296 Physical Chemistry -IV

e.g. determination
1. determination of trace metals in extracted bulk metals,
aluminium, etc.
traces of lead and cadmium in copper, alkalimetals in
2. determination of the composition of alloys, e.g., determination of chromium
in steel.
3. determination of metals other than the principal metal in metallic ores, e.g.,
determination of cadmium, lead and copper in zinc ores.
4. determination of the metal ions present in natural waters, e.g., determination
of Fet in iver and well water samples.
5. determination of electroactive organic compounds, e.g., determination of the
conjugated systems containing a quinonoid structure (say, quinone-hydro
quinone system). redox indicators like methylene blue, etc.
6. determination of components as well as impurities in pharmaceutical and bio
chemical samples, e.g., determination of vitamin Cin a sample, the alkaloid
piperine in pepper, morphine in a prepared sample, etc.
7. determination of contaminants in food samples, e.g., determination of lead in
tinned food, iodide in table salt, etc.
8. determination of the composition of the samples in medical and clinical analy
sis, e.g.. determination of nitrocompounds and benzene in the body fluids of
persons exposed to industrial fumes and vapours.
9. microanalytical determination of the metal ions in inorganic test samples.
e.g., trace determination of Cu²*, Pb*, Cazt, Znzt, etc. ions in a single solu
tion sample.
10. qualitative detection of the presence of metal ions in water-analysis samples
and test solutions.
CORROSION OF METALS
What is corrosion?
Corrosion is the process which involves the chemícal destruction of a metal as a
result of its reaction with the gases and moisture present in the environment. Tno
most familiar example for corrosion is the corrosion of iron which is called 'rusting *
Tarnishing of silver, development of a green coating on copper and bronze, etc. ao
other examples of corrosion.
Types of corrosion
Corrosion comesin many different forms and can be classified into many different
tynes on the basis of the cause of the chemical deterioration of a metal.
 Electromnotive Force 29)

From an engineer's point of view, it is convenient to classify corrosion by the forms in which
it manifests itself, the basis for thisclassification being the appearance of the corroded metal. On
this basis, at least 8 forms of corrosion can be recognised: (1) uniforn, or general attack corToSiOn,
(2)galvanic, or two-metal or dissimilar metal corrosion, (3) ocrevicecorrosion, (4) pitting,
(5)intergranular corrosion, (6) selective leaching, or partíng. (7) erosion corrosion, and (8) stress
corrosion.
Since our consideration of thetopic is from achemist's point of view rather than an engineer'S,
the above kind of classification is not considered here: only the basic chemical aspects are
considered.

On achemical basis,corrosion can be classified broadly into two types: (1) Drect
chemical corrosion or dry corrosion and (2) Electrochemical corrosion or wet
corrosion.
(1) Direct chemical corrosion or dry corrosion
This kind of corrosion occurs by the direct chemical action of atnospheric gases
such as O, halogens, H,S, SO,, Co,,SO,, N,,etc. with metal surfaces in immediate
proximity, in the absence of moisture. This type can further be subdivided into three
main types:
) Oxidation corrosion occurs by the direct action of oxygen at low or high
temperatures on metals in the absence of moisture. Alkali metals and alkaline
earth metals are oxidised even at low temperatures whereas most of the metals
(other than Au, Ag and Pt) are oxidised at high temperatures.
2M + (n/2)0, ’ 2M"* + n0
Metal oxide

