Black Oxide Coating on Fasteners (October 24, 2013)
Black Oxide Coating on Fasteners
Black oxide treatments are a simple, attractive and versatile coating solution for fasteners from both a cosmetic and physical property standpoint.
Black oxide coatings have been used in the fastener industry for many years. These attractive and adherent coatings are popular choices for all types of fasteners as well as springs and many other products such as bearings and tools.
Black oxide coatings are formed by a chemical reaction between the metal surface and oxygen. An oxidizing agent may also be used in the form of an aqueous solution or molten salt bath. The surface layer formed consists of magnetite (Fe3O4), a form of iron oxide, which is black in color and tightly adherent to the metal surface. By contrast, rust is a red oxide of iron called hematite (Fe2O3), which easily flakes off a metal surface and as such is highly undesirable.
Since black oxide coatings adhere so well, they are most often used where tight tolerances are required with minimal change in surface dimensions, typically in the order of 0.013 mm (0.0005"). Fastener steels that benefit from this type of coatings include carbon steels, alloy steels, stainless and tool steels and certain nonferrous materials such as brass.
The decorative black color serves many purposes in addition to providing a strong adhesion between the coating and the substrate. It has good lubrication characteristics and will not peel away from the surface under moderate loading. It improves anti-galling properties, decreases the coefficient of (sliding) friction and improves abrasion resistance, all benefits for fastener applications. In addition, since the process does not involve hydrogen as part of the chemical process, there is no possibility of hydrogen embrittlement.
Additional advantages to black oxide coatings is their anti-glare surface and that they can be exposed to a temperature up to 900°„F (482°„C) before their color begins to change. Black oxide-coated parts can be welded and no harmful fumes are produced.
However, one of the misnomers about black oxide coatings is that they significantly improve corrosion resistance. While slight improvements have been reported, fasteners in corrosive environments will not generally be protected. A rust preventative oil or wax can be applied after the black oxide coating has been applied (and the part thoroughly rinsed) to improve corrosion resistance.
To achieve the best adherence, the metal must be clean so the first steps in the coating process are cleaning, rinsing and thoroughly drying of the metal surface. If rust or scale is a present, additional steps such as acid pickling or alkaline descaling should be done and then the surface may need to be neutralized in preparation for oxidizing. Any other types of coating such as zinc, nickel, chromium, cadmium plating or phosphates must be stripped prior to the black oxide process°™the oxide layer will not be consistent in thickness, vary in color and/or will flake off if attempted over dirty or plated parts.
A variety of black oxide coating chemicals can be used.
The final step in the black oxide process is the application of a supplementary coating also referred to as an afterfinish. This step will dictate the final appearance as well as improve the functionality of the part. To obtain the best corrosion protection, the after-finish is always applied after multiple rinse steps. The most frequently used after-finish is black oxide and oil. In addition to oil, wax or lacquer can also be used, particularly in applications where the fastener is to be used indoors and where corrosion protection is desired. Black oxide coatings on steel fasteners will not protect them in severe outdoor applications or corrosive environments.
The fastener geometry and end-use applications will usually determine the after-finish that is used. Factors to be considered are the final application of the part, the protection needed and for how long, the environmental conditions (humidity, vapor, temperature) and the desired final finish and appearance. An oil after-finish will generally be glossy, while a wax after-finish will be more of a matte finish.
Various testing procedures are used to validate the adherence of the black oxide coatings. The most common is a visual test, which simply involves looking at the component either with the naked eye or under a low power magnificabytion (5X to 50X). The visual test examines whether or not the metal is a uniform shade of black and if all surfaces are covered. In addition, the test looks for evidence of pitting, intergranular attack or etching. It should be noted that although parts will be black in color, pre-existing scratches, tool marks and other surface defects will be visible after finishing because black oxide does not smooth out or fill in these flaws. The °įsmut test°Ī is another test performed prior to the application of the after-finish to determine whether or not a black or dark colored powdery residue is present when the part is rubbed by hand. A humidity test per ASTM D2247 (Standard Practice for Testing Water Resistance of Coatings in 100% Relative Humidity) is another test that can be used when fasteners will be used outdoors.
Black oxide coatings are commonly divided into four distinct classes:
• Class 1: Alkaline oxidizing for wrought iron, cast and malleable irons, common carbon, and low alloy steels.
• Class 2: Alkaline oxidizing for use on certain corrosion resistant steel alloys tempered at less than 482°„C (900°„F)
• Class 3: Fused salt oxidizing for corrosion resistant steel alloys which are tempered at 482°„C (900°„F) or higher.
• Class 4: Alkaline oxidizing for other corrosion resistant steel alloys.