Mild steel rapidly oxidises when exposed to humidity and oxygen in the atmosphere & quickly begins to rust when exposed to humidity in the atmosphere. To avoid rusting, the mild steel material is not allowed to be directly exposed to the atmosphere. Mild steel parts generally have rust and/or grease-oil and/or black scale on the surface. All of these cause severe problems for powder coat bonding if not removed before application of the powder. To avoid this, the material may be coated with a conversion coating or primer, which prevents further oxidation before applying paint and also gives adhesion to the paint.
Conversion coating is a process which converts the metallic surface to a zinc phosphate coating which increases the adhesion and durability of the paint system and inhibits corrosion. This treatment provides clean, corrosion-resisting, non-conductive, non-metallic surface for paint finishes. The complete process for paint bonding zinc phosphate consists of following stages.
Stage 1 - Degreasing : For uniform coating, all metals to be zinc phosphated should be free from grease, oil, any other foreign matter. Alkali and emulsifiable cleaner are used for this stage.
Stage 2 - Water Rinse : Water rinsing is very important to avoid contamination in subsequent stage and to get high performance of coating.
Stage 3 - De-rusting : When the removal of rust and light scale from the surface cannot be achieved by sanding, acidic chemical cleaners are commonly used. All metal to be phosphated should be free from rust & scale. Acidic de-rusting chemicals are used for this purpose.
Stage 4 - Water Rinse
Stage 5 - Activation : This stage is prior to the phosphate process that promotes the formation of a dense, fine grained phosphate coating, which offers superior adhesion and durability to the paint finish, in corrosive environments.
Stage 6 - Phosphating : This stage is the deposition of zinc phosphate crystalline coating on the metal surface. The article to be coated comes in contact with a zinc solution. Due to a complex chemical reaction with metal surface, an insoluble tertiary zinc phosphate coating is deposited. This coating acts as a base to improve paint adhesion to the metal
Stage 7 - Water Rinse
Stage 8 - Passivation : After rinsing, the article is treated in a Passivation solution. This treatment increases the corrosion resistance of the phosphate coating.
Stage 9 - Drying