An Overview of  Phosphating
By Michael Marzano and Tony Oriti
Pavco, Inc. Corrosion protection, paint adhesion, uniform coverage and cost effectiveness are qualities that have seen sought by metal finishers from the infancy of metallurgy and can all be furnished by the use of a phosphate coating. Phosphating is a relatively simple process that has been used for well over a century to protect metal from corrosion. In general, phosphating is the conversion of a metal surface into an insoluble and integrated lattice of metal and crystalline phosphate. This process takes place by the treatment of the surface with a solution of phosphoric acid and other chemicals, which react with the metal to form a slightly protective layer.

A Brief History 

The phosphating process dates back to 1864 when de Bussy obtained a British patent for the treatment of red-hot iron with a mixture of coal-dust and calcium dihydrogen phosphate. This mixture added a corrosion-resistant layer to the metal surface. Other scientists at this time also proposed different treatments, including the immersion of the heated iron into a solution of phosphoric acid and a mixture of sodium and ammonium dihydrogen phosphates. In 1906, T.W. Coslett of Birmingham, England, obtained a U.K. patent for a revolutionary idea involving the treatment of a properly cleaned ferrous substrate with diluted phosphoric acid. This bath operated at temperatures close to boiling, and so to reduce the effect of the intense chemical reaction, Coslett introduced iron filings into the solution. These filings reacted with the phosphoric acid to form ferrous dihydrogen phosphate. Coslett’s most significant improvement, however, took place when he used zinc dihydrogen phosphate added directly to the phosphoric acid. By doing this, he became the originator of a process that remains in use to this day. These early solutions, which operated at temperatures near boiling, have paved the way for the more familiar phosphate baths of today that operate at temperatures ranging from 100 to 180 degrees F and are used industry-wide. 
During World War II, it was discovered that a light phosphate coating could be used as a primary base to provide excellent adhesion for subsequent paint films. In addition to paint, it was found that a heavy phosphate coating had quite an affinity for oils or waxes, and the use of these over the phosphate coating would supplement the corrosion protection of the phosphate. Phosphating plus oil or wax is commonly used to treat cast, forged and hot rolled steel parts. Because of their abilities as lubricant foundations, these integrated phosphate and oil coatings are used to provide resistance to wear, galling or scoring of moving parts. However, on the basis of pounds of chemicals consumed, or tons of steel treated, the greatest use of phosphate coatings remains as a base for paint.

Phosphating … The Process

The conversion of a metal surface to an insoluble phosphate coating provides that surface with a physical barrier against moisture. The amount of corrosion protection that a phosphate coating imparts to a metal surface depends on the uniformity of the coating, as well as the thickness, density and crystal size. Coatings can be produced with a wide range of thickness depending on the method of cleaning before treatment, adjustment in the concentration of the phosphating solution, and duration of treatment. In phosphating, no electric current is used, and formation of the coating depends primarily on contact between the phosphating solution and metal surface and on the temperature of the solution. Consequently, uniform coatings can be produced on irregularly shaped articles in recessed areas and on threaded parts, as well as on flat surfaces. Being a conversion coating, phosphating can be accomplished by immersion or spraying. Small parts with many recessed areas, such as nuts, bolts and other small stampings are immersion coated in a tumbling barrel. Parts that are too large for this barrel process may be immersion coated by the racking method. If the part is still too large for rack application and has very flat surfaces, it may be treated while on a conveyor system by a sprayed phosphate solution. Phosphate coatings can have a thickness ranging from less than 0.1 mil to more than 2.0 mil. Typically, though, the amount of coating is measured by milligrams of coating per square foot, or grams per square meter, rather than by thickness in thousandths of inches. Of the many phosphates proposed, only iron phosphate, zinc phosphate, manganese phosphate, zinc-manganese phosphate, zinc iron (II) phosphate, mnganese-iron (II) phosphate, and zinc-calcium phosphate are of industrial importance. Although aluminum and magnesium can be treated through this process, ferrous and zinc surfaces remain the most commonly phosphate-treated metals in the industry. The three most commonly used phosphate coatings are zinc phosphate, iron phosphate and manganese phosphate. Iron phosphating was the first processes to be used commercially and originally produced a dark gray coating with coarse crystals. 

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 bath 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 neutralized by reaction with the metal. When this solution becomes neutralized at the interface, the solubility of the metal phosphate is reduced and a precipitate 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 widespread method of conversion coating metal surfaces. Phosphates have shown value in their ability to be used to improve corrosion resistance, prepare surfaces for metal finishing, and, most importantly, condition surfaces for painting. With this versatility, the future of phosphates looks very healthy.