Hillco Fastener Warehouse, Inc.

We have received several inquiries recently regarding black oxide finish.  Black oxide is a conversion coating (as opposed to an applied coating) because it results from a chemical reaction with the iron present in the metal fastener and forms an integral protective surface.  It is a color of the base metal, which neither removes nor deposits metal.  Therefore, it adds, at the most, 5 to 10 millionths of an inch to the fastener's dimensions.  Likewise, it cannot chip, peel or rub off, and the item maintains 99% of its conductivity making black oxide a popular finish for electrical parts.

When a black oxide finish is specified, it is called out as "Black Oxide and Oil".  Other supplementary coatings such as wax or lacquer may be ordered, but fasteners with those finishes are usually not available from stock and must be a special order.  Typically, the oil after-finish is dry to the touch, which gives the part a lustrous appearance and improved lubricity without excessive residue.  The color is a deep black rather than the grayish-black appearance of black phosphate, and remains consistent at temperatures as high as 900 degrees Fahrenheit.  Black oxide also resists abrasions better than phosphate, and one of the most important advantages is that there is almost no risk of hydrogen embrittlement because the process does not involve electro-plating nor does it require an acid-activation.  Case hardened parts need not be baked after receiving a black oxide finish.

Black oxide neither enhances nor detracts from a fastener's resistance to corrosion.  The post-treatment oil application offers good indoor corrosion protection, but a zinc-plated part is more resistant to rust.  However, parts with a black oxide finish will not suffer from white corrosion which can occur over time with electro-plated parts.  To avoid the risk of white corrosive particles (which can cause electrical shorts) many in the electronics industry opt for black oxide instead of zinc plated fasteners.

The popularity of black oxide stainless fasteners is also growing.  In addition to being used for decorative purposes, the finish reduces light glare and reflection and makes the surface of the fastener smoother.  The automotive aftermarket utilizes them in tail-light assemblies, grills and windshield wiper assemblies, to name a few.

Fastener threads provide both load holding when tightened and the ability to move during tightening and loosening.  A fastener's pitch and lead, determined by whether it is coarse or fine thread, is a factor in fastener performance.  A coarse thread will move a larger distance with each rotation than a fine thread of the same diameter.  If speed of assembly is a factor, a coarse thread may be preferred as it will assemble faster.  If adjustment is required, a fine thread will provide a less sensitive adjustment and a greater ability to control adjustment.

Each flank of thread carries a percentage of the clamping load.  This load is not distributed equally among the thread pitches in most applications.  It is generally thought that a fine pitch thread will have a more uniform load distribution among its engaged pitches when installed.

Fine threads can be more susceptible to cross-threading if assembly conditions are not controlled.  In general, both coarse and fine threads have advantages depending on the fastening output required.