unit 3 protective measures against corrosion.ppt
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Protective measures against
corrosion
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Modification of the environment
Modification of the properties of the metal
Prevention of corrosion by materialselection & design
Cathode protection
Other methods
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Modification of the environment
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De aeration
De activation
Dehumidification
Alkaline neutralization
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De aeration
consider oxygen concentration type of
corrosion
Here elimination of oxygen from aqueous
solution reduces metal corrosion
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removal of dissolved oxygen is done by
adjustment of temperature & by mechanical
aeration
This method also reduces the CO2
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Deactivation
This involves the addition of chemicals , which
can combine with the oxygen in aqueous
solution.
Sodium sulphite (Na2SO3)
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2 Na2SO3+O2 2Na2SO4
Hydrazine is good over sodium sulphiteN2H4+O2 N2 +2H2O
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Dehumidification
This process reduces the moisture content
of air to such a level
that condensation of water on surface of
the metal should be so small.
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Alumina & silica gels , preferably adsorb
the moisture from their surfaces
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Alkaline neutralization
This is meant to control acidic condition to
avoid corrosion
Acidity includes the presence of H2S, HCl,
CO2, SO2etc
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Alkaline neutralizers are used to control
corrosion
They are ammonia , sodium hydroxide etc
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Inhibitors
Anodic inhibitors
Cathodic inhibitors
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Anodic inhibitors
Alkalies ,molybdates ,phosphates , chromates
are used as inhibitors
The above chemicals react with the ions of the
anode &form insoluble precipitates
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These precipitates form as a protective
layer on metal surface
This layer acts as barrier
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Cathodic inhibitors
In acidic solutions the reaction is
2H+
+2e-
H2
Corrosion can be reduced by slowing down
the diffusion rate of hydrogen ion
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In natural solutions ,the cathodic reaction
is of absorption of oxygen type
1/2O2
+ H2
O+2e- 2OH-
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Modification of the properties ofthe metal
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1.Alloying :
The corrosion resistance property will beincreased by alloying suitable element
Chromium is the best metal for alloying steelor iron
Because the film made is self healing
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2.Refining :
Asstainless possess P,S & C
There is a chance of corrosion
Refining is a process to lower quantities of
above elements
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3. Annealing :this is a process to relieve the metal from
stresses by heating
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3. Use of protective coatings :
Anodic coatings
Cathodic coatings
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ANODIC COATINGS
In this ,the metal used is for coating is
more acidic
Anodic coatings protect the underlying
base metal
For ex: coating of Al, Cd & Zn on steel
surface
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zinc is being more anodic will get attacked
It protects iron /steel under it
Here steel works as cathode
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even if pores or breakages are seen ,still
steel will not get corroded
Because of coating of zinc
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Cathodic coatings
These coatings protect the underlying
base metal by their
Noble character &
higher corrosion resistant character
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Here the good protection is possible only
with plane surface
If pores are seen ,the corrosion will be
speeded up
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Because the coated metal behaves ascathode
Exposed metal acts as anode
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Galvanic cell will set up and localized
corrosion is seen
This results in severe pits
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Prevention of corrosion bymaterial selection & design
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Design & material selection
principles
Avoid L, T, & U shaped profiles in
constructionBecause this may cause corrosion
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weldingwelding
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welding
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Water stands here
Water slips from here
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General facts about designing
Avoid the contact of dissimilar metals in
the presence of corroding solution.
If at all 2 dissimilar metals are in contact,
they should be as close as possible in
electrochemical series.
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When ever direct contact of 2 dissimilar
metals is must
An insulating fitting may be made to avoiddirect contact
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Others
Metal cladding
Electroplating
Painting
Plastic coating
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