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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



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.



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



Types of Metallic coatings Cladding

Flame spraying

Electro-deposition

Hot-dipping

Vapor-deposition

Diffusion

Chemicalconversion

Surface

modification

Ion implantation



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.



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.



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.



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.



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.



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.



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.



High velocity oxy-fuel Spraying Applications

Used in ship-hills,

super structure ,

refrigerationequipment ,

many fabricated steelproducts

Exhaust Stacks



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



ELECTRO-DEPOSITION

Also known as ELECTROPLATING.

Factors affecting quality are:

Time, Temperature, Current Density, Composition of

Bath, Agitation of bath



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.



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)



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





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



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.



Prominent Areas of

Application are :--

Oil & Gas

Chemical Processing

Textile

AutomotiveFood & Pharmaceutical



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.



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.



“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.



Galvanizing forms a metallurgical bond betweenthe zinc and the underlying steel or iron, creatinga barrier that is part of the metal itself.



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.



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.



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'.



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.



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).



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.



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.



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



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)



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)



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.



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,



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.



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.



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.



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

http://www.wikipedia.org/

http://www.wikipedia.org/





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