devashish ppt
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METALLICCOATINGS TO
PREVENTCORROSION
BE IV(MET.& MTRLS ENGG.)
METALLURGICAL & MATERIALS
ENGINEERING DEPARTMENT
FACULTY OF TECHNOLOGY ANDENGINEERING
M.S. University OF BARODA,
VADODARA
DEVASHISH JOSHIRollNo:907
SEMINAR
TOPIC
ON
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Corrosion It is termed as deterioration of the material when it
comes in contact with the environment.
It happens due to the chemical attack from thesurrounding environment and involves transfer ofelectrons.
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Methods for RESTRICTING CORROSION
1. Alter the environment
2. Proper material selection
3. Alter design to optimize geometry
4. Employ cathodic protection
5. Use organic coatings such as paints or powder
coating
6. Use METALLIC COATINGSa) to avoid contact of metal from the environment
b) to provide Galvanic Protection in case of
exposure to environment
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Types of Metallic coatings Cladding
Flame spraying
Electro-deposition
Hot-dipping
Vapor-deposition
Diffusion
Chemicalconversion
Surface
modification
Ion implantation
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Cladding
Formation of surface layer of sheet metal byprocessing two sheets of metal together.
Example Nickel and steel sheet are rolled together giving acomposite sheet of LINCH steel, 1/8 inch Ni.
Ni, Al, Cu, Ti, stainless steel and other metals arecladded over steel.
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In Cladding, inexpensive material is coated withexpensive material having very thin layer but goodcorrosion resistance. Thus, it provides a greateconomic advantage.
Cladding is selected not only to have goodcorrosion resistance but also to be anodic to thecore metal.
Thus, if the cladding becomes damaged byscratches, the cladding will provide cathodicprotection by corroding sacrificially. Thus, we getlower cost metal with good corrosion resistance.
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METHODS OF CLADDING
Prevalent at mill stage by manufacturers of sheet, plateor tubing.
Pressing, Rolling, or Extrusion can produce a coating in
which the thickness and distribution can be controlledover wide ranges, and the coatings produced are free ofporosity.
The hot roll bonding process is used to produce over 90%of clad plate products.
Arc or Gas welding in which relatively thick layers ofweld metal are deposited either by manual or machinemethods on surfaces.
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APPLICATIONS
Principal Uses are Aluminum cladding in the aircraft industry,
Lead and Cadmium sheathing for cables,
Lead-sheathed sheets for architectural applications and
Composite extruded tubes for heat exchangers
The Chemical process, Offshore oil production, Oilrefining and Electric power generation industries.
The interior of pulp digesters or other pressure vesselsrequiring an alloy composition to resist the chemicalconditions can be constructed in this manner.
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Flame or thermal spraying
Also known as METALLIZING.
Four methods based on the form of the coatingmaterial are generally used:
The coating material is in the form of rod which ismelted by an oxyacetylene flame and blown ontothe surface to be coated.
The coating is in the form of a wire. The wire is fedinto the central orifice of a nozzle. It passes throughan oxyacetylene flame and sprayed to the metalsurface.
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The coating material in the form of a wire isheated by passing it through the plasma of anelectric arc. The resulting molten metal is blownout of the arc by an auxiliary gas stream, as
droplets. The coating material is in the form of a powder
which is heated by an oxyacetylene frame,atomized and blown onto the surface.
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The substrate surface should be roughed toobtain mechanical bond.
Sometimes Paint coating is applied oversprayed metal to fill voids and provide abetter barrier.
Flame Spraying is an economical way ofbuilding up worn surfaces such as Shafting.
Coatings are porous and non-protectingunder severe wet condition.
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High velocity oxy-fuel Spraying Applications
Used in ship-hills,
super structure ,
refrigerationequipment ,
many fabricated steelproducts
Exhaust Stacks
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Several fabrication techniques
of Flame spraying Cold spraying
Detonation spraying
Warm spraying
Wire arc spraying
Plasma spraying
High velocity oxy-fuel spraying Plasma Blowing Torch for metallizing
inside pipes and tubes
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ELECTRO-DEPOSITION
Also known as ELECTROPLATING.
