This article is about modern black oxide coatings. For traditional methods of forming black oxide, see Bluing (steel)
Black oxide or blackening is a conversion coating for ferrous materials, stainless steel, copper and copper based alloys, zinc, powdered metals, and silver solder.[1] It is used to add mild corrosion resistance, for appearance, and to minimize light reflection.[2] To achieve maximal corrosion resistance the black oxide must be impregnated with oil or wax.[3] One of its advantages over other coatings is its minimal buildup.[2]
Ferrous material
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A standard black oxide is magnetite (Fe3O4), which is more mechanically stable on the surface and provides better corrosion protection than red oxide (rust) Fe2O3. Modern industrial approaches to forming black oxide include the hot and mid-temperature processes described below. Traditional methods are described in the article on bluing. They are of interest historically, and are also useful for hobbyists to form black oxide safely with little equipment and without toxic chemicals.
Low temperature oxide, also described below, is not a conversion coating—the low-temperature process does not oxidize the iron, but deposits a copper selenium compound.
Hot black oxide
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Hot baths of sodium hydroxide (NaOH), nitrates such as sodium nitrate (NaNO
3), and/or nitrites such as sodium nitrite (NaNO2)[3] at 141 °C (286 °F) are used to convert the surface of the material into magnetite (Fe3O4). Water must be periodically added to the bath, with proper controls to prevent a steam explosion.
Hot blackening involves dipping the part into various tanks. The workpiece is usually dipped by automated part carriers for transportation between tanks. These tanks contain, in order, alkaline detergent, water, sodium hydroxide at 140.5 °C (284.9 °F) (the blackening compound), and finally the sealant, which is usually oil.
The NaOH (caustic soda) and elevated temperature cause Fe3O4 (black oxide) to form on the surface of the metal instead of Fe2O3 (red oxide; rust). While it is physically denser than red oxide, the fresh black oxide is porous, so oil is then applied as post treatment to the heated part, which seals it by "sinking" into it. The combination prevents corrosion of the workpiece. There are many advantages of blackening, including:
μm thick.
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.[citation needed]
Iron(III) chloride (FeCl3) may also be used for steel blackening by dipping a piece of steel into a hot bath of 50% FeCl3 solution and then into a hot boiling water. The process is usually repeated several times.[4][non-primary source needed]
Mid-temperature black oxide
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Like hot black oxide, mid-temperature black oxide converts the surface of the metal to magnetite (Fe3O4). However, mid-temperature black oxide blackens at a temperature of 90–120 °C (194–248 °F),[5] significantly less than hot black oxide. This is advantageous because it is below the solution's boiling point, meaning there are no caustic fumes produced.[citation needed]
Since mid-temperature black oxide is most comparable to hot black oxide, it also can meet the military specification MIL-DTL-13924, as well as AMS 2485.[citation needed]
Cold black oxide
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Cold black oxide, also known as room temperature black oxide, is applied at a temperature of 20–30 °C (68–86 °F).[5] It is not an oxide conversion coating, but rather a deposited copper selenide (Cu2Se) compound. Cold black oxide is convenient for in-house blackening. This coating produces a similar color to the one the oxide conversion does, but tends to rub off easily and offers less abrasion resistance. The application of oil, wax, or lacquer brings the corrosion resistance up to par with the hot and mid-temperature. Applications for cold black oxide process include tooling and architectural finishing on steel. It is also known as cold bluing.[6]
Copper
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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, microscopy and other optical applications to minimise light reflection.[7]
In printed circuit boards (PCBs), the use of black oxide provides better adhesion for the fiberglass laminate layers.[8] 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]
Stainless steel
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Hot black oxide for stainless steel is a mixture of caustic, oxidizing, and sulfur salts. It blackens 300 and 400 series and the precipitation-hardened 17-4 PH stainless steel alloys. The solution can be used on cast iron and mild low-carbon steel. The resulting finish complies with military specification MIL-DTL–13924D Class 4 and offers abrasion resistance. Black oxide finish is used on surgical instruments in light-intensive environments to reduce eye fatigue.[citation needed]
Room-temperature blackening for stainless steel occurs by auto-catalytic reaction of copper-selenide depositing on the stainless-steel surface. It offers less abrasion resistance and the same corrosion protection as the hot blackening process.[citation needed]
Zinc
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Black oxide for zinc is also known by the trade name Ebonol Z.[10]
See also
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References
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There are several differences between carbon steel piping and black steel piping. These differences also account for why these pipes are used for different applications.
One of the main differences between carbon steel and black steel is that carbon steel piping and carbon steel pipe fitting are susceptible to corrosion. Black steel piping, on the other hand, is made from non-galvanized steel and is not susceptible to corrosion.
Carbon steel, as its name suggests, has a high carbon content. On the other hand, black steel gets its name from the dark-colored iron oxide that is used to coat its surface. Apart from that, black steel piping is made using several grades of malleable iron. Unlike this technique, when making carbon steel piping, it is made in a seamless fashion. That means carbon steel pipe fitting can be used for applications that require a very high capacity to withstand pressure.
Black steel piping is mainly used to carry gas into commercial buildings. This pipe is seamless, and this makes it perfect for carrying gas. Also, this pipe is more fireproof than carbon steel piping. Therefore it is also used in fire sprinkler systems. Carbon steel piping, on the other hand, is mainly used for carrying water into homes and commercial properties. This pipe is susceptible to corrosion, which is why it requires galvanization. Hence, it is also used as a scaffolding frame.
Making use of steel for pipes is both safer for people and the environment. Research shows that black steel is more eco-friendly than carbon steel when it comes to CO2 emissions during manufacturing. There are more than 100 major projects where flanges, piping, and various pipe fittings will be needed in the next five years. This means that if the environment is to be considered, most of these projects might use black steel instead of carbon steel pipe fitting due to its low carbon content.
For any offered pressure, carbon steel piping is cheaper than black steel. This is because it can be made much thinner. It also has a higher carrying capacity than black steel pipes with the exact same diameter. Also, its versatility makes it cost-effective. However, there are a lot of other considerations that are made when choosing the right type of steel piping.
As always, for best results, partner with a trusted distributor with a history of supplying projects with the right parts for the job. Reach out to us today at Lynco Flange & Fitting and we will get you taken care of!