which protects the rest of the metal.
(i) The Electrochemical Theory Refer to Q.18.
(ii) The Acid Theory According to this theory, corrosion occurs
Pesence of an acid. This theory is particularly applicable to rusting of iron
he atmosphere. Rusting of iron is due to the continued action of oxygen,
carbon dioxide and moisture, converting the metal into asoluble ferrous
bicarbonate which is further Oxidised to basic ferric carbonate and finally to
hydrated ferric oxide.
Fe+0 +2CO, +H20 ’ Fe(HCO;)2
Fe(HCO,)2 +H,0+0 ’ 2Fe(OH)CO, +20O, +2H,0
2Fe(OH)CO, +2H,0 ’ 2Fe(OH); +2C02
This theory is supported by the following facts -
(a) Rust analysis generally shows the presence of ferrous and
fer ic
the water
carbonates along with hydrated ferric oxide.
(b) Retardation of rusting in presence of added lime or NaOHto
in which iron is
immersed.
0.21. Describe chemical corrosion and electrochemical lcorrosion with
sultable examples. (R.GPE, June 2023)
Ans. Refer to Q.20() and Q.18.
 154 Engineering Chemistry
0.22. Dierentiate between chemical
corrosion. corrosion(RGPV,
and EelecBTtroecchhemi
Ans. Refer to Q.18 and Q.20 (). (R.GRV, June 01
0.28. Name the different types of corrosion.
corrosion. Explain any one type
Ans. Different types of corrosion are as follows -
() Galvanic corrosion (i1) Pitting corrosion
(ii) Erosion corrosion (iv) Crevice corrosion
(v) Stress corrosion (vi) Water-line corrosion
(vii) Intergranular corrosion (vii) Underground or soil
(ix) Microbiological corrosion (x) Passivity.
Water-line Corrosion - When water is stagnant in a
Cor osien
steel the
concentration of oxygen above the water surface is greater than tank,
surface. This generates an oxygen concentration cell. In this that under te
cell, the metal ju
above the water level is cathodic with respect to the metal below the waterlt
Highly Oxygen ated
Cathodic Part (Protected)
Poor Oxygenated
Anodic Part is
Corrected
Fe ’ Fett + 2e H0’ H+OH
Fett + 20OH ’ Fe(OH)2 4H* +0; +4e’2H0

