1. Corrosion!
Corrosion is the gradual destruction of material,
usually metal>>this mean oxidation of metals with
oxygen.
Corrosion occurs in the presence of moisture. For
example when iron is exposed to moist air, it
reacts with oxygen to form rust (Fe2O3.XH2O).
i.e., rust is hydrated iron (III) oxide
2. Corrosion is a natural process…..tends to
the lowest possible energy states.
e.g: Iron has a natural tendency to
combine with other chemical
elements( such as oxygen) to return to
lower E states:
3.
4.
5. WET & DRY CORROSION
Wet / aqueous corrosion is the major form of
corrosion which occurs at or near room
temperature and in the presence of water
Dry / gaseous corrosion is significant mainly
at high temperatures
7. The hot dip galvanizing process
Loading-jigging
The steel items need to be supported during the hot dip
galvanizing process.
Can the item be easily jigged?
Will touch marks be a problem?
Solutions and molten zinc must be able to flow into all
sections and drain off all sections.
Steel items must be designed to prevent air,solutions and
molten zinc becoming trapped
8. The hot dip galvanizing process
Degreasing
A hot (90 C) caustic bath is used to remove oil,grease,paint
another organic compounds
If these contaminating materials are not removed,the next
stage of the process(pickling) is affected
Check if any paint is on item-some paints are not easily
removed and may require abrasive blasting before galvanizing
some pipes and pipe fittings have a black varnish coating that
cannot be removed in the caustic bath.
9. The hot dip galvanizing process
Acid pickling
the steel items are immersed in hydrochloric acid to remove-
rust , mill scale and other metal oxides
The steel surface must be perfectly clean of these oxides for
the molten zinc to react with the steel
very heavy rust may not be easily removed by pickling badly
rusted items should be abrasive blasted or mechanically
cleaned first.
10. The hot dip galvanizing process
Pre-fluxing
steel items are water rinsed after pickling and
immersed in hot (70-80 C)zinc ammonium
chloride(ZAC)solution.
The ZAC solution conditions the clean steel surface
ready for hot dip galvanizing
Good pre-treatment = good quality galvanizing
11. The hot dip galvanizing process
Hot dip galvanizing
The steel is immersed in molten zinc(temp 450 C)
The clean steel surface reacts with the molten zinc to form a zinc-iron alloy
which is very strongly bonded to the surface.
The hot dip galvanized coating forms in 3-5 minutes , depending on the
steel thickness.
12. The hot dip galvanizing process
Quenching
After hot dip galvanized steel item is removed from
the galvanizing bath , it is immediately quenched in a
sodium dichromate solution
The dichromate quenching cools the item so that it
can be quickly handled and conditions the surface of
the galvanized coating to maintain its bright
appearance.
13. The hot dip galvanizing process
Characteristics
Typically 65-150 microns thick (1mm=1000microns)
Zinc iron alloy layers make up 80% -100% of the coating.
The zinc-iron alloy layers are harder than 250 grade steel
Micrograph of hot-dip galvanized coating
14. The hot dip galvanizing process
Coating comparisons
Compared to zinc electroplating(ZEP) and continuous
galvanizing (CG),hot dip galvanizing (HDG) is better
because….
-HDG is thicker and will last over 50 years or more without
rusting
-HDG is harder and is 5 times more abrasion resistant than
ZEP and CG
-HDG coats all external and internal surfaces and edges . All
CG coatings have cut , exposed edges after processing.
16. Corrosion removal
It is possible chemically to remove the
products of corrosion to give a clean
surface. For example phosphoric acid.
Phosphoric acid may be used as a "rust
converter", by direct application to rusted
iron, steel tools, or surfaces. The
phosphoric acid converts reddish-brown
iron(III) oxide, Fe2O3 (rust) to black ferric
phosphate, FePO4.
17. Corrosion of metal can be prevented if the metal is
coated with something which does not allow moisture
and oxygen to react with it.
Coating of metals with paint, oil, grease or
varnish prevents the corrosion of metals.
Coating of corrosive metals with non-corrosive
metals also prevents corrosion. Some of the
methods by which metals can be coated with non-
corrosive metals are:
18. Electroplating
Electroplating is a plating process in which metal ions
in a solution are moved by an electric field to coat an
electrode.
The process used in electroplating is called
electrodeposition. It is analogous to a galvanic
cell acting in reverse. The part to be plated is
the cathode of the circuit.
19. Electroplating
Electroplating: In
this method of a metal is
covered with another
metal using electrolysis.
Silver-plated spoons,
gold-plated jewelry, etc,
are electroplated.
Silver electroplating cell>>>
The anode is a silver bar
and the cathode is an
iron spoon>>>
27. Polymer,heal thyself
A new polymer-based
system that can heal it self
when it becomes
damaged….the material
relies on network of vessels-
similar to blood
capillaries>>that carry a
healing agent to areas that
damaged.
28. Polymer,heal thyself
The healing agent –monomeric
dicyclopentadiene….fill into 200
micrometer channels.
the catalys:
Benzyldienebis(tricyclohexylphosphine)dichlo
rorutheniumincorporated into the outer
coating.
29. When a damage occurs at the coating,
healing agent wicks from the
microchannels into the crack(s) through
capillary action>>>then the healing agent
interacts with the catalyst particles in the
coating to initiate polymerization ,
rebonding the crack faces autonomically
30. The reaction between monomer and catalyst
to produce solid compound
After a sufficient time period the cracks are healed and the
structural integrity of the coating restored.
31. The catalyst lowers
the energy barrier
of the reaction and
allows the
monomer to
polymerize without
the addition of heat.
32. There are many challenges in designing this type of material:
1) the reactivity of the catalyst must be maintained even after
it is enclosed in wax
2) the monomer must flow at a sufficient rate (have low
enough viscosity) to cover the entire crack before it is
polymerized
3) the catalyst must quickly dissolve into monomer in order
to react efficiently and prevent the crack from spreading
further.
