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Powder Coating

PRETREATMENT


Introduction to pretreatment
Pretreatment is a process. which is required prior to any powder coating system just to ensure-
(a) A clean, oil free surface
(b) To impart adhesion
(c) To reduce the corrosion rate by stopping the rust creep age.
(d) To have a neutral layer in between the powder coating and surface
It is very important to note that the performance of any painting system is 100% governed by the correct base pretreatment. As of today, Indian finishing industry is using following surface pretreatment processes prior to powder coating.

Declaimer :
 
Please Ask Your Chemical Advisor / supplier for specific details and guideline.
 
These specification guidelines are given just for your reference, but in specific conditions may vary, no responsibility can be taken for coating system performance. It is the responsibility of the user to ensure that the product and specification meet the particular application.



Mechanical Process
    Wire Brushing /Flame Cleaning /Sand-shot Blasting
 
*  Wire Brushing: Generally used to remove loose rust/dust like impurities from the surface This is a manual process which should be followed by solvent cleaning.

*  Flame Cleaning: Generally used to remove the mill scales/rust from the surface. Surface to be cleaned is subjected to the flame to remove mill scales rust.

*  Sand/Shot Blasting: This method is used to get surface profile; industry is using either glass shots/grits, river sand or iron    grits.
Chemical Processes
General pretreatment

sequence For ferrous surface Degreasing
 (vapour/emulsion/solvent wipe/alkali)
          · Water rinse
          · Derusting HCL,H2SO4,H3PO4
          · Water rinse
          ·  Activation (Titanium/oxalic acid)
          · Phosphate (Mono / Di / Tri)
          · Water rinse
          · Passivation (CrO+6, Based)
For non ferrous surface (Al / zinc/zinc alloys etc.)
          · Degreasing (Emulsion/alkali)
          · Water rinse
          · Desmuting (10% NaOH wash)
          · Water Rinse
          · Chromating
          · Water rinse
 

Chemical pre-treatment details
 
Details of each stage
1. Degreasing - This stage removes oil, grease, dirt and other dust particles to give a clean, surface for next stage in the process.
2.Water rinse - To remove the chemical from the earlier stage and make the surface totally free from any contamination prior to entering into next stage in the process.
3.Derusting - This stage removes rust from the metal and at the same time make the surface little reactive for the next stage in the process.
4.Water rinse - As per point No. (2)
5.Activation - This process provides fine active crystal centres on the surface of the metal which ultimately results into fine phosphate coating layer in the phosphate stage. Process with activation gives uniform, compact crystalline coating with good corrosion resistance.
6.Phosphating - It gives a deposition of insoluble metal phosphate by chemical reaction. Since the deposited coating forms by a chemical reaction it gives good adhesion and good corrosion resistance to the powder coating film.
7.Water rinse - As per point No. 2
8.Passivation - This stage helps in sealing the cavities in the phosphate coating and thus increases the corrosion       resistance of the coating.


Pre-treatment stages in details
1. Degreasing - This can be done by the below given 3 ways.
A. Solvent wipe
This is the cheapest and best method to remove heavy and sticky oil/grease like substances from any surface. The parts to be cleaned are wiped with a rag of cotton soaked in a solvent.
Advantages
Cheapest method of industry to remove heavy oils/sticky press compounds, etc. Does not require skilled labour and costly plant installation. Suitable from smallest job coater to large OEM.
Disadvantages
Labour intensive process. Hazard of fire risk due to use of solvent.
Frequent cleaning of cloth/solvent is necessary to maintain the cleaning standard. Disposal of waste solvent can cause effluent problem.
B. Emulsion Cleaning
They are based on mild alkalies and operate at lower temperatures. They are generally based on kerosene - emulsifier or turpentine emulsifier. These cleaners are generally milky in appearance. They have a limited cleaning tendency compared to alkaline cleaners and they leave a very thin film of solvent/emulsifier over the substrate even after water rinsing . They should be followed by alkali cleaning.
Advantages
Energy Saving as bath operates at lower temperature (45-50°C)
Suitable for all (Ferrous/non ferrous) metals.
Disadvantages
Limited cleaning tendency, cannot remove heavy oil/greases, etc.
C. Alkali cleaning
Alkaline cleaners enjoy superior position in the pretreatment world due to following
·         They operate at room temperatures.
·         Bath is more stable compared to other degreasing processes.
·         Bath control is very simple
·         Costly plant installation is not always required
·         Higher capacity to absorb oil.
These cleaners are based on strong alkalies like sodium hydroxide/sodium silicate with surfactants and fillers.The parts to be cleaned are immersed in the bath at specified temperatures and time.