Fig. 5.9 Water-line Corrosion Occurs Just Underneath the Meniscus and
the Water Level
Corrosionoccurs at the anodic part, the metal just below the water leve
The cathodic area completely unaffected by corrosion.
0.24. Write short note on intergranular corrosion.
Ans. Intergranular corrosion occurs due to the structures and detècs
due to metal surface heterogenity. In most of the metals, there are
surti
boundaries meeting the surface at different places. The plane of metalseparaeV
will usually be low atomic density plane in which various grains are
by kinks. The grain boundaries become active conditionsaN
under certain
localized attack at these boundaries causes corrosion. disadvantage
The hofit
this type of corrosion is that it leads to mechanical strength
metal. In some cases the metal convertreduction in due
to powder to disintegrationsUscepóbl
separate grains. One theimportant parameterthat makes the metalsl100K T
to intergranular is, heating it in the temperature range 800to arecook
metals also become susceptible to corrosion whentheya
intergranular
 Phase Equilibriumand Corrosion 155
believodthat at high temperature,the carbon is completely dispersed
his
om the metalor alloy, whereas, in the sensitizing temperature range it
rvgoh
thegrains
boundaries and form precipitate of chromium
i so
Nundaries.The chromium depleted zone nearthe grain boundary acts
andcorrosion begins.
What is galvanizing ?Galvanization of iron articles is preferred
ning Ghereason.
The process of coating steel or iron sheets with zinc, generally by
nesonina hof zinc, covered with aflux, at atemperature of 425-500°C.
bath
Galvanization (coating iron with zinc) is preferred to tinning(coating iron
ttin)dueto the
t following reason
h is more electropositive than iron, so zinccoating acts as anode, while
eOSed iron portions of coating acts as cathode. If by chance, the zinc
ting is broken at some place, the zinc (being more anodic than iron)
iergoes corrosion, protecting iron (which is cathodic) from rusting. Thus,
atack, on the iron occurs, untilpractically all the zinc has been corroded in
icinity of the exposed iron spot. So zinc coating protects iron 'sacrificially'.
On the other hand, tin is a noble metal (.e., having higher electrode potential
n iron), so it protects the iron due to its higher corrosion resistance than
Such acoating provides effective protection to iron only when the coating
completely continuous. If tin coating is punctured, much more corrosion
nages can be done to the base metal (iron) than to iron metal without it.
e tin becomes cathode, while the exposed iron (which is above tin in the
ctomotive series) acts as anode, so a galvanic cell is set-up, thereby an
eIse corrosion at the small exposed iron part occurs.
Q26. Write a brief note on passivity.
Ans. Passivity is the phenomenon in which a metal or an alloy exhibits a
higher corOsion resistance than expected from its position in the
Scltochemical series. Passivity is the result of the formation of ahighly
loy,ective, but very thin and quite invisible film on the surface of metal or an
which makes it more noble. This film is insoluble, non-porous and of
a'self-hoxiealdinizging
osure to , nature that when broken it will repair itself on the re-
conditions.
Examples of passive metals and alloys are-Ti, Al, Cr and awide variety
slsanceainlessin steel alloys, containing Cr. These exhibit outstanding corrosion
Seically,theactoxiidvie.zingBasedenvironments,
on experim ent , conducted in areated 0.5 M NaCI
but in reducing environments,they become
passivity of certain metals talls in the
following order -
’ACr’Cu.
SPb’ Be-’ Mo ’> Mg’ Ni ’ Co’ Fe ’ Mn > Zn ’ Cd
156 Engineering Chemistry
(R.GPV.,
E
Passivity is not a constant state, but
exists only in certain
condition, which tend to maintain thin protective oxide B.Tech., -Year
repairedenvironsumrteacnets.
In the presence of oxygen, the oxide
film is
any damage occurs, but in oxygen absence,automatically
films on their
the passive
example, wheneavleonys
become chemically active and are rapidly corroded. metals
stainless steels are quite good resistant to the action For
of
and
the:
acid, but show low resistance in air-free acid.
The action of more
concentrated solution of HNO, On
aerated dilute asuulsptheunrtiicc
and Al) produces athin protective oxide active
film, thereby
and making them passive. stiflingthe anodicmetCals (Fe
0.27. Describe the
mechanism, causes andprevention Teaction
Ans. Pitting corrosion is, ofpit ing
(R.GP.V., Junecorro2016)sion,
usually due to the breakdown or
protective film on a metal at specific of the
points. Pitting corrOSion is a localivel
accelerated attack, resulting in the formation of cavities around
cracking
metal is relatively which
unattached. Therefore, pitting
formation pinholes, pits and cavities in the corrosion results in the
of
formation of small anodic and large cathodic metal. This gives rise to the
film may be caused by - areas. Breakdown of theprotectiye
() Surface roughness or
(ii) Scratches or cut non-uniform finish
(iii) Sliding under load
edges
(iv) Alternating stresses
(v) Local straining of metal
(vi)) Chemical attack due to non-uniform stresses
(vii) Impingement attack.
Fe2+ + 2OH-’ Fe (OH)2
Corrosion Product
More Oxygenated Cathode ¬
rMore Oxygenated Cathode
H,0+0, +2e’ 2OH
H;0+0,+2r ’ 20

Iron \Pit Anode is Attacked

Fe’ Fe2+ +2e

Fig. 5.10
A pure and homogeneous metal with a highly polished surfacewillbe
much more resistant to pitting than defectsand
the one with inclusions, knowt
a rough surface. many materials
Surface cleanliness and selection of proper
Sianh2024) Phase Equilibrium and Corrosion 157

