IB.

Tech Engineering chemistry Question with answer complete material

PROTECTIVE COATINGS AND THEIR APPLICATIONS (Unit-IV)

Q.No.1 Write a short note on a) Sheradizing b) Chromising c) Colorizing and

Cementation.
Answer: a. SHERADIZING: Sheradizing is the process of cementation, using zinc powder as
coating metal. The iron articles to be coated are first cleaned and then packed with “zinc dust” in
a drum. The drum is then sealed tightly so that oxidation of zinc is minimum. The drum is slowly
rotated for 2 to 3 hours and its temperature is kept between 350 to 3700C, either by gas heating or
electricity. During this process, zinc gets diffused into iron forming Fe-Zn alloy at the surface. At
the end of the process, the surface of the articles becomes covered with a thin film of iron-Zinc
alloy.
Application: sherardizing is used especially for small steel articles like bolts, screws, nuts,
threaded-parts, washers, valves and gauge tools. The main advantage of sherardizing is that
coating is quite uniform, even if the surface has crevices or depression. Moreover, there is
practically no change in the dimension of articles.

b. COLORISING: Colorizing is carried out by first sand-blasting the metal objects and then
heating then in a tightly-packed drum with a mixture of aluminium powder and aluminium
oxide, together with a trace of ammonium chloride as a flux. Air is excluded and in some
process, a reducing atmosphere of hydrogen is used. The layer formed has an approximate
composition of Al3Fe2, corresponding to about 25% al by weight. Colorizing is frequently
applied, especially for the protection of furnace parts.

c. CHROMIZING: Chromizing is carried out by heating together a mixture of 55% chromium

powder and 45% alumina, together with the base metal parts at about 1300 to 14000C for 3 to 4
hours. The use of alumina prevents the coalescence of chromium particles. Chromizing is also
produced by the interaction of a mixture of volatile chromous chloride and hydrogen with steel
parts at about 10500C. The diffusion of chromium into iron surface is more rapid than the above
powder methods. The process is believed to occur in three stages:
1. Fe + CrCl2 FeCl2 + Cr (Displacement)
2. CrCl2 + H2 Cr + 2HCl (Reduction)
3. CrCl2 Cr + Cl2 (Thermal decomposition)
The corrosion-resistance of chromized coatings corresponds to that of ferrite stainless steels. The
chromium content in the diffusion layers varies, generally, from 10 to 20%. The layers are
supposed to be solid solutions of iron and chromium. Chromizing process is fairly extensively
applied for the protection of gas turbine blades.

d. CEMENTATION: Cementation is the process in which base metal is packed in the

powdered coating metal and heated to the temperature just below the melting point. Resulting in
the formation of layers of alloys of varying composition. Generally an inert or reducing
atmosphere is usually maintained during this process. Due to the diffusion of coating metal into
base metal, an alloy of two different metals is formed.
It is also suitable for coating small articles of uneven surfaces and shapes such as screws, bolts,
valves and gauge tools. The coating metals used in this process such as zinc, chromium and
aluminum are those which can alloy even with iron.

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Q.No.2 Write a short note on a) Tinning b) Metal cladding c) Electroplating d) Metal

