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Why Black Oxide


First, what is black oxide?  Basically there are 2 types of black oxide, room temperature and hot.  Room temperature blackening  is achieved with swab-on  acid chemicals that turn metals various colors from red tones to chocolate –brown; from Verdi-green to simulated pewter; grey to black and gun metal blue.  Color development depends upon the metal substrate, immersion time and chemical concentration.  It is used on copper, brass, bronze, white metal, silver, and tin/lead alloys.  Most chemicals used at ambient temperature are for ornamental purposes.  Steel and stainless steel can also be blackened with swab-on chemicals, maybe for aesthetics or architectural purposes, but certainly not for industrial use.

“Hot” black oxide is the only blackening process accepted for military and industrial purposes.  It is a mixture of alkaline salts and oxidizing agents containing penetrates, catalysts, activators and wetters that when mixed with water and heated to 285 degrees causes the atoms to become extremely agitated and bombard the surface of metal causing the surface to pick up an extra atom of oxygen.  The surface of the metal is then changed from Fe3 to Fe3O4 imparting a black finish with a depth of 5 to 10 millionth of an inch, basically unmeasurable.  So instead of a surface color change with swab-on chemicals you have an actual surface conversion process take place.  The surface now has the ability to retain sealants such as oil or lacquer.  The black oxide has a hardness of 5.5 on a Mohs’ scale.

You have the “what” now the “why”, Black Oxide metals?

 AESTHETIC APPEAL; The next time you look at electronics, tools or furniture notice all the black screws and parts that have been blackened. Chances are they have been black oxided.  The pleasing decorative black finish enhances the salability of many products.  Black oxide also produces a homogenous look to various machined parts that are assembled into one tool.

ANTI-GALLING; When it is necessary to break in mating parts, the anti-galling surface sacrifices the lubricating layer of black during the initial contact and abrasion, while a work-hardened surface is formed.

REDUCED GLARE; There is less eye fatigue with black moving parts and is a must when lasers are used during surgery.

DIMENSIONAL STABILITY; The blackening process produces essentially no dimensional change which means the blackened parts retain their surface properties.  Polished surfaces retain their gloss and heat-treated parts retain their Rockwell hardness.  Tool integrity is preserved and critical sized parts can be finished.  A blackened surface can be formed without flaking or peeling and can withstand temperatures of 350 degrees.

CORROSION PROTECTION; Adds years of service life to parts and extends the shelf life of stored parts.

ECONOMICAL; A much faster process which saves time and money versus painting or plating processes.  A simple, cost-effective means of providing corrosion protection to iron and steel parts.

This covers a majority of reasons for the black oxiding of metal parts.