Black oxide
Encyclopedia
Black oxide or blackening is a conversion coating
for ferrous
materials, copper
and copper based alloys, zinc
, powdered metals, and silver solder. It is used to add mild corrosion resistance and for appearance. To achieve maximum corrosion resistance the black oxide must be impregnated with oil or wax. One of its advantages over other coatings is its minimal buildup.
(Fe3O4). 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, making this a dangerous process.
Hot blackening involves dipping the bolt into various tanks. The workpiece is usually "dipped" by automated part carriers for transportation between tanks. These tanks contain, in order, alkali
ne 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:
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.
, 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 (204.4 °C); above this temperature the coating degrades due to oxidation of the base copper. To increase corrosion resistance the surface may be oiled, lacquer
ed, or wax
ed. It is also used as a pre-treatment for painting
or enamel
ling. The surface finish is usually satin
, but it can be turned glossy by coating in a clear high gloss enamel.
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.
In Printed circuit board
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.
Conversion coating
Conversion coatings are coatings for metals where the part surface is converted into the coating with a chemical or electro-chemical process. Examples include chromate conversion coatings, phosphate conversion coatings, bluing, black oxide coatings on steel, and anodizing. They are used for...
for ferrous
Ferrous
Ferrous , in chemistry, indicates a divalent iron compound , as opposed to ferric, which indicates a trivalent iron compound ....
materials, copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
and copper based alloys, zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
, powdered metals, and silver solder. It is used to add mild corrosion resistance and for appearance. To achieve maximum corrosion resistance the black oxide must be impregnated with oil or wax. One of its advantages over other coatings is its minimal buildup.
Hot black oxide
A hot bath of sodium hydroxide, nitrates, and nitrites, at 285 °F (140.6 °C), are used to convert the surface of the material into magnetiteMagnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
(Fe3O4). 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, making this a dangerous process.
Hot blackening involves dipping the bolt into various tanks. The workpiece is usually "dipped" by automated part carriers for transportation between tanks. These tanks contain, in order, alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
ne cleaner
Detergent
A detergent is a surfactant or a mixture of surfactants with "cleaning properties in dilute solutions." In common usage, "detergent" refers to alkylbenzenesulfonates, a family of compounds that are similar to soap but are less affected by hard water...
, water, caustic soda at 455.4°C (the blackening compound), and finally the sealant, which is usually oil
Oil
An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....
. 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)
- no significant dimensional impact (the blacking process creates a layer about a micrometreMicrometreA micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
thick) - it is far cheaper than similar corrosion protection systems, such as paintPaintPaint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...
and electroplatingElectroplatingElectroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal...
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
Black oxide for copperCopper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, sometimes known by the trade name
Trade name
A trade name, also known as a trading name or a business name, is the name which a business trades under for commercial purposes, although its registered, legal name, used for contracts and other formal situations, may be another....
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 (204.4 °C); above this temperature the coating degrades due to oxidation of the base copper. To increase corrosion resistance the surface may be oiled, lacquer
Lacquer
In a general sense, lacquer is a somewhat imprecise term for a clear or coloured varnish that dries by solvent evaporation and often a curing process as well that produces a hard, durable finish, in any sheen level from ultra matte to high gloss and that can be further polished as required...
ed, or wax
Wax
thumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...
ed. It is also used as a pre-treatment for painting
Painting
Painting is the practice of applying paint, pigment, color or other medium to a surface . The application of the medium is commonly applied to the base with a brush but other objects can be used. In art, the term painting describes both the act and the result of the action. However, painting is...
or enamel
Vitreous enamel
Vitreous enamel, also porcelain enamel in U.S. English, is a material made by fusing powdered glass to a substrate by firing, usually between 750 and 850 °C...
ling. The surface finish is usually satin
Satin
Satin is a weave that typically has a glossy surface and a dull back. It is a warp-dominated weaving technique that forms a minimum number of interlacings in a fabric. If a fabric is formed with a satin weave using filament fibres such as silk, nylon, or polyester, the corresponding fabric is...
, but it can be turned glossy by coating in a clear high gloss enamel.
On a microscopic scale dendrites
Dendrite (metal)
A dendrite in metallurgy is a characteristic tree-like structure of crystals growing as molten metal freezes, the shape produced by faster growth along energetically favourable crystallographic directions. This dendritic growth has large consequences in regards to material properties.Dendrites form...
form on the surface finish, which trap light and increase absorptivity
Molar absorptivity
The molar absorption coefficient, molar extinction coefficient, or molar absorptivity, is a measurement of how strongly a chemical species absorbs light at a given wavelength...
. Because of this property the coating is used in aerospace
Aerospace
Aerospace comprises the atmosphere of Earth and surrounding space. Typically the term is used to refer to the industry that researches, designs, manufactures, operates, and maintains vehicles moving through air and space...
applications for light rejection.
In Printed circuit board
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...
use the black oxide provides better adhesion for the fiberglass laminate layers. The PCB is dipped in a bath containing Hydroxide, Hypochlorite, and Cuprate
Cuprate
Cuprates are chemical compounds containing copper anion. Cuprates have been known for centuries and are widely used in inorganic and organic chemistry...
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.