Conversion Coating
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Phosphating Chemicals
Mostly used pretreatment method is known as “phosphating”. Phosphate coating is the layer form on surface which includes iron, zinc or manganese crystals.
Phosphating consist of deposition on the metal surface of insoluble metal phosphate which are actually chemically bonded to the surface. It gives good adhesion for paint film.
These process provide following;
- Clean, grease/oil surface
- A corrosion inhibitive base for powder coating.
- Non conductive bond between base metal and powder coating
- A chemically inert surface which prevent the reaction between the base metal and powder paint.
Here are three types of phosphate coating; iron phosphate, zinc phosphate and manganese phosphate. Generally it is used for coating iron and steel surfaces. Immersion and spraying are the most common application methods
Here are three types of phosphate coating;
1. Iron Phosphate
This is the conventional and easiest method of phosphating. Corrosion resistance is lower than other phosphating methods. Generally, it leaves a blue-colored layer on the surface. Total acid, pH, temperature, and application time must be under control.
Iron phosphate conversion coating can be applied in three different ways:
- Wipe Iron Phosphating
- Immersion Iron Phosphateing
- Spray Iron Phosphating
2. Zinc Phosphate
Zinc phosphate Conversion coating is use to provide long lasting corrosion protection. all Automotive industries, Hardware, home appliance industries use this type of conversion coating.
Zinc Phosphating bath solution contain saturated solution of Phosphoric acid along with zinc phosphate. Do not keep parts or component in bath more than 10 minutes because pH of Bath is acidic.
Coating quality is better than iron phosphate coating. Application, set up and control of this process are more difficult than other methods and can be applied by immersion or spray.
Zinc phosphating is sub divided in to 3 types:
Mono Cationic: Bath solution contain only zinc as cation.
Di Cationic: This type of bath solution contain of Zinc and Nickel as cation or Zinc and Calcium as cation.
Tri- Cationic: This type of bath solution consist of Zinc, Nickel, and Manganese as cation.
3. Manganese Phosphate
This type of Phosphating is use in Piston and Weapon Industries. It makes easy the sliding of moving parts which are running in lubricant by reducing the friction between metal parts.
This type of phosphate coatings has more stable structure than zinc phosphate under high pressure and high temperature.
Temperature, application time, concentration, free and total acid points are the parameters to be cared. The process generally works at 70-98ºC, 15-20% concentration and with 5-20 minutes immersion time.