resistantto pitting in the given environment are the usual methods to

e
nhetthisproblem.
Owingtothe differentialamount of oxygen in contact with the metal (see
S10),thesmall part (underneath the impurity) become the anodic areas
thesurrounding large parts become the cathodic areas. Intense corrosion
Ctart just underneath the impurity. Once a small pit is formed, the rate of
increased.
rOsion will be
0.28. Exxplain pitting corrosion and stress corrosion.
(R. GPV., June 2022)
Ans. Pitting Corrosion Refer to ).27.
Stress Corrosion Stress corrosion is the combined effect of static
sile stresses and the corrosive environment on a metal. Stresses that cause
ncking result from residual cold working and quenching, welding, thermal
atment or due to applied loads during service. In such cases, the metal
nder stress become more anodic and tend to increase the rate of corrosion.
for stress corrOsion to occur presence of tensile stress and a specific corrosive
vIronment are necessary. The corrosive agents are highly specific and
selective for example-()) caustic alkalis and strong nitrate solution for mild
el, (i) traces of ammonia for brass, and (iii) acid chloride solution for
Sainless steel.

Cathode Cathode
Grain B

Aqueous
Solution
Stressed Part Anodes
Grain Boundary
Anode) where (Strained Part)
where Corrosion
Corrosion Takes Takes Place
Place
Grain A