spraying

Answer: a. TINNING: The process of coating tin over the iron or steel articles to protect it
from corrosion is known as tinning. Tin is a nobler metal than iron, therefore, it is more
resistance to chemical attack. In this process, at first Iron sheet is treated in dilute sulphuric acid
(pickling) to remove any oxide film if present. A cleaned Iron sheet is passed through a bath
molten flux. Like Zinc chloride, then through molten tin and finally through a suitable vegetable
oil.
b. METAL CLADING: The surface to be protected is sandwiched between two thin layers
of coat metal and pressed between rollers. The finished product may be welded at the edges or
riveted at some points. The coat metal has to be anodic to the base metal and only plain surfaces
can be cladded. This method is used for coating Al, Cr, Ni, Duraluminium, etc.
c. ELECTOPLATING; Electroplating is the process or method of coating metals and non-
metals, to change their surface properties such as to improve the appearance, to properties such
as to improve the appearance to corrosion and wear or chemical attack. Electroplating is the
electro-deposition of metal, by means electrolysis over surface of metals, alloys or non-metals.
The wear resistance of a metal part can be improved by electroplating a harder metal on its
surface. The metals most often plated on base metals or materials are chromium, Nickel and
Rhodium. For example metals like Iron which are easily corroded by atmospheric air, moisture
and CO2 are coated electrolytically with base metals such as nickel or chromium which are more
resistant to wear or chemical attack.
Some of the applications of electroplating are,
1. Plating for protection from corrosion and chemical attack.
2. Plating for decoration.
3. Plating for special for special surface and engineering effects.
4. Electroforming
5. Plating on non-metallic materials.
Therefore, this process is widely used in automobiles, aircrafts, refrigerators, jewellery,
radios, cameras, type-writers, umbrellas, watches etc.
d. METAL SPRAYING: In this process, the coating metal in the molten state is sprayed on the
roughened surface of the base metal. The metal thus sprayed adheres t be the base metal surface.
The sprayed-coatings are continuous, but some what porous. Consequently, a sealer-oil or paint
is applied on such a coating to provide a smooth surface. The process offers certain advantages
over the other methods, due to its: 1. greater speed of work
2. Applicability to large surfaces and
3. Ease of application, even of thick coatings to restricted area. However, adhesion
strength of such coatings is, usually, lesser than those obtained by hot-dipping or
electroplating. It is, therefore, essential to have a clean and roughened metal surface on
which a proper adhesion is to be secured. The surface is, generally, roughened by sand-
blasting. Sprayed-coatings can be applied by the following 2 techniques.
1. wire-gun method is more widely used for common metals. In this, the wire of the coating
metal is melted by an oxy-acetylene flame and atomized by a blast of compressed air.
2. Powder metal method: in this, finely divided powdered metal is sucked from the powder
chamber and then heated as it passes through the flame of the blow-pipe. The blow-pipe
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disintegrates the metal into a cloud of molten globules, which are then adsorbed on the base
metal surface. This method is, however, limited to low-melting metals like Zn, Pb, Sn, et
Q.No.3 Write a short note on a) Anodizing b) Galvanization c) Phosphate coating
and Chromating.
Anodizing: The process of formation of conversion coating on a metal surface by anodic
oxidation is known as Anodizing.
This formation of an oxide coating by anodizing may be used to improve the wear resistance of
certain metals. This process is usually applied to aluminum, magnesium, zinc and their alloys.
And in anodizing, the work is the anode, and oxide layer are built up on the base metal.

Galvanization: Galvanization is a process in which the iron article is protected from

corrosion by coating it with a thin layer of zinc. In this process, at first iron or steel is cleaned by
pickling with dilute sulphuric acid solution (H2SO4) at a temperature range of 60-900C for 15
to20 minutes. Therefore, it removes scale, rust and other impurities present if any and then
washed well and dried. Then after, dipped in the bath molten zinc which is at 425-4500C. To
prevent it from oxide formation, the surface of bath is covered with a flux (NH4Cl). When the
iron piece is taken out it is coated with a thin layer of zinc. And to remove excess zinc, it is
passed through a pair of hot rollers; lastly, it is annealed at a temperature of 4500C and then
cooled slowly. For the protection of iron and steel products, it is most widely used in the form of
wires, pipes, nails, bolts, screws, buckets, tubes, roofing sheets etc.,

PHOSPHATE COATING: Phosphate coating is obtained by the chemical reaction of aqueous

solution of phosphate of iron, manganese and Zinc with base metals such as Iron, Steel and Zinc.
The chemical reaction produces a surface film consisting of manganese iron phosphates or Zinc-
iron phosphates. For increasing the reaction rate, accelerators are also added such as nitriles,
chlorates or nitrates. Application of phosphate coating on the base metal surface can be done by
brushing or spraying. This coating so not offers corrosion resistance but offer excellent bases for
painting, impregnation with oils etc. Iron and Manganese coatings are generally used to reduce
wear caused by friction. Phosphate coating can be done on other metals such as Aluminum,
Cadmium and Tin. The colour of phosphate coating is usually grey.
1. Write a short note on anodizing & galvanizing.

