PASSIVATION

What is the passivation :

Passivation is called in normal terms but it is technically a chromate conversion coating process which  mostly apply on zinc or cadmium coated surface ,aluminium,  etc . That particular metal react with chrome base chemicals and form the that metal chromate film . 

Why we do passivation :

Passivation we do on zinc or cadmium coated surface for corrosion  resistance . On aluminium surface we do chromate conversion lubricity , or  foundation coatings for subsequent coating like powder coating, painting, oil etc. 

Types of Passivation :   

  

HEXAVALENT PASSIVATION FOR ZINC & CADMIUM COATED SURFACE

          

1.    Blue passivation – Sodium di chromate base thin coating , which give approx. 24 hours in salt spray for white rust. Appearance will be bright blue finish .

2.   Yellow  passivation – chromic acid hexavalent base , which give approx. 96 hours in salt spray for white rust.Appearance will be bright irridescent  finish .

3.   Black  passivation - chromic acid hexavalent and silver base , which give approx. 96 hours in salt spray for white rust.

4.   Green  passivation - chromic acid hexavalent base , which give approx. 120 hours in salt spray for white rust.

TRI VALENT PASSIVATION FOR ZINC

  

1.    Blue passivation – Trivalent chromium base thin coating , Appearance will be bright blue finish .

2.   Yellow  passivation – Trivalent chromium base , which give thick coating and approx. 96 hours in salt spray for white rust.

3.   Black  passivation - Trivalent chromium and Non silver base , which give approx. 96 hours in salt spray for white rust.

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 CLEANING : 

    ALUMINIUM  ACID DEGREASER 

For aluminium pretreatment ,  normally we clean in mild alkaline cleaner then do etching in the alkaline etchant , then we do further treatment for electroplating ,anodizing , chromatising  etc .

For chromatising purpose , now we can use the acid degreaser which will clean the parts and do mild etching also. By doing this we can avoid the alkaline mild etching and nitric acid dip , which will save chemicals ,  water, labour  and in acid degreaser process there will be no fumes like sodium hydroxide fumes in alkaline etchant . This acid degreaser will be good for shop floor compare to alkaline etchant .

  Process Sequence of Chromating :

1.   Acid Degreasing  :  Concentration : 50 ml/litre , time : 3 – 10 min , Ambient temperature

2.   Water rinse

3.   Water rinse

4.   Passivation

5.   Water rinse

6.   Water rinse

7.   Dry

PHOSPHATING

What is the phosphating :

When Deposition of  Metal phosphate like zinc phosphate, Iron phosphate, Manganese phosphate in a acidic bath by precipitation take place that process called phosphating .

Why we do phosphating :

Phosphating we do on steels for corrosion  resistance, lubricity , or  foundation coatings for subsequent coating like powder coating, painting, oil etc. 

Types of Phosphate coating :

1.    zinc phosphate : 

2.    Iron phosphate

3.   Manganese phosphate

4.   Calcium modified zinc phosphate

Types of Phosphating Process :

1.   Cold phosphating : Cold zinc phosphating process is for undercoat for powder coating , paint etc. In this  process coating will be medium weight between 200- 600  mg/sq feet .

2.   Hot phosphating : Hot zinc phosphating process is for Heavy zinc phosphate coating . This process we do on steel for corrosion  resistance and lubricity ( To retain the oil) purpose . Coating weight may be between 1000 -  2000 mg/sq feet . In the production process it will be very difficult to monitor the coating weight of each and every peice so for quality control we can check the thickness of zinc phosphate coating which can be between 12 - 20 microns .

3.   Trication Phosphating : Trication phosphating is a microcrystal mixed metal phosphate coating having small grain and give high corrosion resistance properties. Coating  weight may be between 200 - 500 mg/sq feet .

                                         

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