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STAINLESS

STEEL PLATING

(ALLOY PLATING)

KRUNAL VORA---------947

201

1

SAMSUNGDEPARTMENT OF METALLURGICAL & MATERIAL SCIENCE

ENGINEERING.FACULTY OF TECHNOLOGY & ENGINEERING.

THE M.S.UNIVERSITY OF VADODARA.9/26/2011



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PLATING

Plating is a surface covering in which a metal is deposited on a

conductive surface. Plating has been done for hundreds of years, but it is

also critical for modern technology. Plating is used to decorate objects, forcorrosion inhibition, to improve solder ability, to harden, to improve wear

ability, to reduce friction, to improve paint adhesion, to alter conductivity

and for many other purposes. Jewelry typical uses plating to give a silver or

gold finish. Thin film deposition has plated objects as small as an atom;

therefore plating finds in NANO TECHNOLOGY.

There are several plating methods, and many variations. In one

method, a solid surface covered with metal sheet, and then heat and

pressure are applied to fuse them. Other plating techniques vapor deposition

under vacuum and sputter deposition. Recently, plating often refers to usingliquids. Metalizing refers to coating metal on non-metallic objects.

ELECTROPLATING

In electroplating, an ionic metal is supplied with electrons to form a

non-ionic coating on a substrate. A common system involves a chemical

solution with the ionic form of the metal, an anode which may consist of the

metal being plated or an insoluble anode, and finally, a cathode where

electrons are supplied to produce a film of no- ionic metal



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Electroplating is used in many industries for functional & decorative

purpose .Some well- known examples for chrome-plating of steel parts on

automobiles. Steel bumpers become more corrosion-resistant when they

have been electroplated with first Nickel and then Chromium.

Hard chromium is used in services where frictional wear must be a

minimum, such as hydraulic pistons & camshaft bearing diameters.

Plain steel or aluminum parts in light fixtures glisten when they are

electroplated with nickel and then decorated with chromium or brass.

Nickel, in the form of a nickel sulfate, is used to restore dimensions on

worn parts, and as under the plate for hard chrome. The nickel sulfate bath is

unsuitable for decorative work.

Steel bolts last much longer because they are sold with a coating of zink

or cadmium that has been applied by electroplating. These electroplating

and conversion coating provide a double protection system for steel

components.

Virtually all types of steel can be protected including casting. Newly

developed electrolytes and process methods are able to provide greatly

increased corrosion prevention and brilliant finishes. Specially developedprocess produce improved metal distribution over complex shapes.



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ALLOY PLATING

In some cases , it is desirable to co-deposit two or more

metals resulting as an electroplated deposit. Depending on the alloy system,

an electroplated alloy may be solid solution strengthened or precipitation

hardened by heat treatment to improve the plating’s physical and chemical

properties. Nickel-Cobalt is a common electroplated alloy. Nickel-Cobalt is a

common electroplated alloy.

PROCESS OF PLATING

Three main stages of alloy deposition are,

1 .IONIC MIGRATION:

The hydrated ions in electrolyte migrate towards the cathode under the

influence if applied potential as well as through diffusion and/or convention.



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2. ELECTRON TRANSFER:

At the cathode surface are, the hydrated metal ions enter the diffusion

double layer where the water molecules of the hydrated ions are aligned bythe field present in this layer. Subsequently the metal ions enter the fixed

double layer where because the higher field present, the hydrated shell is

lost. Then on the cathode surface , the individual ion may be neutralized and

is absorbed.

3. INCORPORATION:

The absorbed atom wonder to a growth point on the cathode and is

incorporated in the growing lattice.

PURPOSE OF ALLOY PLATING:

1. Denser and finer grained

Electro –deposited alloys are denser and finer grained than their constituentmetal deposited under similar condition.

2. Decorative purpose

For example, gold alloys of various colures are used in jewelry. Cu - Son

alloys has silvery white appearance and therefore can be used for decorative

purposes.

3. Resistance to wear and abrasion

Electro-deposited alloys offer advantages in respect to resistance to wearand abrasion. E.g. Tin bronze which is harden than silver.

4. Protective purpose



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Alloy deposition has great application for protective purposes because of its

larger corrosion resistance. Tin-nickel & Cu- Son alloys have got extreme

resistance to tarnishing.

5. Permits depositions of metal that cannot be plated as single

metal

Co which cannot be plated from aqueous solutions. But it can be deposited

as alloy

Stainless steel. What is it ?

The term "stainless" was used initially in the development of the steel for the cutlery business in

the Production of knives, forks, spoons etc. There are now many stainless steel types and grades

used in applications requiring resistance to staining, oxidation and general corrosion resistance.