Factors affecting quality are:
Time, Temperature, Current Density, Composition of
Bath, Agitation of bath
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Hydrogen evolution should not take place.
Parameters can be set accordingly to make thin ,thick, dull, bright, brittle, soft or hard(for preventingerosion corrosion).
Gold, Platinum and Silver plates are common.
Zinc, Nickel, Tin, Chromium, Cadmium are coated on
largest tonnage basis.
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Electroplate can be a single layer, layers of
several metals or alloy composition(e.g.brass).
For example in Automobile bumper:
Flash Plate is of Copper( for good adhesion)
Intermediate layer of Nickel ( for corrosionresistance)
Upper coating of Chromium ( for appearanceand smooth surface finish)
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Plating process that auto catalytically depositsnickel alloyed with Phosphorous on substrate.
No current = Uniform coating thickness
Electrical flux fields tend to be non-uniform and therefore
can result in non-uniform thickness of the electrodeposits.
Reducing Agent Sodium HypophosphiteNa(H2PO2).
Electro-Less nickel (EN) plating
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The electro-less deposit are very uniformin thickness all over the part’s shape and
size.
Hence used in plating irregularly shapedobjects, holes, recesses, internal surfaces,
valves, threaded parts, and so forth.
Main Advantage
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Advantages over Electrodeposition:
• The corrosion resistance of electro less nickel issuperior to that of electrodeposited nickel.
• The coating may contain 6-12% Phosphoruswhich increases the resistance to corrosion.
• The adhesion of electro less nickel coating tothe substrate is excellent.
• The coatings have high strength, limited ductilityand a high modulus of elasticity.
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Prominent Areas of
Application are :--
Oil & Gas
Chemical Processing
Textile
AutomotiveFood & Pharmaceutical
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HOT DIPPING
Applied to metals by immersing them in a moltenmetal bath of Low Melting Point.
Oldest example of coating of the metal.
Chief examples are coatings of Zn ,Sn , Pb andAl.
“GALVANISING” is the most popular example.
Thickness of coating thicker than electroplatesbecause very thin dip are difficult to produce.
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Galvanizing
Process of applying a Zinc coating to fabricated ironor steel material by immersing the material in a bathconsisting primarily of molten Zinc.
During galvanizing, the molten zinc reacts with theiron in the steel to form a series of zinc/iron alloylayers.
Galvanized steel is used in chemical process, pulpand papers, automotive and transportation industry.
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“Zn” which forms a coating over the substrateforms a passive layer of “ZnO2” thus protectingsteel in dry condition.
In case of exposure of metals due to drilling orother activities.
Zn acts as Anode while steel acts as cathode.
Thus, dissolution of Zn takes place asit acts as Sacrificial anode.
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Galvanizing forms a metallurgical bond betweenthe zinc and the underlying steel or iron, creatinga barrier that is part of the metal itself.
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VAPOUR DEPOSITION
Process in which materials in a vapor state arecondensed through condensation, chemicalreaction, or conversion to form a solid material.
Used to form coatings to alter the Mechanical,Electrical, Thermal, Optical, Corrosion resistance,and Wear properties of the substrates.
Accomplished in a high vacuum chamber.
Very expensive and generally used in coating ofhigh strength parts of ROCKETS AND MISSILES.
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There are two categories of VD process
Physical vapor deposition:-Work piece subjected toplasma bombardment
Chemical vapor deposition:-Thermal energy heats
gases in coating chamber and drives the deposition
reaction.
The most useful CVD coatings are nickel,
tungsten, chromium, and titanium carbide.
Titanium carbide is used for coatingpunching and embossing tools to impartwear resistance.
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DIFFUSION COATING or PACK
CEMENTATION
In this method, the surface of the metal to becoated is modified by diffusing into it at a high
temperature, a metal or an element, which wouldprovide the required corrosion resistance whencombined with the parent metal. Such coatingsare called 'diffusion coatings'.
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Process is also known as “SURFACE ALLOYING”.
