MODULE 3

CORROSION SCIENCE AND METAL FINISING

Introduction:
When the metal comes in contact with gaseous or liquid environment [or surrounding] they undergo a
spontaneous, but gradual destruction which starts at the surface of metals. In chemical terminology destruction
is known as corrosion.
Explain rusting of iron based on electrochemical phenomenon.
According to electrochemical theory, corrosion of metals take place due to the formation of anodic and
cathodic region on the same metal surface or two different metals are in contact with each other in the presence
of a conducting medium. At the anodic region oxidation and at the cathodic region reduction takes place. Metal
undergoes corrosion at the anode region and cathodic region are unaffected by the corrosion
b) Anodic reaction;- At the anodic area, oxidation take place m mn+ + ne -
2+
Resulting in the corrosion of iron. Fe Fe + 2e -
c) Cathodic reaction ;– The electrons flow from the anodic to cathodic aera and cause reduction. There are
three possible ways in which the reduction can take place.
i) In neutral or alkaline medium and the presence of oxygen , hydroxide ions are formed
O2 + 2H 2 O +4e 4OH -
ii) In neutral or alkaline medium and in the absence of oxygen. 2H 2 O + 2e H2 + 2OH-
iii) In acidic medium and in the absence of oxygen , 2H+ + 2e - H2
-
The metal ions produced at the anode combines with the OH ions to form the corresponding metallic
hydroxide, which undergoes aerial oxidation to form iron oxide
Fe Fe2+ +2e-
Fe2+ + 2OH- Fe(OH)2 Ferric hydroxide
4Fe (OH )2 + O2 + 2H2O 2{Fe2O3 3H20} Iron oxide
Galvanic Series
The galvanic series is a chart showing the relationships and a
guide for selecting metals that can be joined, with an aim of
helping in the decision-making process. This is done by
illustrating which materials have a minimal tendency to a
galvanic interaction, or the need or even degree of protection
that can be applied to reduce the expected probable
interactions. In general, the further apart materials are, the
higher the level in the galvanic series, and hence the higher the
galvanic corrosion risk. The galvanic series serves as a simple
qualitative guide only, and does not give any information on
the galvanic corrosion rate.
Differential metal corrosion
When two dissimilar metals are in contact with each other in a
corrosive conducting medium. The two metals differ in their tendencies to undergo oxidation; the one with
lower electrode potential or the more active metal acts as anode and the one with
Higher electrode potential or the less active metal act as cathode the potential difference between the two metals
is the cause corrosion. The anodic metal under goes corrosion and the cathodic metal is protected from
corrosion. A anodic reaction A cathodic reaction
2+
Fe Fe + 2e O2 + 2H2O + 4e 4OH-

Zn metal Fe metal Fe metal Cu metal

Anode Cathode Anode Cathode

Exampals; 1.Steel pipe connected to copper. 2. Zinc coating on maild steel. 3. Tin coatingon copper vessels.

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Differential aeration Corrosion
Corrosion of a metal due to difference in air or oxygen
concentration on its surface is called differential aeration
More
aerated corrosion.When a metal is exposed to different oxygen
concentration. The part of the metal exposed to higher oxygen
concentration acts as cathodic region and the part of the metal
exposed to lower oxygen concentration acts as a anodic region.
Less
Consequently, the poor oxygenated region undergoes corrosion.
aerated
The reactions may be represented as follows:
Water
At anode : Fe → Fe + 2 e- (oxidation)
2+

At cathode : 1/2 O2 + H2O + 2 e- →2 OH- ( Reduction)