The main element of stainless steel is iron (Fe) hence it is an iron alloy containing a minimum of

12% chromium. Additional alloying elements are added to provide strength, cold working abilityand toughness for example:-

o Copper

o Molybdenum

o Titanium

o Nickel

o Other non-metals are added primarily Nitrogen and Carbon.

The main objective in the selection of a type and grade of stainless steel is to meet thePerformance requirements for strength and durability in its particular intended application.

The two common grades of stainless steel used for construction fixings in the stone and masonrysectors are from the Austenitic group commonly referred to as follows:-

Grade 304 or Grade 316

Grade 304 is generally specified for normal environmental applications.

Grade 316 is specified where a higher level of corrosion resistance is required e.g. marine

environment.

Why choose stainless steel?



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The cost of stainless steel is very competitive compared with other nickel or titanium based

alloys and offers a range of corrosion resistant properties suitable for large number of industrial

applications. Their strength is superior to plastics materials stainless steel can be hot or coldworked and fabricate using standard and traditional engineering techniques. Unlike certain

plastics stainless steel is fully recyclable.

What makes stainless steel "stainless”?

The reason for the high level of corrosion resistance of stainless steel is due to its self repairing properties.

This is achieved by the formation of naturally occurring chromium-rich "passive" chromium

oxide film on the surface of the steel. The film is extremely adherent and chemically stable providing sufficient oxygen is available to the surface for the chromium rich oxide film to form.

It is important to remember that stainless steel can corrode under certain conditions and it is

essential therefore to select the correct grade for a given application.

Stainless steel and its corrosion and oxidation

resistance.

As a general rule, the higher the chromium content of stainless steel the higher the corrosion

resistance. When nickel is added to create the austenitic steel the oxide film is strengthened andincreases the durability in more aggressive environments.

If molybdenum is added to either austenitic or ferrite stainless steel the pitting corrosion

resistance is greatly improved..

The family group of stainless steels.

Stainless steels are divided into four main group types namely

Austenitic, Ferrite, Martensitic and Duplex.These names describe the crystalline structure of the steel

AUSTENITIC STAINLESS STEEL



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Austenitic stainless steels have typically 18% chromium and contain nickel. This changes the

metallic structure from ferritic to austenitic and improves the corrosion resistance. They cannot

be hardened by heat treatment.

They are non-magnetic however a small amount of local magnetism is produced after cold

forming such as bending and rolling.

FERRITIC STAINLESS STEEL

Ferritic stainless steels contain chromium as their main alloying element of between 13% - 17%

and have a low carbon content. They are magnetic and cannot be hardened by heat treatment.

MARTENSITIC STAINLESS STEEL

Martensitic stainless steels have typically 12% chromium with higher carbon content than the

ferritic types.

They are magnetic and can be hardened by normal quenching and tempering techniques like

medium and high plain carbon steels.

Martensitic steels are commonly used in the production of cutlery and are also used in the

aerospace industry.

DUPLEX STEEL

Duplex stainless steels are used where both corrosion resistance and strength are required. They

cannot be hardened by heat treatment. Their metallic structure is a combination of austenite and

ferrite.

"Super" Austenitic and "Super" Duplex stainless steels have a greater resistance to pitting or

crevice corrosion-than the ordinary austenitic or duplex steels. This is due to additional

chromium, nitrogen and molybdenum.

Precipitation Hardening stainless steels can be hardened and strengthened like the martensitic

group by heat treatment. The metallic structure has a different mechanism to the process of themartensitic types consequently either austenitic or martensitic precipitation hardening structures

can be formed.

International material Standards: EN, ASTM and

JIS



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EN 10088-2 (Previously UK BS 1449 part 2) Stainless steel - Sheet, plate and strip for

general purposes.

EN 10151- Stainless steel strip for springs.

ASTM A240- Heat-resisting Cr and Cr-Ni stainless steel plate, sheet and stripfor pressure

vessels. ASTM A167- Stainless and heat-resisting Chromium-Nickel steel plate, sheet and strips

ASTM A176- Stainless and heat resisting Chromium steel plate, sheet and strip.

ASTM A666 -Austenitic stainless steel sheet, strip, plate, bar for structural And

architectural applications.

JIS G4308 -Stainless steel wire rods.

JIS G4303- Stainless steel bars

STAINLESS STEEL PLATING

In stainless steel main alloying element is Chromium(Cr) and Nickel(Ni) and it is a alloy of Iron(Fe).

