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General. Protective coatings composed of insoluble phosphate crystals or iron oxides are applied to ferrous metal parts by a number of component processes in which varying types of chemical solutions are used. (The iron oxide coatings are commonly known as black oxide coatings.) The following three stages are normally used to apply the coatings. These are:

Cleaning and pretreatment

Coating

Preservation Treatment

The objective of the coating process is to provide an economical base for subsequent treatment which will protect parts from corrosion resulting from abrasion or exposure to moisture and perspiration.

The zinc or manganese phosphate coatings are formed on the parts by dipping the iron or steel parts in a solution of zinc or manganese dihydrogen phosphate containing an oxidizing agent such as nitrate. The pH of the solution ranges from 2.0 to 2.5. As iron is dissolved from the part by the acidic solution, the pH of the solution adjacent to the part increases until the insoluble phosphate coating is deposited on the part. In the spray operation the reactions are similar and occur at the interface between the solution and the surface of the parts being treated.

The insoluble phosphates involved in the phosphate processes are of three types. An insoluble phosphate consisting of zinc, manganese or iron monohydrogen phosphate is precipitated at a pH of about 4. This material is found in the sludge in the bottom of the tanks and in the scale on heating surfaces and tank walls. The monohydrogen phosphate salts are soluble in an excess of phosphoric acid and serve as a buffer to prevent the accumulation of excess phosphate acid in the processing bath. The phosphate coatings, normal phosphate salts, are formed at a PH of approximately 5.8 and are not easily dissolved in phosphoric acid even when exposed to an excess of that acid.

The zinc base phosphate coatings consist primarily of two crystals [zn₂Fe(PO₄)₂:4H₂O and Zn₃(PO₄)2:4H₂O]. The proportions of the two crystals vary depending upon many factors. These factors include: The composition of the phosphating bath, the temperature of the phosphating bath, and the surface preparation which determines the number of sites on which the crystals form.

The manganese base phosphate coatings have not been as well characterized as the zinc base phosphate coatings but it is believed that the crystal composition is similar.

Water of Crystallization will be lost when the phosphate coating is exposed to elevated temperatures. This loss results in a non-adherent powdery coating and a subsequent decrease in corrosion resistance. Exposure of no more than 15 minutes to temperatures in air of 225°F (107°C) will not adversely affect a zinc phosphate coating Corresponding temperature for manganese phosphate coatings is 375°F (190°C). Exposure at these temperatures for longer times or for shorter times at higher temperatures will cause a decrease in corrosion resistance.

The black oxide coatings are formed by immersing the iron or steel parts in a solution where the iron or steel surface is converted to an oxide, generally believed to be Fe₃ O₄. The bath for Class 1 treatments is a concentrated solution of sodium hydroxide and sodium nitrate. The bath for class 3 treatments is a molten dichromate salt, usually potassium dichromate. The bath for class 4 treatments is a concentrated solution of sodium hydroxide and proprietary sulfur compounds which form an oxide-sulfide coating.

The phosphate coatings are intended to provide supplementary resistance by holding a corrosion resistant finish such as an oil in the voids of the crystalline coating. Recent work has demonstrated that the phosphate coating itself provides some temporary corrosion resistance independent of the oil.

To obtain coatings (phosphate or black oxide) with the maximum corrosion resistance, it is necessary to remove all foreign matter from the part and process the part in a properly controlled chemical bath. On parts to be phosphated the surface must be conditioned to ensure that the crystalline coating has the desired structure. The preferred system is to removed all grease and oil, clean with abrasive blasting and process in a properly controlled bath.

Phosphate coatings meeting TT-C-490 are commonly suggested as the base for a paint specified by the procuring agency. The presence of the phosphate coating under the paint film aids in preventing underfilm corrosion and increases the durability of the paint film. The black oxide coatings are commonly finished with a corrosion resistant oil. Both phosphate and black oxide coatings are used a variety of military part. Some examples of applications are listed in the Appendix.

Any of these protective coatings will be ineffective if their continuity is broken by surface defects that serve as points of entry for corrosive substances. Such imperfections are unavoidable unless the metal surface is completely free of dust, grit, oil, acid and alkaline residues, rust, and other contaminants before the protective coating is applied.