2. Derusting
This process used to remove rust and mill scales from the substrate. It has following advantages and disadvantages.

Advantages
Cheaper process than blasting or flame cleaning to remove rust.
Costly plant/installations are not always needed.
Also makes the surface reactive for next phosphating stage.

Disadvantages
Use of HCL/H2SO4 can create corrosive atmosphere in the plant.
Not suitable for Spray process
Chemical carry over in next stage (phosphating) can cause problem

Type of Derusting
 
Hydrochloric acid based Sulphuric acid based Phosphoric acid based
Used mainly--to remove
mill scales
Used for heavy rust Used for mild Rust
Can create corrosive plant atmosphere due to fuming tendency Can cause corrosive plant atmosphere No Harm
Carry over in phosphate
stage can poison the bath
Carry over in phosphate stage can poison the bath No Harm
Has severe pitting tendency Has severe pitting tendency No pitting tendancy
Cheapest acid for derusting Cheaper than phosphoric Costly but more used by the industry
3. Activation
 This stage provides fine active centres on the surface of the metal which results into fine phosphate coating in the phosphating stage. Finer the phosphate coating better the corrosion resistance. This stage also controls the phosphate coating weight thereby controlling the phosphate chemical consumption. There are two types of activation processes.

a) Acidic activation
This is a cheaper process in which bath control is very easy. Bath can be
prepared in hard water. This process gives coarse and bigger crystalline coating. Not suitable for non ferrous metals. Bath is more stable.

b) Basic activation
This process is based on Ti compounds and suitable for both ferrous/non ferrous substrates. It gives more compact/microcrystalline, Uniform phosphate coat with better corrosion resistance.
 The major disadvantages are:
 (i) The bath cannot be made in hard water
 (ii) Shelf life of bath is very low (One week maximum)


4. Phosphating
 It is the universal method of pre treatment. Phosphating consist of the deposition on the metal surface of the insoluble metal phosphate which are chemically bonded to the substrate. This process provides the following.
a) A clean, oil grease free surface.
b) A corrosion inhibitive base for powder coating
c) A non conductive bond between base metal & powder coating.
d) A chemically inert surface which prevent the reaction between the best metal and powder ingredients.


Types of zinc phosphating
 
Chemical Processes
Phosphate Processes
Zn Mono
Cationic
ZN DI
Cationic
ZN Tri
Cationic

 
Mono Cationic Di Cationic Tri Cationic
1 Contains only zinc as a basic cation Contains zinc and nickel as basic cations Contains zinc nickel and manganese as basic cations
2 Zinc imparts good adhesion and crystal formation Nickel contributes to corrosion resistance Manganese improves the wear and tear properties of the coating.
3 Generates hard sludge Generates hard sludge Generates soft sludge
4 Operates at higher temperatures Operates at lower or even at room temperatures Operates at lower or even at room temperatures.
5 Superior in terms of adhesion, corrosion resistance and wear and tear properties.
Comparison
Mechanical surface treatment Chemical surface treatment
1 Suitable for large structures when dipping in tanks is not possible 1 Suitable for all kind of applications from small appliances to automobile except bigger structures like ship
2 Good for site jobs/exterior application. 2 Good for interior use
3 Faster process. 3 Can be automised for faster speed.
4 Not suitable for local job. Coating application as process requires skilled labour, costly installations, safety clothings for operators. 4 Suitable for job coating market and hence more popular in powder coating industry.
5 Should be immediately followed by primer application (storage of treated jobs for longer time is not possible). 5 Parts can be stored for 48 hours if done correctly.
6 Does not seal the metal surface as in chemical process and hence rust creepage is much faster if surface gets exposed due to rupture of coating. 6 It chemically seals the surface there reducing the rate of creepage in the event of powder film damage.
7 Suitable for high temp, curing coating like HR 7 Not suitable for higher temp.

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