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Dacrotizing produces a coat of Zinc-Aluminium flakes in a trivalent chrome polymer matrix in an aqueous media by drip process. The high efficiency of corrosion protection is due to combination of:
1. Controlled (sacrificial) galvanic protection by zinc.
2. Passivation by chrome.
3. Barrier effect of metal particles and the matrix.
Grauer & Weil (India) Limited has joined hands with M/s. Nippon Dacro Shamrock Company, Japan, to bring Dacrotizing process in India. G&W can supply process know-how chemicals, plants and equipments for dacrotizing.
Process Description :
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- Characteristics - Application - Properties - Limitations - Advantages
Process Description :
Shot Blasting
Using steel balls of dia 0.1 to 0.2 mm
Coating
Depending upon the size and the shape of the component, the following methods are used to coat the surface. Plants can be made according to specific requirements.
1. Dip Spin Process
Specialized equipment is used for coating small parts in bulk. After surface cleaning, parts are loaded in a basket and dipped in the water based chemical, raised and spun in order to remove excess coating liquid. Parts are then transferred to an oven for curing at temperature between 300 deg. C to 340 deg. C. Two coats and two bakes are standard for smaller parts.
2. Dip Drain Process
Parts are fitted on racks and transported from the surface cleaning stage to the coating section (dip tanks and drain zone) and then to the curing zone.
3. Electro Static Spray Process
Coating liquid is applied by electro static gun fitted with mini-cups or discs within a fully automated enclosed and ventilated spray booth - Parts which do not allow removal of excess coating are coated in this way.
4. Curing
Coated parts are sent to a heated oven and cured at temperatures between of 300 deg. C to
340 deg.C.
- Characteristics of the process
Good throwing power - Parts with complex shapes can be effectively coated.
No Hydrogen Embrittlement
Application of the coating does not generate any hydrogen embrittlement. Since acid pickling is not used and mechanical surface preparation (degreasing + abrasive cleaning) is used, there is no chance of hydrogen embrittlement during preparatory stage also.
Eco-Friendly
No Pollution :
The process for treatment in bulk does not create any pollution. Zn, Al, and Chrome used in the solution are totally transferred from the bath to the surface of the component. The only effluents are those which may arise from the periodic cleaning of coating equipments and racks. Since solution is mainly water-based there is no evaporation of organic solvents to the air. In addition, fumes from the curing oven is generated which mainly consist of water vapour.
- Applications
Dacrotizing is mainly used for the protection of ferrous metals, including high tensile steels.
Dacrotizing is particularly suitable for :
- Bolts and screws
- Wheel bolts.
- Spring Pins.
- Parts in the engine compartments.
- Brake discs.
- Self drilling and tapping screws.
- Properties
- Appearance :Silver grey finish.
- Corrosion resistance : At equivalent zinc coating weight dacrotizing is far superior to many other systems.
Grade A : Coating weight 200 to 220 mg / dm sq. i.e. approx. 5 microns.
Neutral Salt Spray Test 500 hours without red rust.
Grade B : Coating weight 240 to 260 mg / dm sq. i.e. approx. 8 microns. Neutral Salt Spray Test 1000 hours without red rust.
- Superior temperature stability :No change in properties upto 250 deg. C.
- Resistance to stone chipping : Good resistance, No significant adverse influence on corrosion protection.
- Resistance to solvents offers good resistance to fuels, brake fluids and other liquids associated
with vehicle operation.
- Limitations
- Dacrotized parts cannot be plastically deformed.
- Case hardened or work hardened components may undergo some modifications of mechanical properties following curing at 300 deg. C.
- Wide range of finishes offered by electro plating and painting are not possible.
- Advantages over Zinc Plating
- No chromate rinsing
- No hydrogen embrittlement
- Corrosion resistance in salt fog test far superior to zinc plating
- Can withstand exposure upto 250 deg. C temperature
- Excellent throwing power
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