Sometimes,aprotective oxide layer is formed on the surface of the metal (e.g.,

Al) which prevents further attack by O, on the metal.
(u) Corrosion by gases other than 0, 0ccurs by the direct chemical action of
atmospheric gases such as halogens, H,S, SO,, C0,, SO,, N,, etc. with metal
surfaces, the extent of which depends upon the chemical affinity between the
metal and the gas involved.
If a protective and adherent layer ofa metallic compound is formed on the
surface of the metal (e.g., whendry chlorine attacks Ag. a protective AgCl
layer is formed on Ag), further corrosion is prevented. On the other hand. ifthe
compound formed volatilizes off (e.g., the layer of SnC1, formed on the surface
of Sn by the action of dry chlorine volatilizes off), fresh metal surfacee a
exposed for further attack.
() Liquid metalcorrosion occurs due to the action of a flowing liquid metal t
high temperatures on another solid metal or alloy (e.g., in nuclear power devicecls
 298 Physical Chemistry -IV
the liquid metal may either dissolve the solid metal or penetrate into the solid
metal,thereby causing a weakening of the solid nmetal.
(2) Electrochemical corrosion or wet corrosion
This kind of corrosion occurs in the presence of water or aconducting liquid l
involves the formation of separate anodic and cathodic areas in the presence of a
conducting medium. The electrons set free at the anode flow through the metal are
consumned in the cathodic reaction; thus, coTOsion always occurs at the anodic areas
M ’ Mit + ne
Dissolves into solution state.
’ Forms a compound such as oxide.
Such corrosion can occur when a conducting liquid is in contact with the metal or
when another metal or alloy is dipped partially along with the metal in a solution.
The electrochemical theory of corrosion is briefly discussed below.
Electrochemical theory of corrosion
According to electrochemical theory, corrosion is basically an electrochemical
process. e.g., In the ´rusting of iron', it is understood that electrochemical cells are
formed on the surface of the metal in the presence O, (of the air) and water contain
ing dissolved CO,, i.e., H,C0,, which acts as the electrolytic solution.
Atone spot of the iron object (the spot in contact with water), oxidation of Fe2t
occurs and that spot behaves as anode. The electrons released move through the
metal to another spot (the spot in contact with air) and reduce O, in presence ofH*
(produced by the dissociation of H,C0,); this spot acts as cathode. The electrode
reactions and the overall reactions of the miniature cell may be given as follows:
At anode 2Fe(s) ’ 2Fe2*(ag) + 4e- (Oxidation)
At cathode O,&) + 4H"(aq) +4e’2H,O () (Reduction)
Cell reaction : 2Fe(s) +0,(g) +4H*(aq) ’ 2Fe?t(aq) +2H,0)
Fe ions move through water on the surface of the iron object and is further
oxidised by atmospheric oxygen to yield rust which is hydrated ferric oxide
(Fe,0,xH,0).
4Fe*(aq) +4H,0() +O,e)-’ 2Fe,0,(5) +8H(aq)
Fe,O,(s) +xH,0 ’ Fe,0,xH,0 (Rust)
 Electromotive Force 299

0.(g) WATER DROPLET

RUST

CATHODE ANODE
0,+4H*+ 4e
Pe’ Fe + 2e
’ 2H,0
IRON

Fig. 3.14: Rusting of iron

Corrosion monitoring

The needfor corrosion monitoring:

