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
