Powder coating chemical pretreatment, nine times out of ten, refers to phosphate pretreatment.
WHY PHOSPHATE ?
Phosphate coatings are used to increase corrosion resistance and improve paint adhesion.
WHAT IS PHOSPHATE ?
Phosphate means the formation of a layer of zinc, iron, or manganese phosphate crystals on the surface of the part to be painted. It is used in the finishing of iron and steel surfaces and occasionally on aluminum and zinc. The amount of phosphate on the surface is usually expressed as milligrams (mg) per square foot of surface rather than units of thickness. The phosphate layer is generally applied by spray or dipping into a phosphating solution.
A phosphate coating is a transition layer in several respects. It is less dense than most metals but more dense than coatings. It has thermal expansion properties which are intermediate between that of the metal and the coating. The result is that phosphate layers can smooth out the sudden changes in thermal expansion which would otherwise exist between the metal and the paint. Phosphate coatings are porous and can absorb the coating. Upon curing, the paint solidifies, locking into the phosphate pores. Adhesion is greatly enhanced.
3-STAGE PHOSPHATE SPRAY PROCESS
1. Combined cleaning and phosphating. 1.0 to 1.5 minutes at 100 degrees F to 150 degrees F.
2. Water rinse 1/2 minute
3. Chromic acid rinse or deionized water rinse. 1/2 minute.
6-STAGE ZINC PHOSPHATING SPRAY
1. Alkaline pre-cleaning 1.0 minutes. 100-150 degrees F.
2. Water rinse 1/2 minute.
3. Titanium activator rinse, 1/2 minute 100 degrees F.
4. Phosphating stage, 1 minute 100-150 degrees F.
5. Water rinse 1/2 minute.
6. Chromic acid rinse, 1/2 minute, 100-150 degrees F.
The chromic acid rinse stage was commonly used in the finishing industry, however the environmental regulations of today forbid the dumping of chromic acid into the sewer system. When applied, diluted chromic acid is allowed to soak down into the defects in the phosphate crystal surface. When it reaches the iron below the phosphate, it forms an inert layer which resists corrosion. This phenomena is called passivation.
IRON PHOSPHATE
This is the oldest phosphating method. It is the most economical to install and operate, but is less corrosion resistant than zinc phosphating. Iron phosphating is usually deposited in the range of 25-75 mg per square foot. Control of the iron phosphate solution is somewhat easier than that of zinc phosphating. Also, iron phosphate lends itself to combining the cleaning and phosphating stages, thus saving time and floor space.
ZINC PHOSPHATE
This is often the method chosen when long life under corrosive conditions is required. Coating is also very good with zinc phosphate because the crystals form a porous surface which can soak up and mechanically trap the coating film. On the other hand zinc phosphate systems usually require more treatment stages, are more difficult to control and more expensive to install and operate. The zinc film is usually deposited at 200-500 milligrams per square foot. Total time required is about 4 minutes for a spray system.
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Powder Coating Chemical Pretreatment
