You don’t have to be a metallurgist to appreciate the value of a good finish on hardware. Yes, a good finish can sometimes hide less than perfect metals, but in general, an attractive finish indicates a high-quality component. A good example are the finishes of Makers Bolt fasteners, available to hobbyists, enthusiasts, and manufacturers alike. We offer fasteners in a number of black finishes. We’ll discuss four: Black Oxide, Black Ruspert, Black Zinc, and Black Xylan.
Frequently used on precision fasteners, black oxide is a conversion finish, not an applied coating. Black oxide forms an integral protective surface created during a chemical interaction with the metal fastening. The conversion solution is applied to the surface in a bath either chemically or electrochemically. The black oxide process was originally developed during the early 1900s. The modern bath became commercially prevalent during the later 1930s and has remained so.
Due to the absence of electroplating and acid activation, one of the most important benefits of black oxide is that there is essentially little possibility of hydrogen embrittlement. It is not necessary to bake case hardened pieces after applying a black oxide finish. The other consideration is that black oxide doesn’t alter the dimensional properties of the finished part, making it ideal for precision hardware.
Makers Bolt uses a hot bath for black oxide parts, immersing them in an alkaline aqueous salt solution at approximately 285 degrees F. This process can also be used on some non-ferrous metals and alloys; it is favored on iron, steel, and stainless steel. Magnetite (Fe3 O4) is formed on the surface of the components as a result of the reaction between the iron in the ferrous alloy and the hot oxidizing bath. The process is performed in the USA to assure the highest quality.
The Five Step Process for Black Oxidizing
To prevent black oxide coating from having a less than uniform finish in color or coverage, the process includes five steps: clean, rinse, black oxide bath, rinse, and after-finish. Oil and wax are the most prevalent after-finish. Lacquers are also used. If no after-finish is specified, the default will be oil.
Black oxide has been measured to be only five-millionths of an inch, with no discernible dimensional change. The finish does not chip, peel, crack, or rub off since it becomes a permanent part of the surface of the base metal.
Base material will determine the characteristics of the finish. Mechanical methods can be used to attain the appropriate gloss and shade level; machining, polishing, line-graining, abrasive blasting, polishing, and buffing. Black oxide is a non-sacrificial coating that provides just a modest level of corrosion protection and is typically specified when the dimensional build-up of a more corrosion-resistant finish cannot be accepted. For this reason, it’s crucial to choose stainless steel as a foundation material. Little to no corrosion prevention is necessary for stainless steel.
Parts having a black oxide finish will not erode with white corrosion, however, which can occur over time with electroplated parts. Engineers in the electronic industry often use zinc plated hardware to avoid the risk of white corrosive particles that can cause electrical shorts.
Black oxide stainless steel needs no supplementary coatings to be superior. Black oxide stainless fasteners are growing in popularity. It’s ideal for decorative purposes because the finish reduces light glare and reflection and makes fasteners smoother.
Types of Black Oxide Finish for Precision Machined Parts
Black oxide can be produced using hot or cold blackening procedures. To complete the surface treatment, each of these categories has its own proprietary coating composition. Because the Cold Black Oxide finish method employs an additive coating rather than conversation coating, which alters the part’s dimension significantly, it can be carried out at room temperature. This method differs significantly from the hot blackening method in that it does not entail the conversion of oxide. Instead, it calls for the modification of its end product using deposited copper selenium. Although this method is simpler to use, it could not produce the best aesthetic qualities.
It’s What is Underneath that Counts
Regardless of the finish, products should be evaluated by their base material, namely ferrous metals, steel, stainless steel parts, copper, and zinc. Black oxide neither enhances nor detracts from a fastener’s resistance to corrosion. If corrosion protection is required, a stainless steel fastener is preferred. Nearly all the Makers Bolt products labeled stainless steel is 304 aka 18-8 stainless steel and 410 stainless steel for self-drilling screws marketed as stainless steel.
Black oxide stainless fasteners are growing in popularity. They are ideal for decorative purposes because of the reduced light glare and reflection. The fastener surface is smooth and attractive, and in general renders the fastener unnoticeable.
Black oxide is frequently used for the following purposes but is not limited to them:
- Fasteners, gauges, shafts, bearings, springs, and other hardware tools used in assembly.
- Parts for military applications that must adhere to strict standards, such as rifle parts and shotgun barrels.
- Surgical equipment in a light-sensitive environment.
- Electrical parts include wire strippers, timer and electrical switch gears, and cutters.
- Automobile components such as oil filter cans and fasteners for the hood, tail-light assemblies, luggage racks, grills, and windshield wiper assemblies.
Enter Black RUSPERT®, the Four Layer Alternative
Ruspert metal finish is a high-quality coating with exceptional corrosion resistance. The proprietary process consists of three layers, but Makers Bolt adds a fourth layer to deliver 1000hr no red rust. The 1st layer is a metallic zinc layer, the 2nd layer is a high-grade anticorrosion chemical conversion film, and the 3rd and 4th layers are baked ceramic surface coatings.
The tight joining of the outer coating and the chemical conversion film make Ruspert noteworthy. These layers are bonded together through chemical reactions. The result is a rigid combination of the coating films.
Black Zinc and Black Xylan®
Other processes used to create a black corrosion finish include black zinc and black Xylan®. Zinc plating of .0002″ thickness is followed by a black chromate conversion coating. This results in the corrosion protection of zinc, along with a good appearance. It does affect the dimensions enough to be a problem on very tight-fitting parts.
Often applied by spraying, Black Xylan® is a fluoropolymer coating designed for use on various types of OEM components and fasteners to prevent corrosion and increase the life of the product. Most Xylan® coatings contain PTFE or other types of lubricants and are applied in thin films.