Phosphating Process

Phosphating process are used very widely in industry for three distinct purposes, for each of which a somewhat different formulation is necessary. These end uses are:

1. Application of a simple and cheap corrosion preventative for machine and mechanical steel parts. As formed, the phosphate coating is dull matt and grey black; it is unattractive in appearance and has only slight protective properties. It is, however, moderately rough and absorbent. It is therefore usually treated with an aqueous solution of a black dye and then, after drying with hot oil.
2. Rather thicker phosphate coatings are used alone or impregnated with oil to provide an exceptionally good lubricant in the processes of heavy pressing, drawing, forming and even extrusion, of steel. Similarly, they are often applied to gears and other sliding parts to reduce friction. The coating not only holds the oils, but itself prevents the metal-to-metal contact which is the cause of scoring and seizure when metal works on metal.
3. The most widespread application of phosphate coating is as a basis for industrially applied paints, lacquers and enamels. The phosphate coating greatly increases the adhesion of the organic coating to the base, and also largely prevents the lateral extension of the corrosion from inevitable points of damage, which would otherwise cause blistering of the adjacent paint film.
4. Phosphate coatings are sometimes applied to zinc-plated or galvanized steel, the zinc coating supplying even more assurance against rusting. Phosphate coatings are also applied to zinc base die-castings as an aid to paint adhesion. Thick coatings are in the range 700 to 2000 mg/ft2; light coatings are 150 to 400 mg/ft2. A process using a solution containing manganese gives thicker coatings, and is preferred for the first two application. A zinc process is more rapid, and can even be applied by spraying the liquid onto the work.