PHOSPHATING & POWDER COATING
PHOSPHATING
SURFACE CLEANING
CHEMICAL PRE-TREATMENT (PHOSPHATING)
*Pretreatment- The preparation of a part prior to the
application of a coating in order to improve adhesion
& corrosion resistance.
• Phosphating is a surface treatment process by which
the virgin steel surface is converted to metallic
phosphate & is widely used for preparing metal
surface before painting. (Thickness lies between 3 to
50 micron)
• Phosphate coatings are produced by the chemical
reaction of base metal with aqueous solution of
phophoric acid and phosphates (Ions of Bivalent
metals…Zn+2, Mn+2, Fe+2).
• Such coatings do not offer complete resistance to
atmosphere & principally used as an adherent
base/primer coat for paints.
MECHANISM OF PHOSPHATING
• The free phosphoric acid in phosphating solution reacts with
the Iron surface undergoing treatment in the following
manner: Fe + 2H3PO4 
Fe (H2PO4)2 + H2 
..(1)
• Thus producing soluble primary phosphate and liberating
Hydrogen. Local depletion of phosphoric acid occurs at metalsolution interface. As the primary phosphates of Zinc
dissociate readily in aqueous solution, the following reactions
takes place: Zn (H₂PO₄)₂
 Zn HPO₄ + H₃PO₄ ……..(2)
3 Zn HPO₄
 Zn₃ (PO₄)₂ + H₃PO₄ ……(3)
3 Zn (H₂PO₄)₂  Zn₃ (PO₃)₄ + 4 H₃PO₄ .....(4)
• The neutralisation of free phosphoric acid by reaction (1)
alters the position of equilibrium of equations (2), (3) & (4)
towards the right and there by leads to the deposition of
sparingly soluble secondary phosphates and insoluble tertiary
phosphates on the metal surface.
PHOSPHATING
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The conventional seven-tank Hot Zinc
Phosphating system is used.
• The following are the different steps involved in
the process:– Alkali degreasing
– Cold swilling
– De – rusting
– Cold swilling
– Hot phosphating
– Cold swilling
– Sealing/Passivation
PHOSPHATING EQUIPMENT
ALKALI DEGREASING
• Tank:…………………………………… Mild steel
• Capacity: ……………………………..7000 L
• Chemicals used:
–Sodium hydroxide………6 – 8%
–Tri sodium phosphate….2-3%
• Operating temp: ………………….60 – 700C
• Duration:…………………………….. 15 – 20 mts.
*continuous air agitation is provided for effective
cleaning.
COLD SWILLING
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Tank:…………………………….......... Mild steel
Capacity:………………………………. 7000 L
Cold swilling medium:…………… Static water
Operating temp: …………………….Room temp.
Duration:………………………………. 2-3 times
*Continuous agitation is must for effective cleaning
(It brings fresh chemicals in the bath to the substrate
& cleaning is very effective ).
DE- RUSTING
• Tank:………………………………………… Mild steel
• Capacity: …………………………………..7000 L
• Chemicals used:
– Phosphoric acid……………………25-30%
• Operating temp:………………………. Room temp
• Duration: ………………………………….5 - 7 mts.
*Oxide films & rust stains are dissolved in acid.
*The tank is lined with FRP to avoid corrosion of the
tank.
COLD SWILLING
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Tank: …………………………………………..Mild steel
Capacity: …………………………………….7000 L
Cold swilling medium: ………………..Static water
Operating temp: …………………………Room temp.
Duration: …………………………………….2-3 times
*continuous air agitation is provided for effective
cleaning.
HOT PHOSPHATING
• Tank: ………………………………………….Mild steel
• Capacity: ……………………………………7000 L
• Chemicals used:
– Phosphoric acid & Zinc phosphate..30%
– Accelerators (Sodium nitrite, an oxidising agent)
• Operating temp:………………………..60 – 700 C
• Duration:……………………………………10 - 15 mts.
*Iron dissolves in phosphate solution & hydrogen is
evolved due to chemical reaction & adheres to the
metal surface. Sodium nitrite reacts with hydrogen &
forms water.
*The tank is lined with stainless steel
COLD SWILLING
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Tank: …………………………………………..Mild steel
Capacity: …………………………………….7000 L
Cold swilling medium:………………… static water
Operating temp: ………………………….Room temp.
Duration: …………………………………… .2-3 times
*continuous air agitation is provided for effective
cleaning.
SEALING (PASSIVATION)
• Tank: …………………………………………….Mild steel
• Capacity: ………………………………………7000 L
• Chemicals used:
– Chromium tri oxide…………….. …..0.025 – 0.05%
• Operating temp: ……………………………60 – 70oC
• Duration: ………………………………………25 -30 sec.