Cathode
Fig., 5.1I Stress Corrosion
This type of corrosion involves in a localized
0Crring along narrow paths, forming anodic areas with electrochemi
respectcalto corrosion,
the more
cathodic
Tesult in
areas at the metal surface. Presence of stress
zones of higher electrode produces strains, which
localized potential. These become so
chemiticnalgly
Tesul
active that they are attacked, even by a mild
in the formation of a crack, which grows andcorrosive environment,
(perpendi
Stop, aftercular
to the operating tensile stress), until
progressing a finite distance.
propagates
failure in a plant
occurs or it may
rosion.
type taking Oxygen) electronsand as
conducted further.
reactions
convert environment, 1Ons,
electL. rons the reGe Chemi
158 sty
aso Eroieening
given BThis
example,For anodic Ans clectrroemaichemins cal acts and
asacts Ans
Such placeneutralIn the Thus, 20 0.30, anodi c .
hydrogen below to(i) Fe Fe(s) cathodethe at continues the to and
Electrochemi completely
corrosicalon Mrite asThis Q20
type wil the metal extentthe In M(s)
Liberation (b)of deposit (a) Cathodic
- (aq) > environment Anodi c
cathodi c So is
cathodic, Corrsion
of 2H,0' and
be (aq) Cu'(aq) cathode t
case
hey
ar e the because
corrosion 2H,0 alkaline ions Electroplating + Fe ions as ’ mechaniunafsm fecorcterdosion, of
on Reactions 20H (aq) formed long M"Reactions areas. anodi c while
+ acquire the and electrons
mop-upwhich of the the wa te r
2e’H,1 in + are (aq) + corrOSion as(aq) and of whi le part
media + 2e’cathode 2e the at not the + pass by fu l y area fil ed
which 2e-’clectrons
Hydrogen - responsible
- -’ ne - electrochemical hele ow above
(btimmersed
and - electrons
removed, th e At involves corosion. steel
hydrogen Cus)surface. At TheFe(OH), anode of (oxidation) into the
theis
inH,(g) the
cathodic thwater-line e tanks
the theanode.
20H +absencethe and cathode, electrons for into and flow corroded water. part
+ metal solution partr-ine) occurs
i
is 2H,0hydrogen
0) acidIn the insoluble
flowingcorrOsion removedions are the corrosion.
of is
anode electron-current (above poorly B.Tech.
(RGPV, E
evolved the various released metal
in (R.GPV., 2015%
Dec. below
(in gets
of solution, the highly-oxygeh
gas met al from tlhe
is oxygen, cathodic corrosiondepends will form atoms he
known is ions at the water-ine water-ine
formed. the (oxidain not of
asbyd the abyence collste reactin anode anodeupon postve be
rescio produg procelr
a ate
impossible
corrosion non areas pools legscorrosion use lo shoulddissimilar and that ansolution
the
their active
absence given
Corrosion Ans,
the Explain0.32.the Ans.
stressed of or to a be dectrochemical .31. Such
action is etum,
structure permit
dam more cathodic should Refer
at inserted metals oxidation metal below cont(i) rol. Thereof type the gianb2024
control or(i) e of
Cathodic areas () thactive (e) (d) (c) (b) isthis (a) Design What ? of reaction
impractical (g)arcas. free expense
If be - are I
to fcorrosion o4H" (2) 2H,0 (1In
)
to A As Ifa
between If are
material The used, avoided. The corrosion
Q.1 5 is
may ofproper circulation third contact potentials tw oprinciple, Somevarious In Formation
stress.
the far structure toanodic contact and corrosion +0; is
and
be metal. as of be metals it corrosion
methods of and presence +0; presence
metal them in should involving
to Suchdesign possible, the ofshould Corrosion Material methods the by
achieved Anodic
change consists dissimilar contact. material are while of Q.3 0. + +4e of
of third in to are different important oxygen 4e of
a air contact reduce as be of of Hydroxyl of
areashould have near to the
insulated ’dissolved f dissolved
Protection
bythe e
and thmetal. be is Selection fo r metals oxygen 2H,0 ’
could ofmetals should
as as in localized less absorption. 40H Phase
cathodicnature equipment
prevent prevent o
tsowcurrent contact, metals protecting
small possible. active methods lon-
setup dissimilar
that is have from
is oxygen oxygenEquilibrium
unavoidable,
flow in - control. Describe known
the the area they onthImportant
e
otection the of In- condition
a should as more metal are and
situation galvanic formation structure and as the
presence metal (R.GPV,
2017)
June as
s and
corrosion metals, large should discussed Oxygen in and
attack possible cathodic remains more the in
subjecting be suitable area design from acid neutral
or is it on be active ofa mechanism type Corrosion
by where couple supported as
corrosion
of saved is the protected.
so medium,
medium,
anodic stagnent beneficial insulator whenpossibleas metals. corrodingprinciplesfollows corosion. or
anode. selected metal alkaline
it with some trom 159
is on two the
If in - or of
T60 Engineering Chemistry (R.GPV.,
B. Tech.
protection. When electrical current flows between the anodic or ,1Yean
areas on a coroding metal surface, the higher the current, the
faster will bethe corrosion at the anode. The rate of corrosion greatcatehrodiandc
can
be
by imposing additional current on the metal using an external controled
opposing current is applied to nullify corrosion, it is known ciasrcuit. If an
protection. If the potential of the metal is so adjusted that the cathodic
appreciably suppressed because the metal is rendered passive, then corrOSi
it is on
is
as anodic protection. The principle involved in cathodic protection istoknown
f
the metal to behave like a cathode. Since there will not be any anodic are
the metal, corrosion will not occur. While in anodic protection, metal is
passivated by applying current in a direction that renders it more anodic
(ii) Inorganic Non-metallic Coatings-The inorganic non-metallie
protective coating include surface conversion or chemical dip coating, anodizai
oxide coating and vitreous enamel coating.
(v) Metallic Coatings -Metallic coatings are mostly applied on iron
and steel because they are the cheap and most commonly used construction
materials and are also the most susceptible ones for corrosion. The metallic
coatings often used are of Zn, Sn, Ni, Cr, Al, Pb and Cu. In general, the
following methods are used for metallic coatings
(a) Electroplating (b) Metal cladding
(c) Metal spraying (d) Hot dipping
(e) Vapourising () Cementation.
() Organic Coatings - Protection ofa metal surface from comosi0n
by using organic protective coatings is an established practice. Imporant
organic protective coatings include paints, varnishes, enamels and lacques
inert barriers
When applied on cleaned metal surfaces, they act as effective
which not only protect the metal from corrosion but also afford decor
and aesthetic appeal.
natureof the
(vi) Modifying the Environment - The corrosive constituents
environment can be reduced either by, (a) the removal of harmful effectof
or (b) the addition of specific substances, which neutralizethe
corrosive constituents of the environment. corrosion
0.33. What is corrosion ? Discuss various theories toexplain
with suitable examples. How corrosion may be prevented June 2020
2018,
(R.GPV, Now.
Ans. Corrosion - Refer to Q.15.
Theories of Corrosion - Refer to Q.20.
Prevention of Corrosion - Refer to Q.32.