Metallic corrosion is a major problem in many industries. The rate of corrosion
dictates how long any process equipment can be usefully and safely operated.a
Uncontrolled corrosion can cause leaks and component failures, bringing about
reduction in both the performance and reliability of important equipment. In extreme
terms of repair
cases, corrosion can lead to unexpected failures that can be costly in
necessitates what
costs, environmental damage and potential harm to humans. This
is known as corrosion monitoring.
The meaning of corrosion monitoring:
critical
Corrosion monitoring means the comprehensive monitoring of all theinvolves
components of industrial equipment and plants for signs of corrosion. It
diagnosing a particular corrosion problem and identifying its location, rate, extent,
the
Tate-controlling. parameters such as pressure, temperature, pH, etc. as well as
underlying causes.
Methods of corrosion monitoring:
corrosivity of a
CoTOsion monitoring comprises the practice of measuring the "probes" can be
uetallic sample by the use of "probes". Corrosion monitoring give a direct
echanical, electrical, or electrochemical devices. Some techniques corosive
on the
casure of metal loss or corrosion rate while others yield information
environment.
(1) The This is the best known and the simnplest of all
Weight Loss Technique: a specimen of material
vOrosion monitoring techniques. It involves exposing duration and then
(Called the coupon")to the process environment for a given
typical monitoring program, coupons
Temoving the specimen for analysis. In abeing removed for alaboratory analysis.
are exposed for a 90-day duration before the"average" value of
Tne weight loss resulting from the exposure yields
300 Pirysioal Chemistry -1V
corrosion oocuring during that exposure and this can then be easily expres ed
as corrosion rate. The simplicity ofthe coupon technique makes;it the
method of measurement in many corrosion monitoring programs, baseline
(2) The Electrical Resistance Monitoring technique: It involves
measuring
change in Ohmic resistance of a corroding metal that is exposed to the the
environment. Surface corrosion of the metal produces a decrease in its process
crosS-
sectional surface area with a corresponding increase in its electrical
resistance.
This can be related directly to metal loss and thereby to the corrosion rate
Other sophisticated methods include the use of ultrasonic, radiographic and
electromagnetic probes.
Advantages of corrosionmonitoring:
Corrosion monitoring provides an early warning of the existence of damadine
conditions that may lead to acorrosion-induced failure of industrial equipment. The
measurement of corrosion and the consequent action for remedying high corrosion
rates permit the most cost effective plant operation to be achieved while reducing
maintenance costs associated with the operation.
Prevention of corrosion
Some of the methods used to prevent corrosion of metals are given below:
i) Barrier protection: Toprevent corrosion, the metal surface is protected from
direct contact with the surroundings by coating it with paint, oil, enamel or athin
protective layer of a corrosion-resistant metal. e.g., Rusting of iron can be pre
vented by painting the iron articles or plating them with a more corrosion-resis
tant metal like Cu or Ni.
ii) Sacrificial protection: The corrosion of ametal can be prevented by covering its
surface with a layer of a more active metal like Zn, Mg or Al. The more active
metal acts as the anode and iron acts as the cathode in the electrochemical cell
set up. The more active metal thus, loses electrons preferentially to go into ule
ionic state, thereby preventing the loss of electrons from iron. The covering metal
gets consumed in the course of time, but as long as it is present, even the nearby
exposed surfaces of iron do not rust.
The process of covering iron articles with a thin layer of zinc is called
galvanisation.
ifi) Electrical protection (Cathodic protection): The metal to be protected is cor
nected to a more active metal either directly or through a wire. e.g., For protect-
ing underground iron pipes from corrosion, they are connected to more active
magnesium or zinc plates buried in the subsoil water nearby. The iron object:acts
as the cathode whereas the protecting (active) metal acts as the anode and Joses
 Electromotive Force 301

electrons preferentially. The anode gets

consumed gradually, but as long as some .(ANODE) IRON PIPE
of it is present, the iron pipe is protected (CATHODE)
Mg
from rusting.
iv) Use of anti-rust solutions: Anti-rust so- (Mg ’ Mg +2e) (0, +4H° + 4e
lutions, which are alkaline phosphate ’ 2H,0)
and alkaline chromate solutions, are
Jused in car radiators to prevent the rust Fig. 3.15: Cathodic protection
ing of the iron parts of the engine. These
prevent the availability of H* ions and thereby prevent rusting. Further, when an
alkaline phosphate solution is used, an insoluble sticky layer of iron phosphate is
formed on the surface of iron which protects iron from the action of surround
ings.

UNIVERSITY MODEL QUESTIONS

Section A
(Questions which require short answers)
1 What is a galvanic cell ? Give an example.
2. Indicate the electrode and cell reactions in the cell :
Zn()|Zn (aq)l|Cu(aq)|Cu(s)
3. Depict the standard galvanic cell by combining Cu<Cu* electrode (E = +0.34 V) and
AulAu* electrode (E=+1.50 V).
4. Why is KClcommonly used in asalt bridge ?
5. Give an example for a cation reversible electrode.
6 Give an example for an anion reversible electrode.
7. What are redox electrodes ? Give an example.
8. What is meant by a reversible cell ? Give an example.
2 What is meant by a reference electrode ? Give one example.
10. What is a calomel electrode ?Give the electrode reaction.
11. What is quinhydrone electrode ?
12. Define single electrode potential. Can its absolute value be determined?
13. What is meant by standard electrode potential1?
14. Knowing that copper can displace silver from an aqueous silver salt solution, Write the
Nernst equation for acell constructed from Culs)|Cu"(ag:c, M) &Ag()]Ag*(aq; c, M).
15. The standard electrode potentials of some electrodes are given below:

=-0.76 V; Eo =0.34 V E =0.8 V; EH, =0 V.