A ppt on dry corrosion

1. DRY CORROSION

2. CORROSION
 Corrosion is the deterioration of a metal as a result
of chemical reactions between it and the
surrounding environment.
 Both the type of metal and the environmental
conditions, particularly what gases that are in
contact with the metal, determine the form and rate
of deterioration

3. TYPES OF CORROSION
1. Wet corrosion – It is also known as electrochemical
corrosion . Such type of corrosion is due to the flow of
electron from metal surface anodic area towards cathodic
area through a conducting solution.
2. Dry corrosion – It is also known as chemical corrosion .
Such type of corrosion takes place due to direct chemical
action of atmospheric gases and anhydrous liquids on the
metal surface.

4. DRY CORROSION
 It takes place due to direct chemical action of atmospheric
gases like CO2 , SO2 .
It is of three types
1) Oxidation corrosion :- It takes place by direct action of
oxygen on metal. It occurs in absence of moisture . It mostly
occur at ordinary temperature.

5. Mechanism:
When a metal is exposed to air it gets oxidized by loosing its valence
electrons & reduction of oxygen take place
M+n/4 (o^2) => M^n+ + O^2-
At point of contact of m^n+ & o^2- metallic oxide will form & that
metallic oxide scale forms a barrier to restrict further oxidation of
inside metal . Since size of cation (M^n+) is smaller than anion . Hence
cation will diffuse much faster than anion through the scale for
continuation of oxidation , it can be possible if the metallic oxide
barrier is sufficiently porous .
The nature of oxide film plays very important role in oxidation
corrosion.
a. When oxide film is stable and tightly adhering , it will act as
protective coating and corrosion is prevented.
b. When oxide film is unstable and has tendency to decompose back to
metal and oxygen , it does not go into oxidation corrosion.

6. c. When film is volatile then metal surface again come into
contact with air and oxidation take place
d. If film is sufficiently porous then continuous oxidation take
place.
2. Liquid metal corrosion - When a liquid metal is allowed to
flow over solid metal at high temperature is called liquid metal
corrosion.
Due to this solid metal gets weak .
Example- In nuclear reactor sodium metal is used as coolant &
it leads to corrosion of cadmium .

7. 3. Corrosion by other gases :-
Some gases react with certain metal and forms a protective
or non-protective layer on metallic surface .
Due to chemically combination of metals with gases metals
undergo corrosion .
The extend of corrosion depends upon –
A- Nature of environment .
B- Nature of oxide film formed .

8. PREVENTION
 Corrosion requires three conditions: moisture, a metallic surface, and
an electron acceptor. If you remove any of these three criteria,
corrosion is inhibited.
 Corrosion can be prevented by giving the metal to be protected a
negative charge.
 One way to protect metals from corrosion is by sacrificing a metal
with a more active reduction potential through sacrificial coating or
sacrificial anodizing.
 Developing a thick layer of corrosion in a process known as
anodizing protects underlying metals from corrosion.