CHROMATING: Chromating is the process of coating a surface film with a mixture of

trivalent and hexavalent chromium for protection from corrosion.
Chromating is generally used for the protection of Zinc, Aluminum, Magnesium and Cadmium-
plated parts. They are produced by immersion of the article in a bath of acid potassium chromate,
followed by immersion in a bath of metal chromate solution. The properties of chromate coatings
are amorphous, non-porous and more corrosion resistant than phosphate coatings. These are also
sometimes used as base for paints, lacquers and enamels.

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Q.No.4. To write about surface preparations.

Answer: SURFACE PREPARATIONS:
SOLVENT CLEANING: Solvent cleaning is used to remove oils, greases, buffing compounds
and fatty substances. This involves in cleaning the surfaces by the application of organic solvents
like naphtha, chlorinated hydrocarbons (CCl4), toluene, xylene or acetone. This is followed by
cleaning with steam and hot water, containing wetting agents and alkalis. This treatment provides
a metal surface, readily wetted by aqueous solution, which is particularly required for
electroplating.

ALKALI CLEANING: Alkali cleaning is particularly well-adapted for the removal of old paint
coating from metal surfaces. Alkali cleaning agents are trisodium phosphate along with soaps
and wetting agents like caustic soda. An alkali treatment is always to be followed by a very
thorough rinsing with water and then immersion in a slightly acidic solution of 0.1% chromic
acid or sodium or potassium chromate to remove the last traces of alkalis. Alkali cleaning
method can be made more effective by the application of an electric current, and making the
“metal cathodic” in alkaline medium. The copious evolution of hydrogen at the cathode metal
results in strong agitation, which helps to dislodge the oily substances.

SAND-BLASTING: Sand blasting is used or removing oxide scales, particularly when a slightly
roughened surface is desired. Paint coatings on sand-blasted surface are, particularly, more
durable than those on metal surfaces cleaned by other methods. Sand-blasting is especially
suitable for large steel surfaces and should always be used, whenever maximum protection from
the coating is required. However, the proves required expensive equipment and the sand-blasting
operation is dangerous to health of workers, because of the possibility of disease, silicosis, unless
special protective measures are taken. The process consists in introducing the sand an abrasive
into an air stream, under pressure of 25 to 100 atmospheres. The blast is impacted on the metal
surface to be cleaned. The sand-blast impact removes any scale present and also causes a certain
degree of hardening of the cleaned metal surface

MECHANICAL CLEANING: Mechanical cleaning removes loose rust and other impurities
from the surface. This is, generally, done by hand cleaning with a bristle brush plus some
abrasive like sand and detergent like soap. Impact tools like dull chisels, knife scrapers, wire
brushes, grinding wheels and cutters are also used for removing strongly adhering scales, etc. the
remaining dust and loose particles of dirt are then removed by solvent cleaning, followed by
steam or hot water treatment.

PICKING AND ETCHING: Acid picking is more convenient method of scale removal in many
cases than mechanical cleaning and sand-blasting. It is, usually, accomplished by immersing the
metal except Al in an acid-pickling solution. Al is picked in alkaline solution.
Acid-pickling of steel is accomplished by dipping in warm dil.H2So4 or in cold HCl solution to
which some “inhibitor” has been added. For cleaning of articles made of copper, brass or nickel,
the pickling bath consists of dilute HNO3 or a mixture of dil Nitric acid and dil. Sulphuric acid.
If the cleaning operation is carried out in an efficient manner, it provides a clean, smooth surface

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for electro-deposition. Moreover, the deposit obtained is adherent, tough, and bright in
appearance.

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