corrosion part 2
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Unit IICorrosion
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Corrosion Inhibitor
Corrosion can be reduced by using corrosion
inhibitor, which is a substance when added in small
quantities to the aqueous corrosive environment,
effectively decreases the corrosion of a metal.
Inhibitors are mainly anodic inhibitors and cathodic
inhibitors.
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Anodic Inhibitors
Anodic inhibitors are the substances which reduce the
corrosion occurring at anode by forming sparingly soluble
salts with the metal ions produced. Eg. tungstates, chromates,
phosphates etc.
Metal surface adsorbs metal ions to form a protective layer to
reduce the corrosion rate. Even though the corrosion control
is highly effective, local corrosion may occur in case ofexposure of even small area.
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Cathodic inhibitorsCathodic inhibitors are mainly added to reduce the diffusion
of hydrated H+ ions to the cathode. The H+ ions formed arecombining to form hydrogen gas, which is evolved.
2H+(aq) + 2e- H2(g)
The formation of H2 gas can be controlled by increasing the
overvoltage of hydrogen evolution.
Organic inhibitors like amines, mercaptans, heterocyclicnitrogen compounds, substituted ureas, thio ureas, heavy
metal soaps reduce the diffusion of H+
ion considerably.
These inhibitors are being adsorbed at the metal surfaces andform an adherent film of metals at the cathodic area thereby
increase the hydrogen overvoltage considerably.
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However, in neutral solution, the cathodic reaction isH2O + O2 + 2e- 2OH-
Hence, the corrosion can be controlled either by removingoxygen from the corroding environment or by reducing its
diffusion to the cathodic areas.
The oxygen can be eliminated by adding reducing agents likeNa2SO3 or by deaeration. The diffusion of oxide ion is reducedby using inhibitors like Mg, Zn or Ni salts.
These react with hydroxyl ions forming corresponding
insoluble hydroxides, which are deposited on the cathodeforming impermeable self barrier.
Cathodic Inhibitors
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ElectroplatingIt is a process by which a coating metal is deposited on the
base metal by passing direct current through an electrolyticsolution, containing the soluble salt of the coating metal.
Electroplating is done both for protecting the metal fromcorrosion and for decorative purposes.
A well cleaned and properly pre treated surface of anymaterial to be electroplated is necessary for obtaining thecoating of long life.
To get an adherent coating, base metal should be free fromany dirt and extraneous matter like grease.
To get good film complexing agents, organic additives,levelers, structure modifiers and wettin a ents are added.
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Electroplating
Current density is adjusted to get an adherent film. The
electrolyte is selected in such a way that it is a good
conductor and highly soluble.
It should not undergo hydrolysis, oxidation, reduction and
other chemical changes. It should possess sufficient
covering power.
Hence mixture of two or more electrolytes is used for
preparing electrolytic bath.
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For a good electrodeposit, the pH of the bath must be
properly maintained. For most plating baths, pH ranges
from 4 to 8.
Electroplating method depends upon the type of metal to be
electroplated, the size and type of article to be electroplated,
its main objectives and economics involved.
Normally plating is carried out in rectangular tank made of
wood or steel with a ceramic or polymer layer inside so as
to provide thermal insulation. The volume varies between
25 to 2000 L.
Electroplating
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Electroplating
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Plating bath solution
It is a highly conducting salt solution of the metal which isto be plated. However, non-participating electrolytes are
added to the bath solution to increase the conductivity and
the throwing power.
The level of the plating bath should cover completely the
cathode and sufficient area of anode. Heating if required is
provided by heating coils or hot gases. Air sparger or
nitrogen sparger is employed to introduce convection current
in the plating bath solution.
DC source is used for electroplating with a voltage of 8-12V
and a current density of 1-200 mACm-2.
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In large plating operations, pumps and filters are
employed to filter out regularly the metallic particle.
The anode sludge can be retained by anode in cotton bags.
Copper, brass orAl bus bars are usually employed for
supplying the power to electrodes. The anodes and
cathodes are suspended alternatively from the bus barsthrough the entire width of the tank. Usually, anodes are
made of the plating metal, but in some cases insoluble
anodes are used.