Diffusion coatings causes alloy formation bydiffusion of one metal into another.
A surface product entirely different from the metal isproduced.
Diffusion coatings can be applied to a range of
metals and alloys, such as Nickel, Titanium andMolybdenum, but the widest use is on Ferrousmetals.
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Process
It is also known as Pack Cementation Process.
Components to be coated are packed with metal
powders and flux in sealed, heat-resistant retorts and
heated inside a furnace to precisely led controltemperature-time profiles.
The coating forms at temperatures ranging from 700 to
1100 degree C over a period of several hours.
Pack cementation processes include Aluminizing,
Chromizing (Cr), and Siliconizing (Si), Sherardizing (Zn).
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Advantages and uses
Provides Good Corrosion resistance
Hard, abrasion resistant surface of some 20 to 50 µm in thickness is produced.
These coatings are used to impart abrasion and/or
high-temperature resistance to Gas turbine bladesand vanes, among other similar uses.
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Aluminizing provides strength to the metal and
Corrosion resistance to air and Sulphur gases athigh temperature.
Aluminum coatings are frequently used on heat
exchanger surfaces subject to attack by sulfurgases (H2S, SO2, and SO3) as it is highly corrosionresistant.
Carbon Steels, Low Alloy Steels and StainlessSteels are Aluminized.
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Chemical conversion
Adherent and protective coatings are formed by“corroding” the metal surface.
Examples
Parkerizing ( Phosphatizing in a bath of PhosphoricAcid)
Used in Automobile Industry
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Chromatizing (exposure to chromic acid and Di-chromates)
Applied mainly to Zn and Mg parts
Anodizing (Anodic oxidation in acid bath to buildup an oxide layer, generally on Al and Mg)
Sealing in boiling water required to removediscontinuity
Used for architectural purpose (e.g. Building wallpanels)
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Advantages of Chemical coating
I. Great improvement in Corrosion resistance(
by produce uniform surface thus, chances ofstress corrosion or crevice corrosion or furtheroxidation decreases.
I. Gives Aesthetic look ( using Colored Oxide)
II. Increases Wear resistance ( as they are hard)
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SURFACE MODIFICATION
Modification of surface of a material by bringingphysical, chemical or biological characteristicsdifferent from the ones originally found on thesurface of a material.
Laser, Electron, Microwave, Solar beams,Synchroton radiation, Pulsed arc, Pulsedcombustion, Spark, Friction, Induction etc. are
latest processes in this field.
Almost all types of materials, includingceramics, polymers, and composites can becoated on metals.
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Laser Treatment improves resistance to uniform
corrosion in HCL and Citrate solution and has noeffect on Pitting potential.
Lasers are used to impregnate metals with hardparticles for improving wear resistance.
Applications:
Automotive, Aerospace, Missile, Power, Electronic,
Biomedical,
Textile, Petroleum, Petrochemical, Chemical,
steel, Cement,
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Ion implantation
Thin surface alloy coatings produced bybombarding the metal with ions in vacuum.
Ion implantation can be used for anyelement that can be vaporized and ionizedin a vacuum chamber.
Since material is added to the surface,rather than onto the surface, there is nosignificant dimensional change or problemswith adhesion.
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ADVANTAGES
Implantation is used to alter surface properties,such as hardness, friction, conductance, opticalproperties, corrosion resistance, and catalysis.
Requires no post treatment
Generates minimal waste.
Such coatings of, for example, Ti, B, Cr, or Y havespecialized applications for wear and high -temperature oxidation resistance.
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LIMITATIONS
If exposed to high temperatures, however,implanted ions may diffuse away from the surfaceso depth of coating decreases.
Commercial availability is limited by generalunfamiliarity with the technology, scarcity ofequipment, lack of quality control and assurance,
and competition with other techniques.
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REFERENCES
Corrosion Engg. by Mars .G. Fontana
Electrochemistry and Corrosion science-
N. PEREZ
Corrosion Engg. Principles and Practice-Pierre R. Roberge
Corrosion and Protection – Einar Bardal
www.wikipedia.org
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