Corrosion

1. di INTRODUCTION -, ••..
~ oon as the metals are extracted from their ores, the reverse process also starts i.e., nature tries to
E,
1
:ert them back into the form in which they occur. This is due to the chemical attack of the gases
cor h f: f h 1 · · ·
present in the atmosphere on t e sur ace o t e meta converting 1t into compounds such as oxides,
sulphides, carbonates, sulphates etc. It can also take place due to electrochemical interaction ofinetal
,,ith the environment.
The process of slow destruction of metal due to chemical or electrochemical attack ofthe
environment at their surfaces resulting into the formation ofcompounds such as oxides, carbonates,
sulphides, sulphates etc., is called corrosion.
The most common example of corrosion is the rusting of iron. Rust is hydratedferric oxide,
Fe2
03
xH2
0. Some other examples include tarnishing of silver, development ofgreen coating ofbasic
carbonate [CuCO3
+ Cu(OH)2] on copper and bronze etc.
Corrosion occurs at the surface ofmetal and is a slow process, but its ill effects are numerous. It
destroys metallic equipments, machines and instruments and reduces their efficiency. It is, therefore,
important to study its mechanism and measures to control it to minimize the loss due to corrosion.
Inthis chapterwe would learn types ofcorrosion,their mechanism ofoccurrence, factors effecting
corrosion and measures which are usually adopted for their prevention.
305

2. I
- I A TEXTBOOK OF ENGINEERING CHEMISTRY
DI TYPES OF CORROSION ---......
. b low in the fonn ofa table. The shaded areas re
Various types ofcorrosion processes ar~ ~ive~ t:the types related to each main type. Present
the main types, which have been sub-d1v1des 10
Corrosion
Dry or
(Chemical)
Wet or
(Electro-chemical)
. r:=1s
.
1
Other
Atmosphenc LJ forms
By O er Liquid
Intergranular Erosion Microbiological
oxygen gases metal
Galvanic Pitting Crevice Concentration
cell or
differential
aeration
Waterline Stress
The details of only some important types will be discussed here.
■:fl DRY OR CHEMICAL CORROSION
The direct chemical action ofatmospheric gases such as oxygen, halogens, sulphur dioxide and
hydrogen sulphide with metals resulting into the formation of compounds such as oxides, halides,
sulphates and sulphides is referred to as chemical corrosion.
It is mainly ofthe following three types.
(a) Corrosion by oxygen (oxidation corrosion): Atmospheric oxygen reacts with metals in the
absence ofmoisture fonning their respective oxides.
Alkali metals (Li, Na, K, etc.) and alkaline earth metals (Be, Ca, Sr, etc.) are oxidized at low
temperatures, whereas all other metals (except, Ag, Au and Pt) are oxidized at high temperatures.
Mechanism: The process ofoxidation starts at the exposed surface of metal. The metal loses
electrons and changes to metal ions, whereas oxygen accepts the electrons and changes to oxide ion.
overall reaction
2M ~ 2M11
+ (Metals ions)+ 2ne-(loss ofelectrons, oxidation)
n
20 2 + 2ne- ~ no2
- (Oxide ions) (Gain ofelectrons, reduction)
2M+!!:..o ~ lM11
+
2 2
+ tno2- ~ MO
2 n
Metal ion Oxide ion (Metal oxide)
This oxide layer formed acts as a barrier towards further oxidation. The continuation of the
oxidation process depends upon two factors: (i) the nature of the oxide film formed (ii) the rate of
diffusion of the metal ion and the oxide ion through the layer formed. '

3. ------~--=--~~~----------~C=O~R~RO~S~IO~N~A~N~D~ITS~C~O~NTR~O~L~·
(i) Nature of oxide film formed on the surface
(a) When the oxide film is stab/ t' ht] dh . .
. . e, ig Y a ermg and impervious in nature, 1t acts as a
protective coatmg and furthe t· f • · d
. r ac 10n o atmosphenc oxygen on the metal 1s prevente •
For example, oxide films formed on Al, Cu, Sn and Pb are of this type.
(b) When ~he film formed is unstable and decomposes back to metal and oxygen, oxidation
corroswn can not continue further. Metals like Ag, Au and Pt form this type of oxide
films.
(c) Wh~n the volatile.(i-lm isformed, it gets volatilized immediately making the metal surface
available for contmuous attack. For example, Mo forms volatile oxide and it is corroded
completely with passage of time.
2Mo + 302 ~ 2Mo03
(Volatile)
(d) When the oxidefilm is porous in nature, the metal is attacked by oxygen through the pores
and cracks and the corrosion continues till the entire metal is converted into its oxide.
Piling-Bedworth Rule: This rule is helpful in deciding whether the oxide film fom1ed on the
metal surface is porous or non-porous. According to this rule:
"An oxide film is non-porous i.e., protective, if the volume of oxide is at least equal to or
greater than the volume of metal from which it is formed. However, if the volume of the oxide is
less than the volume of the metal, the oxide film is porous i.e., non-protective."
Metals like Al form oxide having greater volume as compared to the metal, so they form
non-porous oxide layer and further oxidation of the metal is prevented. On the other hand, alkali and
alkaline earth metals form oxides having lesser volume as compared to the metal, they form porous
oxide films which allow further attack of atmospheric oxygen through the pores and the metals are
completely corroded.
(iz) Rate of diffusion of metal ions: The metal oxide film formed at the surface acts as an
interface. For the oxidation to continue further, the metal ions (Mn+) must diffuse outwards
and the oxide ion (02- ) inward through the interface for further action benveen them. This
happens through the pores ofthe porous film only and not through the non-porous film. The
metal ions being smaller in size as compared to the oxide ions diffuses outward at a faster
rate than the oxide ions diffusing inward.
(b) Corrosion by other gases: Besides oxygen, gases like Cl2, S02, H2S, CO2 and F2 also react
with metals. The corrosive effect of these gases depend upon their chemical affinity for the
metal and also upon the nature ofthe compound formed by them i.e., whether the film formed
is protective or non-protective. For example, Cl2 reacts with silver forming a protective (non-
porous) film ofAgCl which prevents further attack of Cl2 on Ag. But Cl2 forms a volatile
chloride oftin. Similarly, H2
S gas at high temperature reacts with iron forming a porous FeS
film and corrodes iron.
(c) Liquid metal corrosion: When liquid metal flows over solid metal at high temperature,
corrosion takes place which is called liquid corrosion. It weakens the solid metal due to
(i) penetration of the liquid metal, (ii) dissolution of the solid metal in liquid metal.
A common example of this type of corrosion is the corrosion of cadmium rods taking place
due to sodium metal acting as coolant in nuclear reactors.