Black oxide

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This article is about modern black oxide coatings. For historical uses, see Bluing (steel).

Black oxide or blackening is a conversion coating for ferrous materials, copper and copper based alloys, zinc, powdered metals, and silver solder.[1][2] It is used to add mild corrosion resistance and for appearance.[3][4] To achieve maximum corrosion resistance the black oxide must be impregnated with oil or wax.[5] One of its advantages over other coatings is its minimal buildup.[4]

Ferrous materials

Hot black oxide

A hot bath of sodium hydroxide, nitrates, and nitrites,[5] at 285 °F (141 °C),[6] are used to convert the surface of the material into magnetite (Fe3O4).[3] Water must be periodically added to the bath, with proper controls to prevent a steam explosion. Explosion of these caustic baths has repeatedly claimed lives[citation needed], making this a dangerous process.

Hot blackening involves dipping the bolt into various different tanks. The workpiece is usually "dipped" by automated part carriers for transportation between tanks. These tanks contain, in order, alkaline cleaner, water, caustic soda at 455.4°C (the blackening compound), and finally the sealant, which is usually oil. The caustic soda bonds chemically to the surface of the metal, creating a porous base layer on the bolt. Oil is then applied to the heated bolt, which seals it by "sinking" into the applied porous layer. It is the oil that prevents the corrosion of the workpiece. There are many advantages of blackening, mainly:

  • blackening can be done in large batches (ideal for small bolts)
  • zero dimensional impact (the blacking process creates a layer about a micrometre thick)
  • it is far cheaper than similar corrosion protection systems, such as paint and electroplating

The oldest and most widely used specification for hot black oxide is MIL-DTL-13924, which covers four classes of processes for different substrates. Alternate specifications include AMS 2485, ASTM D769, and ISO 11408.

This is the process used to blacken wire ropes for theatrical applications and flying effects.

Cold black oxide

Cold black oxide is applied at room temperature. It is not an oxide conversion coating, but rather a deposited copper selenium compound. This coating produces a similar color, but tends to rub off easily and offers no corrosion protection.

Copper

File:Specular reflectange of cupric oxide.svg

Black oxide for copper, sometimes known by the trade name Ebonol C, converts the copper surface to cupric oxide. For the process to work the surface has to have at least 65% copper; for copper surfaces that have less than 90% copper it must first be pretreated with an activating treatment. The finished coating is chemically stable and very adherent. It is stable up to 400 °F (204 °C); above this temperature the coating degrades due to oxidation of the base copper. To increase corrosion resistance the surface may be oiled, lacquered, or waxed. It is also used as a pre-treatment for painting or enamelling. The surface finish is usually satin, but it can be turned glossy by coating in a clear high gloss enamel.[7]

On a microscopic scale dendrites form on the surface finish, which trap light and increase absorptivity. Because of this property the coating is used in aerospace applications for light rejection.[7]

In Printed circuit board[8] use the black oxide provides better adhesion for the fiberglass laminate layers. The PCB is dipped in a bath containing Hydroxide, Hypochlorite, and Cuprate which becomes depleted in all three components. This indicates that the Black Copper Oxide comes partially from the Cuprate and partially from the PCB Copper circuitry. Under microscopic examination, there is no Copper(I) Oxide layer.

An applicable U.S. military specification is MIL-F-495E.[9]

Zinc

Black oxide for zinc is also known by the trade name Ebonol Z.[10]

Notes

  1. ↑ What metals can be black oxided?, http://www.clevelandblackoxide.com/whatmetals.htm, retrieved 2009-09-03
  2. ↑ Black Oxide for Non-Ferrous Metals, http://www.heatbath.com/heatbath/products/black_oxide_finishing/black_oxide_for_non-ferrous_metals, retrieved 2009-09-03
  3. ↑ 3.0 3.1 What are the advantages of Black Oxide?, http://www.clevelandblackoxide.com/advantages.htm, retrieved 2009-09-03
  4. ↑ 4.0 4.1 Oberg 1996, p. 1444.
  5. ↑ 5.0 5.1 Oberg 1996, p. 1442.
  6. ↑ Black oxide FAQ, http://www.epi.com/black-oxide/blackoxide-faq.htm, retrieved 2008-09-14
  7. ↑ 7.0 7.1 Ebonol 'C', http://www.jacksonplating.co.uk/ebonol-c.php, retrieved 2009-09-04
  8. ↑ http://www.finishing.com/260/16.shtml
  9. ↑ The specification can be found here [1].
  10. ↑ Enthone, http://www.enthone-omi.com/functional/index.aspx, retrieved 2009-09-04

Bibliography

nl:Chemisch zwarten pl:Oksydowanie ja:黒染め sv:Svärtning


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