Phosphate Coatings,Phosphate Coatings,Phosphate Coatings,Phosphate Coatings
PHOSPHATE COATING is the treatment of iron, steel, galvanized steel, or aluminum with a dilute solution of
phosphoric acid and other chemicals in which the surface of the metal, reacting chemically with the phosphoric acid
media, is converted to an integral, mildly protective layer of insoluble crystalline phosphate. The weight and crystalline
structure of the coating and the extent of penetration of the coating into the base metal can be controlled by:
· Method of cleaning before treatment
· Use of activating rinses containing titanium and other metals or compounds
· Method of applying the solution
· Temperature, concentration, and duration of treatment
· Modification of the chemical composition of phosphating solution
The method of applying phosphate coatings is usually determined by the size and shape of the article to be coated. Small
items, such as nuts, bolts, screws, and stampings, are coated in tumbling barrels immersed in phosphating solution. Large
fabricated articles, such as refrigerator cabinets, are spray coated with solution while on conveyors. Automobile bodies
are sprayed with or immersed in phosphating solution. Steel sheet and strip can be passed continuously through the
phosphating solution or can be sprayed.

Phosphate coatings range in thickness from less than 3 to 50 μm (0.1 to 2 mil). Coating weight (grams per square meter of
coated area), rather than coating thickness, has been adopted as the basis for expressing the amount of coating deposited.
Phosphate Coatings
Three principal types of phosphate coatings are in general use: zinc, iron, and manganese. A fourth type, lead phosphate,
more recently introduced, is operated at ambient temperatures.
Zinc phosphate coatings encompass a wide range of weights and crystal characteristics, ranging from heavy films
with coarse crystals to ultrathin microcrystalline deposits. Zinc phosphate coatings vary from light to dark gray in color.
Coatings are darker as the carbon content of the underlying steel increases, as the ferrous content of the coating increases,
as heavy metal ions are incorporated into the phosphating solution, or as the substrate metal is acid pickled prior to
phosphating. Zinc phosphating solutions containing active oxidizers usually produce lighter-colored coatings than do
solutions using milder accelerators.
Zinc phosphate coatings can be applied by spray, immersion, or a combination of the two. Coatings can be used for
any of the following applications of phosphating: base for paint or oil; aid to cold forming, tube drawing, and wire
drawing; increasing wear resistance; or rustproofing. Spray coatings on steel surfaces range in weight from 1.08 to 10.8
g/m2 (3.5 × 10-3 to 3.5 × 10-2 oz/ft2); immersion coatings, from 1.61 to 43.0 g/m2 (5.28 × 10-3 to 0.141 oz/ft2).
Iron phosphate coatings were the first to be used commercially. Early iron phosphating solutions consisted of
ferrous phosphate/phosphoric acid used at temperatures near boiling and produced dark gray coatings with coarse
crystals. The term iron phosphate coatings refers to coatings resulting from alkali-metal phosphate solutions operated at
pH in the range of 4.0 to 5.0, which produce exceedingly fine crystals. The solutions produce an amorphous coating
consisting primarily of iron oxides and having an interference color range of iridescent blue to reddish-blue color.

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