;Fe2+ + 2 OH- → Fe(OH)2
2Fe (OH)2 + 1/2 O2 + H2O → 2 Fe(OH)3 → Fe2O3+ 3 H2O→Fe2O3 .x H2O(rust)
Examples
1.Windows rods inside the frame suffer corrosion but not the exposed region
2.paper pins inside the paper gets corroded, and the exposed part is free from corrosion
Factors affecting the rate of corrosion [Factors influencing the rate of corrosion]
The rate of corrosion depends on the following factors.
a) Nature of metal b) Electrode potential c) Nature of corrosion product
d) Relative areas of anode and cathode e) Nature of environment f) PH of the medium g) Temperature
Electrode potential;
1.In general the metal with lower electrode potential values are more reactive than the metal with higher
electrode potential values,
2. The more reactive metals are more susceptible for corrosion, thus the tendency of metal to undergoes
corrosion decreases with increase in electrode potential. For example,
3.The active metals like k, Na, Mg, Zn etc, with low electrode potential values are highly susceptible for
corrosion,
4.The noble metal such as silver, gold, platinum etc, with higher electrode potential values are less susceptible
for corrosion. However, there are few exception for this general trend as some metal show the property of
passivity.
Nature of corrosion products;- The rate of corrosion depends on the nature of corrosion product which is
formed during corrosion. For example- In case of metal like iron, zinc and magnesium, The oxides which are
formed during corrosion are, highly porous, Non-stoichiometric, highly soluble and good ionic conductivities,
As a result, corrosion is continued till the metal is completely destroyed. On the other hand, In case of metal
like aluminum, titanium, chromium etc, The oxides formed are non-porous, stoichiometric, highly insoluble
with low ionic and electronic conductivities, As a result , the corrosion is stopped as soon as oxide film is
formed.
Anodic and cathodic area;- Smaller the anodic area ,larger the cathodic area faster will be the rate of
corrosion, conversely larger the anodic area, smaller the cathodic area slower will be the rate of corrosion.
If an iron sheet is coated with tin is not properly coated, then tin is cathodic with respect to iron, i.e. iron is
more reactive, the small anodic area has to supply electrons to large cathodic area for corrosion takes place,
therefore corrosion rate is faster.
On the other hand, If iron sheet is coated with zinc is anodic with respect to iron, Even of the part of plated Zn
peels off, intense corrosion is not observed because it has small cathodic area, large anodic area can supply
electrons to small cathodic area and therefore rate of corrosion is slow which shown in fig (A) , (B) and (C).
pH of the medium: In general, lower the pH of corrosion medium, higher is the corrosion rate. (Exception:
Metals like Al, Zn etc. undergo fast corrosion in media with high pH.)
Iron does not undergo corrosion at pH greater than 10. This is due to the formation of protective coating of
hydrous oxides of iron. Between pH 10 and 3, the presence of oxygen is essential for corrosion. If the pH is less
than 3, severe corrosion occurs even in the absence of oxygen due to continuous evolution of hydrogen gas.
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Temperature;- In general increase in temperature increase in corrosion rate, For many metals the corrosion rate
almost exponentially rises with increase in temperature. This is attributed to
1)Increase in conductance of the medium - ions migrate fast at higher temperature.
2) Increase in the rate of chemical reaction 3) Increase in solubility of the corrosion products in the medium
4) Decrease in the polarization effects at the anode and cathode sites, However ,if corrosion is due to dissolved
gases like O2, CO2 etc. The rate of corrosion decreases with increase in temperature as the solubility of these
gases in the medium decreases.
CORROSION CONTROL
Metal coating ; Galvanization and Tinning
Hot dipping – (Galvanization) It is defined as coating anodic metallic layer over the surface of base metal.
Eg ;- Galvanization;- is a process of coating Zn over the surface of iron. The method is carried out by hot
dipping process, It involves the following steps.
1) The iron sheet is washed with solvent to remove
greasy & oily materials.
2) The iron sheet is washed with dil H2SO4 to remove
oxide film.
3) The sheet is then treated with a mixture of aqueous
solution of zinc chloride and ammonium chloride &
dried
4) The dried sheet is dipped in molten zinc at 430 –
450 0 c
5) Excess Zn is removed by rolling, wiping or air
blast.
Application;-1) Galvanized iron pipes are exclusively used for domestic purpose all over the world 2)
Galvanized iron drum are used to preserve variety of chemicals, drugs & organic solvents.
3. Used in Roof sheets, fencing wires, barbed wire, buckets, pipes, etc