* Phosphate coating is porous. Therefore, oil or other
sealers such as Chromium tri oxide, are used to
achieve corrosion resistance.
POWDER COATING
INTRODUCTION
• An advanced method of applying a decorative and
protective finish to a wide range of materials and
products that are used by both industries and
consumers.
• The youngest & fastest growing surface coating
techniques
• First introduced in USA in the mid of 1950 & used first
in LLH(W) under ER of IR
• Environment friendly (not emitting any VOC’s & HAP’s)
• Over sprayed powder can be recycled
• Uniform, durable, high quality and attractive finish
A QUICK OVERVIEW
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Very simple process
More jobs can be tackled at a time
No significant skill is essential
Lasts for a longer period
Material cost is very less
WHY POWDER COAT ?
• Produces a hard, abrasion resistance & tough
coating
• Can be applied over a wide range of
thicknesses (20-25 mils/500-600 microns)
WHAT IS POWDER COATING
“A mechanical mixture of resin, pigments &
hardening agents in powdered form, when
applied on to a surface as a thin/thick layer,
forms an adherent film after curing”.
CONSTITUENTS OF POWDER COATING
• Powder coating is composed of finely ground
particles of:
• Resins: key component
 Thermosetting, Most suitable for thin coating.
(Epoxy, Polyester, urethane, Acrylics, etc) &
Thermoplastic, Most suitable for thick coating. (PVC,
Polyehylene, Polypropylene, nylon, etc)
– Good resistance to chemicals
– Low shrinkage during curing
CONSTITUENTS OF POWDER COATING
• Pigment:
 Titanium dioxide, Carbon black, Zinc oxide, etc.
– Provides desired colour & opacity
– Protects from UV-light
– Improves impermeability to moisture
• Fillers:
 Barytes, French chalk, Talc, Whiting, etc
– Fills voids
– Reduces the cost
CONSTITUENTS OF POWDER COATING
• Degassing agent:
– Eliminates gas bubbles
• Accelerators:
 Copper salts, etc
– Enhances the rate of slow reaction
• Dry flowing agents:
 Poly acrylates, silicone, fludised alkyl esters, etc
– Improves free flow of powders
• Waxes:
– Provides slip
MANUFACTURING POWDER PAINT
• The dry ingredients in exact amounts are blended at
high speed to create a homogeneous mixture &
heated to the resin melting temp in an extruder.
• The extruded mixture is then cooled & rolled before
being broken into small chips.
• The chips are milled to a very fine size ( Average
diameter of powder particle varies in between 30-50
micron) & packed.
• Each individual powder particle contains the entire
coating formulation.
COMPRESSED AIR DATA
• Requires good quality of compressed air, free of oil &
moisture.
• Pressure ------------------- --4 to 5 kg/cm2
• Max. moisture content----1.4 gm/m3
• Max. oil content-----------0.1 ppm
POWDER COATING PROCESS
• Basic steps:– Surface preparation
– Application of powder
– Curing
– Recovery of powder
SELECTION OF GUN ATTACHMENT &
RIGHT POWDER
• No perfect tip for all applications.
• Extension lances help to get into recessed
areas.
• Unwanted areas to be covered with high temp
tape specially designed for powder coating.
• Right powder to be chosen as per the end use
of the component.
POWDER COATING APPARATUS
Powder coating machine
Powder coating gun
APPLICATION OF POWDER
BASIC POWDER COATING PROCESS
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Four basic processes:Electrostatic spraying
Fluidised bed
Electrostatic fluidised bed
Flame spray
*Electrostatic spraying is the most commonly used
OPERATIONAL PROCEDURE OF POWDER COATING
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Ensure proper earthing to the equipment.
Put powder into the hopper.
Connect the equipment to the main air supply.
Switch on the power supply.
Direct the gun into the spray booth & press the trigger.
Adjust the fluid bed air pressure to 4-5 kg/cm² till the powder
in the hopper appears to ‘boil’.
• Don’t keep higher pressure than necessary otherwise powder
may ‘puff’ out the gun.
*By increasing dosing air, powder speed will increase, cloud
density is reduced. By reducing dosing air, powder speed will
reduce, cloud density will increase.
ELECTROSTATIC SPRAYING
• An electrical charge is applied to the dry powder particles
while the component to be painted is electrically grounded.
• The charged powder & grounded work piece create an
electrostatic field that pulls the paint particles to the work
piece.
• The coating deposited on the work piece retains its charge,
which holds the powder to the work piece.
• The powder coated work piece is placed in a curing oven,
where the paint particles are melted on to the surface & the
charge is dissipated.
* The gun gives +ve electrical charges on the powder particles
by frictioning between them & non- conductive material (air).
