Can Rust Be Good? If It’s Black Oxide It Can Be!

If you’re talking about the red-orange growth that seemingly spontaneously appears on your lawn furniture, bike chain, or propane tank the answer is an emphatic NO!

That red-orange compound is chemically speaking hydrated ferric oxide, Fe2O3, formed by exposure of iron to oxygen in the presence of water.

However, iron when exposed to a highly alkaline salt bath at about 285 degrees F forms the other oxide of iron, namely ferrous oxide, Fe3O4, which is an aesthetically appealing black oxide or what could be called black rust.  In this sense, it’s good rust!

The benefits of black oxide, i.e. good rust, include:

  • Deep black color which retains the luster or matte finish of the uncoated part.
  • Unlike painting or plating, black oxide will not chip, flake, peel or rub off.
  • No dimensional buildup as one would have with paint or plating.
  • Can be done in bulk making it more economically attractive than plating.
  • Black finish will reduce light glare reducing distraction and easing eye fatigue.
  • Supplemental coatings can significantly extend the protective nature of black oxide. 
  • Can readily be stripped and reprocessed without damaging the parts.
  • Black oxide finish is high temperature stable up to ~ 900 degrees F.
  • Unlike electroplating, there are no concerns with hydrogen embrittlement.

Black oxide is a conversion coating in that it converts the iron on the surface into a nearly immeasurable, uniform, beautifully appealing iron oxide.

black oxide

Typically the process to do this is referred to as the hot oxide process in that it uses a boiling salt solution at 280-290F.  Prior to the oxide treatment itself, typically parts have to be alkaline cleaned to remove machining coolants, rust preventative oils or other preservatives.

Following cleaning, parts are typically acid treated to remove any rust or other oxide that has formed.  Once cleaned and de-rusted, parts are ready for the oxide treatment.

The time to form a decent, deep black varies but for most steels it is 10-15 minutes.  Care should be taken following oxide treatment to thoroughly rinse parts as retained salts in porosity, welded seams or blind holes can result in ugly white crusty salts forming later on.  Following black oxide, a supplemental coating is typically applied.  This can be light or heavy oil, a dry-to-touch oil, or even wax.

The reason for this post-oxide treatment is to give the finish greater corrosion protection.  As I’ve explained to many a sportsman, what’s the first thing you do with your shotgun when you return from the field? The answer is wipe it down with oil.

Black oxide is nothing more than gun bluing; by itself it provides only minimal protection from corrosion.  With a proper post-oxide supplemental coating, black oxide will exhibit upwards of 100 hours of high humidity / high temperature exposure which provides excellent indoor corrosion protection. 

However, black oxide on steel is really not suitable for outdoor application or corrosive environments.

As a final cautionary note, don’t be fooled by room temperature DIY “black oxide” processes.  They contain selenic acid and deposit black selenium compounds which aren’t an oxide at all. Not surprisingly then, they don’t meet the military or aerospace specifications for black oxide!  Additionally, they tend to be less durable and readily rub off. 

Lastly, from a health and safety aspect, in humans exposure to selenium can result in brittle hair and hair loss, bad breath, fever and nausea.

It is also reported to cause reproductive impairment in aquatic life.  It’s always best off leaving chemical processing to specialists who are trained and experienced in using and disposing of such otherwise hazardous materials. 

In addition to the standard Black Oxide on Steel, we also offer Black Oxide on Stainless steel and copper here at Anoplate. Please contact us today for all your Black Oxide needs!