A Brief History
Phosphating … The Process
Unlike early iron phosphating, the solutions used today produce an amorphous coating of exceedingly fine crystals with an iridescent blue to bluish-brown color. Since iron phosphate crystals are translucent, their color is modified by the surface on which they are deposited. While iron phosphate coatings can be applied to steel to provide a receptive surface for the bonding of fabric, wood and other materials, their chief application remains as a foundation for paint. Under appropriate processing conditions, iron phosphate coatings have excellent adherence and show good resistance to flaking from impact or flexing. Spray application of iron phosphate is most commonly used, although immersion applications are also practical. The typical range of coating weight is 20-80 mg/ft2 (0.21-0.86 g/m2). Exceeding this thickness hasn’t shown to provide a significant amount of benefit, while underachieving this thickness tends to show a non-uniform, or discontinuous, coating.
The zinc phosphate coating, which can be attributed to Coslett’s use of zinc dihydrogen phosphate, possesses a wide range of weights and crystal characteristics. These coatings can be heavy films with coarse crystals or ultra-thin microcrystalline deposits. Depending primarily on the carbon content of the underlying steel, they may vary from light to dark gray in appearance (darker as the carbon content increases). Microcrystalline coatings usually are darker gray than coatings of the same weight and have coarser crystals. Zinc phosphate coatings can be applied by spray or imersion and may be used as a paint, wax, or oil base; an aid to cold forming or rust-proofing; and for increasing wear resistance of moving parts. The weight of spray zinc phosphate coatings over steel can range from 100 to 1000 mg/ft2 (1.1 - 10.8g/m2), while immersion coatings can range from 150 to 4000 mg/ft2 (1.6 - 43g/m2).
The use of manganese phosphate coatings can be traced back to R.G. Richards and his system patent that was published in 1911. Richards, like Coslett, used a solution of phosphoric acid, but instead of zinc, manganese dihydrogen phosphate was added directly into the bath. This type of process is also applied primarily to ferrous parts, most importantly on internal combustion engine parts where the phosphate coating acts as a lubricant carrier to prevent galling. Manganese phosphate coatings are usually black or dark brown, depending on the amount of manganese dioxide included in the coating. Because almost all manganese phosphate coatings are used as an oil base, and oil intensifies the black coloring, manganese phosphate coatings usually appear to be black. This type of phosphate is only applied by immersion and requires immersion times of five to 30 minutes. Coating weights are typically 500 to 3000mg/ft2 (5.4 – 32.3 g/m2), but can be heavier if needed. Usually, the preferred manganese phosphate coating is tight and fine grained rather than loose and coarse grained. Generally, a manganese phosphate crystal is softer and will break down more readily than a crystal of zinc phosphate. Manganese phosphate plus oil or wax is also used on cast iron and steel parts. Although manganese phosphate generally costs more to apply than zinc phosphate, the greater thickness of the coating encourages retention of more oil or wax, and thus may provide greater resistance to corrosion.
The mechanism of all phosphate coatings takes place in an acid
that contains the coating chemicals. The chemicals react with the metal
to be coated, and at the interface, a thin film of the solution is
by reaction with the metal. When this solution becomes neutralized at
interface, the solubility of the metal phosphate is reduced and a
is formed as a crystal. These crystals are attracted to the surface of
the metal by the normal electrostatic potential within the metal.
Even though all phosphate baths are acidic in nature and attack the metal being coated, hydrogen embrittlement seldom occurs as a result of the phosphating process. This is primarily because all phosphate baths contain depolarizers or oxidizers that react with the hydrogen as it is formed and render it harmless to the metal. Typically, zinc and manganese phosphate baths use these depolarizers as accelerators. This can be a mild oxidant, such as a nitrate, or a more vigorous nitrite like chlorate or peroxide. The purpose of these accelerators is to speed up the rate of the coating and to reduce the crystal size. This is accomplished by the ability of the accelerators to oxidize the hydrogen from the surface of the metal being coated. The phosphate solution can then contact the metal continuously, permitting completeness of reaction and uniformity of the coverage. Accelerators also have an oxidizing effect on the dissolved iron in the bath, thus extending the useful life of the solution. Although some iron phosphate baths do not require accelerators, many still incorporate oxidizing agents to accelerate the phosphating process.
The pH of the phosphate bath depends on the type of phosphate compound and its method of application. Manganese and immersion zinc phosphate baths operate in the pH range of 1.8 to 2.4, whereas spray zinc phosphate solutions can operate at a pH as high a 3.0. Iron phosphate baths can operate at a pH of 3.0 to 6.0.
Since 1864, phosphates have proven to be an inexpensive and
method of conversion coating metal surfaces. Phosphates have shown
in their ability to be used to improve corrosion resistance, prepare
for metal finishing, and, most importantly, condition surfaces for
With this versatility, the future of phosphates looks very