POWDER SPRAY BOOTH
ADDITION OF VIRGIN POWDER
• Virgin powder should be added frequently to the
powder hopper because, recycled powder will always
have properties that differ from virgin powder.
• In order to maintain a constant composition, always
keep the volume of re-used powder to minimum.
• To reduce variations in particle size distribution due to
recycling.
CURING
• The curing process requires a certain degree of temp
for a certain length of time in order to reach a full
cure.
– Temp…………….180-2000C
– Curing time……45 minutes.
• The application of energy to the product to be cured
can be accomplished by:– infrared cure ovens (direct exposure to light in the
range of IR-region)
RECOVERY OF POWDER
• Powder coating booth is
equipped with powder
recovery cyclone and filter.
• It recovers as much as 98% of
powder exhausted from the
booth.
• Recovered powder is
progressively re-used.
• After filter, filters fine
powder of powder recovery
cyclone and virtually no
powder goes to atmosphere.
COMPONENTS BEING CURED
COMPONENTS READY FOR DESPATCH
POWDER COATING STORAGE CONDITION
• Powder coating must be:– flowing easily
– capable of maintaining a good electrostatic
charge.
• Proper storage is essential as it:– prevents particles agglomeration
– ensures satisfactory application.
FACTORS AFFECTING POWDER STORAGE CONDITIONS
• Factors:– Temperature
– Humidity
– Contamination
– Direct sunlight
Recommended optimum conditions:• Temperature ………………………….250C (Approx).
• Relative humidity…………………….50 to 65 %.
• Away from direct sunlight.
SAFETY & HEALTH
• Powder & air mixtures can be a fire hazard when an
ignition source is introduced.
• The concentration of powder in air must be
controlled to maintain a safe working.
• Any finely divided organic material, such as dust, can
form an explosive mixture in air.
• Personal cleanliness should be maintained at all
times (soap & water clean up to remove powder
from the skin)
• Don’t use air blast nozzles to blow off powder from
the skin.
SAFETY & HEALTH
• Disconnect the main power supply before cleaning or
maintaining the gun
• Operator should hold the gun in his bare hands.
• Avoid inhalation of the powders.
• Use proper PPE
ADVANTAGES OF POWDER COATING
• Releases zero or negligible amounts of VOC into the
atmosphere, resulting in minimal air pollution.
• Easy recycling of over sprayed powder for reuse
• Provides a superior finish, greater durability,
improved corrosion resistance.
• Lower raw material (powder) costs.
• Unlimited colour selection with high & low gloss.
• Easy to apply thick coatings
• Requires minimum operator training & supervision
• Parts can be racked closer together.
DISADVANTAGES OF POWDER COATING
• Application of large areas not feasible.
• Coating touch up not possible.
• Difficult to apply intricate shapes & assembled
components
• Can be more prone to orange peel.
• Difficult to apply thin coats (< 1.0 mil) .
REMOVING POWDER COATING
• Methylene Chloride is most effective.
NB: Most recently the suspected human carcinogen
Methylene Chloride is being replaced by benzyl
alcohol with great success.
*(Organic solvents such as Acetone, Thinners, etc,
are completely ineffective).
• Can be removed by abrasive blasting.
• Can be removed by using 98% commercial grade
Sulphuric acid.
COST EVALUATION
Calculation of cost of powder coating against each coach on the
basis of 25 nos. of coaches per month:Cost of phosphating chemicals:ITEM
MLY.
CONSUMP.(Kg)
UNIT COST(Rs.)
TOTAL COST(Rs.)
Degreasing
Chemical
10
26/-
260/-
De-rusting
chemical
Phosphating
chemical
Total cost per
month
10
30/-
300/-
8
31/-
248/-
Rs. 808/- only
COST EVALUATION
• Raw material cost per coach for phosphating
A
= 808/25=Rs.32/------------------• Powder required per coach=1.67 Kg
• Cost of powder per Kg = Rs. 270/• Total cost of powder for each coach
B
=Rs. 451/---------------------------------• Total raw material cost for powder coating of each coach=
A+B=32+451= Rs.483/-------------------------------C
• Labour cost=7.5X 13 Man days=97.5 hrs.
• Hourly rate= Rs. 39.60/• Total labour on cost for each coach= 97.5x 39.6=Rs.3861/-• Total Cost for each coach= C + D= 483+3861=Rs. 4344/- only
D
POWDER COATING OF RAILWAY
COACHING COMPONENTS
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Done at Liluah workshop:-----Side wall panel
• Curtain rod
Roof panel
• Bottle opener
Partition frame
• Guard rail bearing
• Tissue paper box
Chair pillar
• Shutter
Kitchen berth frame
• T- bolt assembly
Push plate
• Towel rods
Spring cover plate
• Long hinge. etc
Basin cover
Soap dish
Flushing valve