Anodes can be rod type, plate type or even as pellets kept
in mesh basket. Cathodes are either articles to be
electroplated or jigs connected to the bus bars.
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Physical Vapour Deposition
This is a process of depositing some material by atom by atom or
molecule by molecule or ion by ion.
1. This process is widely used to produce decorative coatings on
plastic parts those are resembling shiny metal.
2. Many automobile parts are plastic with a PVD coating of
aluminium.
3.A
lacquer coating is applied over the decorative coating to providecorrosion protection.
4. This process is also used to apply relatively thick (1mm) coatings
of heat resistant materials on jet engine parts, A special alloy of
chromium, aluminium and yttrium is used for this type of coating.
Applications
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PVD Thermal Evaporation
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PVD - Sputtering
1. High technology coatings such as ceramics, metal alloys and
organic and inorganic compounds are applied by sputtering.
2. The substance to be coated is connected to a high voltage dc
power supply.
3. When the vacuum chamber has been pumped down, a controlled
amount of argon or another gas is introduced to establish a
pressure of about 10-2 to 10-3 torr.
4. On energizing current supply, plasma is established between thework and the material to be coated.
5. The gas atoms are ionized, and they bombard the material to be
coated.
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PVD-Sputtering
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PVD - Sputtering
6. The energy of impinging ions cause atoms of the target material tobe sputtered off, and they are transported through the plasma to
form a coating.
7. Direct current sputtering is used when the target is electrically
conductive.
8. Radio-frequency sputtering, which uses a RF power supply is used
when the target is a non conductor such as polymer.
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PVD Ion Plating1. A third variation of the PVD process is ion plating, which involves the
evaporation.
2. They physically implant into the substrate to produce an extremely
strong coating bond.
3. Sputter and ion plated coatings are used in design for very thin films for
electrical, optical and wear-resistant applications.
4. The wear properties of tools are widely enhanced by hard thin film
coatings.
5. Sputtered coating processes produce microscopic modules of diameter ofseveral micrometers and they are called macros.
6. These macros are undesirable for metal to metal sliding systems.
7. On the other hand they are usually beneficial to cutting tools.
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PVD applications
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Chemical Vapour Deposition
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Advantages and Disadvantages
This coating finds application on glass containers to make explosion or
shatter resistant glasses.
The main advantage of CVD process over PVD is that it is not line of
sight.
All surfaces in the reaction chamber get coated.
Separate process and reaction must be developed for each coating.
Some of the gases are toxic and dangerous.
But greatest disadvantage is temperature should be very high at 700C.
At this temperature, many metals soften.
Chemical Vapour Deposition
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1. A newer process known as plasma assisted chemical vapour deposition.
This process is used to apply diamond and diamond like carbon
coatings.
2. Silicon carbide barrier coatings are applied on plastic films and
semiconductors.
1. Chemical Vapour Deposition is used to produce bulk shapes of high
purity silicon carbide. Reactants are deposited on a chamber wall to a
thickness in terms of millimeters.
2. Thin-film coatings are key to the manufacture of many electronicdevices. They involve the application of dopant, sealant and other
microelectronic paste.
3. Thermal evaporation is a low cost process, but all these processes are
normally batch processes because of vacuum chamber requirements.
CVD Applications
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Ion Implantation
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Ion Implantation1. The common systems in use are ion implantation, laser treatment and
electron beam treatment.
2. In ion implantation ions of specific element with sufficient energy areimpinged on the surface of a material with sufficient energy.
3. The ions so impinged are embedded into the atomic lattice of substrate.
This process is performed in a vacuum chamber.
4. Usually ion gun produce the ions. It is done by passing the gas throughan electron beam or plasma. The gas atoms on collision with the electron
beam or with the species in plasma become ions.
5. The ions are accelerated by an applied magnetic field. These ionsimpinge on the work surface. The surface of the metal is hardened bycreating atomic defects or misfits by the impinged ions.
6. The depth of implantation is usually about 0.1m. However, the sphere
of influence is much deeper, may be a micrometer.
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