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General pretreatment sequence For ferrous surface Degreasing (vapour/emulsion/solvent wipe/alkali)
For non ferrous surface (Al / zinc/zinc alloys etc.)
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.
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
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.
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 plantatmosphere due to fuming tendency |
Can cause corrosive plantatmosphere |
No Harm |
Carryover in phosphate stage can poison the bath |
Carryover in phosphate stage can poison the bath |
No Harm |
Hassevere pitting tendency |
Hassevere pitting tendency |
No pitting tendancy |
Cheapestacid for derusting |
Cheaperthan phosphoric |
Costlybut more used by the industry |
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)
| Mono Cationic | Di Cationic | Tri Cationic | |
|---|---|---|---|
| 1 | Contains only zinc as a basiccation | Contains zinc and nickel as basiccations | Contains zinc nickel and manganeseas basic cations |
| 2 | Zinc imparts good adhesion andcrystal formation | Nickel contributes to corrosionresistance | Manganese improves the wear andtear properties of the coating. |
| 3 | Generates hard sludge | Generates hard sludge | Generates soft sludge |
| 4 | Operatesat higher temperatures | Operatesat 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. |
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