Corrosion Part 2

8/8/2019 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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Corrosion Part 2