david french

Re: The Basics of Black Oxide

The Basics of Black Oxide MIL-C-13924]

Class 1: Alkaline oxidizing. For wrought iron, cast and malleable irons, common carbon, and low alloy steels. 
Class 2: Alkaline oxidizing. Use on certain corrosion resistant steel alloys tempered at less than 900F 
Class 3: Fused salt oxidizing. For corrosion resistant steel alloys which are tempered at 900F (482C) or higher. 
Class 4: Alkaline oxidizing. For other corrosion resistant steel alloys.

What is it?
Black oxide is a conversion coating, formed by a chemical reaction, produced when parts are immersed in an alkaline aqueous salt solution operating at approximately 290 degrees F. The reaction between the iron of the ferrous alloy and the hot oxide bath produces a magnetite (Fe3 O4) on the actual surface of the part. It is also possible to oxidize some non-ferrous metals under suitable conditions to form black oxides. It is possible to apply black oxide at room temperature, however it is not possible to achieve all of the benefits available from the "hot" oxide process. The cold black oxide process routinely shows color variation from part to part and the black material frequently rubs off in your hands. The cold process does not meet military or automotive specifications. Therefore the remainder of this information addresses the "hot" process.
How is the process performed?
The five basic steps for the black oxide conversion coating are clean; rinse; black oxide; rinse; supplementary coating (after-finish). If rust or scale is present on the part, additional steps such as acid pickling or alkaline de-scaling may have to be added before oxiding. Neutralizing and/or further rinsing may be necessary on assemblies and parts with blind holes to eliminate "flowering" or bleed-out. Black oxide cannot be produced over plated parts (zinc, nickel, chromium, cadmium, phosphate). The plating must be stripped prior to the black oxide process.

The supplementary coating (after-finish) will dictate the final appearance and function of the part. When a print or drawing specifies "Black Oxide" without any specific after-finish, it is interpreted as Black Oxide and Oil. It is recommended that an after-finish (per Mil-C-16173) be always applied, as black oxide without an after-finish has very poor corrosion protection. However, with an oil, wax, or lacquer, it is possible to achieve excellent indoor corrosion protection (100+ hrs. in a humidity cabinet). Black oxides on steel are not suitable for severe outdoor applications or corrosive environments, but they can provide superior humidity cabinet results, with proper supplementary coating. Black Oxides on Stainless steels and/or brass alloys will yield excellent corrosion protection, primarily due to their inherent properties.

The after-finish is usually determined by the part configuration and the end-use. If a "dry-to-touch" finish is needed, it is important to specify either Oil & spin, dry to touch, wax or lacquer. The oil after-finish will generally be a glossy finish, whereas the wax will be more matte. It is possible to use a torque/tension wax to provide added lubricity and further reduce drive torque
.
Standard Test Procedures
Visual Test - The black oxide coating shall exhibit a satisfactory appearance. The black oxide coating shall cover the basis metal completely and shall be a uniform shade of black. The part shall show no evidence of pitting, inter-granular attack, or etching. Black oxide does not exhibit a smoothing effect like plating or painting. Pre-existing scratches, tool marks, pits, scribe lines, and other surface defects will be visible after finishing (although they will be black).
Smut Test - Smut is a black or dark-colored, powdery residue, which comes off on the hand when rubbed. Such tests shall be performed prior to the application of the after-finish.
Relative Humidity Test - Testing per ASTM D 2247, Standard Practice for Testing Water Resistance of Coatings in 100% Relative Humidity.

Some advantages of black oxide are:
No dimensional changes: The as-formed dimensions do not change (as they do when plated or painted). Black oxide is a coloring of the base metal; no metal is removed or deposited.
Dark black color: There will be no patina or rainbow effect. The part appearance may be shiny or matte depending on the manufacturing of the part. For example, a polished or blasted surface will retain its pre oxiding finish.
The finish will not chip, peel, flake, or rub off: Black oxide can only be removed by mechanically or chemically removing the finish itself.
Reduces light glare (reflection): Black oxide makes an excellent finished surface for moving parts such as gears, hand tools and machine parts. The reduction in reflectivity reduces eye fatigue and is less distracting.
Can be post-coated for additional protection: Black oxide finishes with supplementary treatments ("after-finishes") improve the appearance, abrasion resistance, and corrosion resistance of the part to which they are applied. The normal after-finishes are Oil, Wax, Lacquer, and Chromic Seals. 
Improved lubrication characteristics: Black oxided parts have improved lubricity and anti-galling characteristics due to the after-finish (oil or wax) resulting in smoother running, mating parts.
Color change resistant to temperature: Black oxide finishes can be exposed to a temperature of 900 degrees F. (482 degrees C.) before the color begins to change.
No hydrogen embrittlement: The black oxide process does not require an acid activation nor is it an electro-process; therefore, hydrogen embrittlement is not a factor. If the parts are scaled or rusty and an acid pickle is required, any hydrogen that may have evolved will normally dissipate in the black oxide tank (running temp. 290 degrees F). Any remaining hydrogen will normally dissipated within 48 hours.
No white corrosion: The finished part does not have a "white-corrosion" state as some electro-plated parts exhibit over time. This makes black oxide an excellent finish for parts used on internal electronic components. With plated parts a risk is run with the small corrosive flecks that are conductive and could cause electrical shorting.
Weldable: Welding black oxided parts does not produce noxious fumes, nor hamper the ease with which a part can be welded, as is often encountered by welding plated or painted parts.

David French & Associates, Inc.
336 Orchard Circle, Charlotte, NC  28217
Manufacturing Broker & Consultant
Serving the Metal Coating & Fabricating Industries
PRO-pHx VP Technical Sales
Environmentally Sustainable Remediation Technologies
www.pro-phx.com
Fax      704-598-1020
davidfrench@pro-phx.com
Mobile 704-607-0191

zurv

Re: The Basics of Black Oxide

Yes, the basics of the dark side... :)

Griffe Youngleson

http://www.zurv.com/Email_Footer.jpg
Visit us on the web at:
www.zurv.com

Nygaard

Re: The Basics of Black Oxide

If the HE bake is not needed after the use of acid to activate why does the spec require lot testing for HE?

david french

Re: The Basics of Black Oxide

Baking to relieve Hydrogen Embrittlement is often required on hardened parts.  Extremes of pH during processing can cause this baking to be a requirement in any process.

tshiloh

Re: The Basics of Black Oxide

What is the difference between the four "classes" of black oxide, is it different for different materials?