Powder coating Painting
electrically grounded surfaces until heated and fused into a smooth coating in a curing oven. The result is a uniform, durable, high-quality, and attractive finish.
The basic purpose is to carry out pre-treatment before powder coating. Normally 7-10 tanks are used for hot process for cleaning before applying the powder coat. General practice is to follow the best of standards in the implementation of “Seven Tank Process”.
Typical size: seven tanks are made of Poly Propylene and the size of each tank is (2600 L X 1400 B X 2500 D) mm
TANK-2 WATER RINSE-I
TANK-4 WATER RINSE-II
TANK-5 SURFACE ACTIVATION
TANK-7 WATER RINSE-III
Room Temperature Process, Duration-30-40 Minutes, For-Cleaning the Sheet Metal Surface of grease, oils, soils, lubricants, oxide films, Heat Treatment/welding Scales etc. by ALKALINE DEGREASER.
2. WATER RINSE-I
Rinsing Process is Carried out.
Room Temperature Process, Duration-20 Minutes, For-Cleaning The Sheet Metal Surface of Rust by Mixed HYDROCHLORIC ACID.
4. WATER RINSE-II
Rinsing Process is carried out.
5. SURFACE ACTIVATION
Room Temperature Process, Duration-20 Minutes, For-Activating the Sheet Metal Surface for Zn & Mn phosphate Coating by ACTIVATION CHEMICALS.
Room Temperature Process, Duration-5-20 Minutes, For-Smooth & Uniform Coating of Sheet Metal Surface with Zn Phosphate by ZINC PHOSPHATE SOLUTION
7. WATER RINSE-III
Rinsing Process is carried out.
Room Temperature Process, Duration-15-20 Minutes, For-Sealing Pores of Sheet Metal Surface for obtaining Maximum Corrosion Resistance over Phosphate Coating by DECXYLITE SOLUTION
150 Deg c Process, Duration-10-20 Minutes, For- Drying Sheet Metal Parts by hot air.
Done within 24 Hrs of Oven Drying, Sheet Metal parts are Coated with Powder of shade desired by client through Spraying in Spray Booths, and then Epoxy-Powder Coated/Sprayed Sheet Metal parts are baked in Electrically Fired Oven at 140-150 Deg c for 10-15 Minutes
Surface preparation includes
Cleaning – mechanical or chemical
Mechanical cleaning includes methods like scratch brushing and sand blasting. This by abrasive action not only removes the surface impurities but also eliminates scratches and surface irregularities. Cleaning is very good; however, coating must be done immediately because the cleaned surface is in a highly reactive state and corrosion occurs very soon. Chemical cleaning includes removal of dirt, oil and grease, and the oxidation products present on the surface by means of chemicals. The chemicals may be applied by wiping, spraying or dipping. The nature of chemicals used depends upon the base metal.
Application of conversion coating:
These are applied for three purposes:
a) To provide temporary in process corrosion protection before application of powder coating.
b) To promote good adhesion of the powder coating to the substrate.
c) To impart under paint corrosion & thereby improve the life of the powder coating.
The type of the conversion coating used also depends upon the base.
PRETREATMENT FOR MILD STEEL
TENDENECY OF MATERIAL: - picking up rust when exposed to atmosphere bare.
NEED OF CLEANING - Presence of oil, grease, rust and black scale on the surface. Mild Steel material is known for its tendency of going under rapid oxidation when exposed to atmosphere bare. The result of oxidation is the formation of oxide layer on the surface which we commonly call as ‘RUST’.
To avoid this oxidation or rusting, the mild steel material is not allowed to get directly exposed to atmosphere. For this, we generally find rust preventive oil on the surface of mild steel material. The oil does not allow the material to come in direct contact with air and thus delays oxidation. However, this doesn’t entirely eliminates the problem of rusting and it only delays the process of rust formation.
Some mild steel materials are hardened for strengthening and similarly some are welded during fabrication. Both these processes leave black scale on the surface.
It concludes that mild steel parts generally have rust and/or grease-oil and/or black scale on the surface. All these cause severe problems of bonding if not removed before application of paint.
NEED OF CONVERSION COATING – The bare metal being extremely susceptible to atmospheric corrosion should be applied with conversion coating Before painting, once the rust, oil and scale are cleaned, the bare metal is again exposed for oxidation. To avoid this, the material has to be coated with some type of conversion coating which prevents further oxidation before applying paint and also gives adhesion for paint.
Removal of oil and grease from the surface-
This can be done by Solvent Based Degreasing:- Petroleum based solvents have excellent degreasing abilities but are not commonly used because they are highly inflammable.
Alkaline degreasing powders:-
This is generally a blend of alkalies and surfactants. This is advised to be used under hot conditions. However, alkaline powders are also used under cold conditions as knock out degreasing stage.
These are solvent based liquids which are acidic in nature. They remove rust as well as grease under cold conditions. The bath of these chemicals is however, much expensive as their concentration is much higher than alkaline degreasing.
DERUSTING – Removal of rust and light scale from the surface.
These are necessarily acidic chemicals unlike degreasing, which may be alkaline in nature. The chemical is generally a blend of mineral acids like phosphoric acid, sulphuric acid and hydrochloric acid with added inhibitors. Higher acidity will result in faster removal of rust. However, this will affect the life of the coating applied later. Phosphoric acid based de-rusting chemicals are most suggested for pre-treatment of mild steel before applying paint.
CONVERSION COATING FOR MILD STEEL
The most common types of conversion coating include:
- Zinc phosphate coating
- Iron phosphate coating
Zinc phosphate coating is crystalline heavy coating gray in colour. While iron phosphate coating is amorphous conversion coating that ranges in colour from iridescent blue to gray.
Iron phosphate coating gives minimum sludge formation and hence smoother finish than zinc phosphate. Apart from this the bonding characteristics are also good. Iron phosphate processes are much easier to operate than zinc phosphate processes and require fewer process stages (zinc phosphate coating requires a pre-dip in activation chemical prior to phosphating to have micro crystalline grain structure), but iron phosphates do not provide the degree of corrosion protection imparted by zinc phosphates.
Iron phosphate systems are therefore used for a range of products requiring a durable finish that are not exposed to severely corrosive environments. Zinc phosphating processes have been developed to provide exceptional painted part durability in corrosive requirements. Typical industries using zinc phosphate processes include automotive, appliance and truck and bus.
After a metal surface receives a conversion coating, the surface is water rinsed to remove un-reacted conversion coating chemicals and a post-treatment is applied. The post-treatment can provide a two- to ten-fold increase in corrosion resistance and humidity resistance when compared to conversion coatings without final rinses. Post-treatments are generally based on chromic acid.
Problems Because Of Improper Cleaning:-
1. Appearance of oily patches.
2. Appearance of pin holes.
3. Appearance of rusty surface beneath powder coating film.
PROBLEMS BECAUSE OF IMPROPER CONVERSION COATING:-
1. Reduced resistance to weather.
2. Reduced mechanical resistance.
3. Poor finish of the powder coating film.
PROBLEMS BECAUSE OF IMPROPER DRYING:-
1. Appearance of pin holes.
2. Poor bonding of the powder coating film.
These problems can be avoided by the choice of right process – right chemicals and right maintenance of the chemicals.