Cathodic metal coating (Tinning);-

It is defined as coating of cathodic metallic-layer over the surface of base metal. This type of coating which are
cathodic to the base metal such as tin, nickel, chromium & copper on Iron.
Tinning;-is a process of coating tin over the surface of iron. Coating of tin over iron is done by hot dipping
process.
HOT DIPPING PROCESS:- The uniform coatings
of tin on iron is called tinning. This method is
superior because tin is a time tested metal and is non
toxic, giving a uniform coating of tin on iron is very
simple and easy and does not undergo corrosion 1)
The Iron sheet is washed with solvents remove
greasy oil materials
2) The iron sheet is washed with dil H2SO4 to
remove acidic film and heated at about 80oC
3) The sheet is then treated with a mixture aqueous
solution of zinc chloride and ammonium chloride &
dried
4) The dried sheet is dipped in molten tin followed by dipping in palm oil, excess of tin is removed by rolling
or wiping
Applications;
1. Manufacture of food storage cans,
2. Making of tin coated copper sheets for cooking utensils

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 Inorganic coating;- The coating are provided by converting the metal surface in to an inorganic compound like
oxide or phosphate either by chemical or electrochemical method is called inorganic coating are called as
chemical conversion coatings. The important inorganic coatings are Phosphating and anodizing
Anodized coating or anodizing
The method involves developing an oxide layer on the metal surface by induced oxidation in an oxidizing
environment, making the object as anode and passing current. This method is mostly done on non ferrous metal
like Al, Zn, Mg, Cr, Ti etc. It is extensively done on aluminum and its alloys. Anodizing of aluminum
involves;-
1) Cleaning the article to remove oil / grease and rust by usual methods.
2) Dipping the article in 3-10% chromic acid solution ( Even H2SO4, phosphoric, oxalic acid or boric acid)
maintained around 40oC

Making the article anode as & applying a voltage of above 40oC,

with Pb or steel as cathode. The net reaction that occurs during
anodizing of aluminum can be represented as follows.
2Al + 3H2O AL2O3 +3H2
Aluminum oxide deposits on the surface of the object, its
thickness can controlled by adjusting the current and the bath
solution. The oxide coating is slightly porous. The pores are
covered by a process known as sealing. Sealing involves dipping
the anodized aluminum in boiling water, Al2O3 gets hydrated to monohydrate form Al2O3H2O Which occupies
more volume and hence covers up the pores giving better corrosion resistance.
Uses;
1. Anodized articles are used as soap boxes, Tiffin carriers, window frames etc,
2. Anodizing is also used to prevent galling of threaded components.
Cathodic protection
Sacrificial anodic protection;-
The metallic structure to be protected is connected to
a more anodic metal using a metallic wire, The more
active metal gets corroded, while the parent structure
is protected from the corrosion. The more active
metal so employed is called sacrificial anode, As
the anodic metal sacrifices its life to protect the base
metal from corrosion is called sacrifical anodic
protection. The commonly used sacrifical anodes are Mg, Zn, Al etc this method is generally used for the
protection of buried pipe lines ship hulls, water tanks
Impressed voltage method ;- In this method, current is applied from apposite direction to nullify the corrosion
current and convert the corroding metal from anode to cathode, Usually, the impressed current is derived from a
direct current source with an
insoluble anode (like graphite or
platinum

Usually, a sufficient DC current is

applied to an insoluble anode and
buried pipe in the soil and
connected to the metallic structure
to be protected (see the fig) to
increase the electrical contact between the anode and the soil, the anode is buried in a back fill such as gypsum
under the earth. Thus electrons are supplied from an external cell so that the object itself becomes cathodic and
is not oxidized. This type of cathodic protection has been applied to open water box cooler, water tanks, ships
etc.
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