Conversion Coatings

Chemical reactions at casting surfaces can produce iron-containing compounds that provide wear resistance or an attractive appearance or that serve as excellent bonding agents for subsequent organic coatings. Table 11 details common conversion coatings and their useful properties (chromate and phosphate conversion coatings are discussed in greater detail in the articles "Chromate Conversion Coatings" and "Phosphate Coatings" in this Volume). Most of the successful processes are proprietary, and reproducibility of consistently good finishes is one of the important features.

Table 11 Chemical conversion coatings, structures, and characteristics

Type

Coating structure

Properties

Uses

Chromate

Nonporous film acts as moisture barrier

High corrosion resistance; inhibits corrosion if surface is broken; can be colored

Marine applications; can be decorative; nonporous bond layer for paint

Oxide

Ferric oxide formed from iron

Inhibits formation of ferrous oxide; highly absorbent; some wear resistance

Decorative blue-black coating; readily absorbs overlays of wax or oil

Phosphate

Iron zinc or manganese phosphates are crystalline structures formed on the surface by deposition from chemical solution

Chemically neutral and high adherence to iron surfaces; highly absorbent

Excellent for bonding paint to iron; prevents abnormal wear or seizing during break-in

Source: Ref 7

Source: Ref 7

Phosphate coating is the treatment of cast iron with a dilute solution of phosphoric acid and other chemicals in which the surface of the metal, reacting chemically with the phosphoric acid medium, is converted to an integral, mildly protective layer of insoluble crystalline phosphate. The coating reduces corrosion, prevents surface seizure on initial wear contact, and provides an excellent base for organic paint bonding.

Gray, ductile, and malleable iron castings all lend themselves readily to phosphating. The ability of a cast iron to accept a phosphate coating is not affected by alloy content, but hinges primarily on two requirements: a clean surface, and a metal temperature approximately equal to that of the phosphating bath. Dry machined surfaces need no further cleaning; cast surfaces can be prepared by blasting or other cleaning methods to remove scale and sand.

Oxide Coatings (Ref 7). Conversion of the surface of an iron casting to a magnetic oxide, Fe3O4, gives a thin black finish. These films have a certain degree of wear resistance, provide a good bond for paint or lacquer, and have pleasing decorative features. The film layer, if oiled or waxed, provides satisfactory corrosion resistance against handling and storage and is useful with lubrication for sliding and rotating wear applications. The original method of forming a strongly adherent black oxide film is still useful today. The process involves use of steam around an iron part heated above 480 °C (900 °F) and yields a tight oxide coating. Steam generally is introduced to furnace atmospheres to completely replace air while the parts are heated to 590 °C (1100 °F). Twenty minutes at temperature in this atmosphere is sufficient for buildup of a thick, wear-resisting film.

Oxide film finishes can also be produced in a wide variety of proprietary basic-nitrate media. Reproducibility of consistent finishes in large-volume operations is one of their prime features. These provide thinner films that are more suitable for decorative purposes. Sodium carbonate and sodium and potassium nitrates constitute the bulk of the bath solutions. Often these are supplemented with additional agents, such as manganese, to give a mixed oxide film.

Chromate conversion coating is accomplished by immersion of iron in an aqueous solution of chromic acid or chromium salts. This solution can be used as a supplement to cadmium and zinc plating to prevent the formation of powder corrosion products (see Table 9) and it also provides an excellent bond for paints.

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