So here we study about the chromium plating

CHROMIUM PLATING

It is widely used for the coating of steel. Electrolytically deposited chromium coating can have amorror finish and are of a silver - colour with a bluish tinge

The potential of chromium is more electronegative than that of iron but owing to the strongtendency of chromium to become passive in air, coating of this metal protect steel only

mechanically.

METHODS FOR PRODUCTION OF CHROMIUM

COATING:



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• Chromium cementation OR chromizing.

• Electrodeposition.

CHROMIZING>>>>>

It is classified into 3 types

Powder process

vapour process

fused salt process

It is known as enrichment of the surface region of steels with chromium by thermochemical

treatment. During this treatment chromium atoms diffuse at temperature between 900 and 1000

C into the surface of the workpiece.

ELECTRODEPOSITION

Chromium plating has found widespread use in two different field of application

1.Decorative Cr plating

2.hard Cr plating

Decorative Cr plating: thin bright coating 0.002 to 0.020 mil in thickness is used over nickel or

copper undercoat. Bright finish of moderate durability is obtained

Hard Cr plating: it is directly applied to steel, without any intermediate deposit of other mrtals.The thickness of the chromium coating can very from 0/075 to 0.25 milimeter, but range can

range from 0.005 to 0.01 mil.it is deposited for high hardness, wear resistance & low coefficientof friction. After such treatment the service life of steel articles is considerably increased.

PROPERTIES OF CHROMIUM DEPOSIT:

a) 1.It has a characteristic bluish white tint b) 2.Shows unusual resistance to oxidation and tarnish

c) 3.Hardness of chromium deposit is of the order of 900 BHN.

d) 4.low coefficient to friction

e) 5.Not affected by alkalies , sulphur & its compounds etc.f) 6.It is non magnetic.

ELECTROLYTIC BATH

Chromic-acid-based bath (Hexavalent Type)



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In plating from this bath, sulfate and fluoride ions act as catalysts. Temperature , currentdensity and bath composition affect the film characteristics and current efficiency. Thickness of

deposits vary from 0.25 to 0.5 micron(0.001 inch), or more for hard chromium plating.

Bath Composition

Chromic acid and sulfate are the necessary ingredients. Chromic-to-sulfate ratio range from

50:1 to 250:1. Preferably at 100:1. The composition depends on whether the bath is co-

catalyzwd, e.g.with fluorides, fluosilicates or fluoborates, and on the application.

Basically 2 types of baths commonly used

1.Concentrated solution bath containing CHROMATE(Cr2O3) 400g/1 & H2SO4 4g/1

Advantage -higher conductivity and lesser voltage required

rate of deposition becomes faster

Disadvantage

• Low throwing power

• Narrow range of current density for bright deposit

2.Dilute solution bath containing Cr2O3 250g/1 & H2SO4 4g/1

Advantage Less costlier

Wide range of current density for bright deposit

Disadvantage

Low conductivity

Requires high voltage



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FACTOR AFFECTED ON THE PLATING

1. TEMPERATURE

It is closely related to current density in its effect on brightness and coverage of deposit.Generally, the brighter the current density, the higher temperature requirement.

For decorative bath the range is 35C to 46C & for hard chromium, the range is 49C to65.5C

Agitation is required to equalize the bath temperature, to produce uniform brightness, and inthe case of hard chromium, to improve deposit hardness.

Preheating of parts to optimum bath temperature may be needed before they introduced into

the tank amd in rare case cooling of parts may be needed

2. CURRENT DENSITY

At given solution composition and temperature, current density affects cathode efficiency,

brightness and hardness. Generally optimum current density is recommended.

At too high current densities, burning or roughness of deposit occurs. At low current

densities, lack of chromium coverage can be expected.

The increase in current density or decrease in it depending on whether the temperature is

below or above 55C. At 55C hardness is independent of current density.

3. CURRENT EFFICIENCY

It depends on temperature and current density. It decrease with increasing current and

increase exponentially with increase in current density.

4. SOLUTION DEPOSITION

Increasing the chromic acid concentration reduces the rate of deposition. Increasing chromic

acid concentration or sulfate keeping the chromic/sulphuric acid ratio constant decrease the

hardness of deposit.



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5. ANODES

Iron anode are used in stainless steel plating.

COMMON PLATING PROBLEM

Faulty bath chemistry

Improper temperature and/or current density

Poorly finished basis-metal surface

TYPICAL APPLICATIONS

Printing plates

Wrist pins

Steering Knuckles

Tube drawing dies

Forming dies

water pipes

surgical applications

Architectural

WORLD WIDE APPLICATIONS

Sant Fruitos pedestrian bridge in SPAIN



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Padre arrupe bridge in SPAIN

REFERENCES

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