Black Oxide Stainless Steel: A Comprehensive Guide

Black finish is often employed in machining. Surface finishing is important in precision machining. It provides non-reflective surface, rust prevention and increases service life of part. Black oxide parts also have minimal dimensional distortion. Further discussion on how important a black finish could be for your project is explained in this article.

Introduction to Black Oxide Coating

Black finish involves conversion reactions. A conversion reaction finish is in which a chemical reaction has occurred to make coating. The term ‘black oxide’ refers to black or matte coating over the metallic surface. the machined metal part is dipped in an alkaline solution which transform the iron on the surface into magnetite. This creates a black finish via a conversion reaction. Metal that can be black coated are steels, copper, silver, zinc etc.

Is Black Oxide Corrosion Resistant?

Black finish provides low rust prevention. But a proper post-treatment can increase its corrosion resistance. This post treatment allows full light absorption into black oxide coating, that increases surface protection. Thus, increasing its rust prevention properties.

Does Black Oxide Steel Rust?

Black oxide parts have little rust prevention and abrasion properties. but steel has its own corrosion protection and black finish retains its surface protection.

Common Stainless-Steel Grades for Black Oxide Coating

Black finish takes place at temperature between 93-98℃ And provide good surface protection to stainless steel.it work best for stainless steel grades of 200, 300 and 400 series. All these grades resulted in high light absorption, rust prevention and non-reflective surface.

Does Black Oxide Work on Stainless Steel?

Yes, black finish is a conversion reaction coating which works in stainless steel effectively. It is used to add mild rust prevention for light absorption and non-reflective surface and for making visually appealing.

Black Oxide stainless steel vs stainless steel

Stainless steel is a naturally corrosion resistant material, but black finish increases its rust protection, increases strength, and enhances its visual qualities, light absorption properties and making non-reflective surface. so black oxide parts of stainless steel are better than stainless steel.

Understanding the Black Oxide Process for Stainless Steel

Machined parts are dipped in alkaline solution like sodium hydroxide, nitrites or nitrates to black finish the parts

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Chemistry Behind

At high temperatures, 90-150℃, ferrous alloys undergo conversion reaction. Magnetite forms over the surface by an oxidation reaction from salt with the iron. In cold black coating, chemical reaction that occurs is a non-conversion reaction. It does not involve any oxidation reaction.

Hot Black Oxide

At higher temperatures, around 140℃, part is soaked into boiling alkaline solution of salts. It normally has nitrites, nitrates and sodium hydroxide. This converts the iron into magnetite which turns the metal into black or matte finish. This surface protection process creates a non-reflective surface of black oxide parts.

Room-temperature Black Oxide

Cold black oxide is carried out at room temperature. The process is like hot blackening and follows the principle of chemical reaction. The difference is it does not involve oxide conversion. It uses copper selenium. This coating is ductile compared to conventional black finish. but it simpler, and easy to apply

Black Oxide Thickness and Uniformity

Black finish has uniformity in thickness. The ideal thickness is 0.000020in for precision machined parts. Black oxide parts are best that cannot tolerate variable thickness of electroplating. The coating adds minimal thickness to metallic substrate which ensures the dimensional stability of black oxide parts.

Black Oxide Color Variations

The color of black finish is because of magnetite or ferrous oxide. The oxide itself is heavy black in shade and opaque. Ferrous oxide contains more iron than red oxide. But it is lesser than yellow oxide.

Advantages and Disadvantages of Black Oxide Coating for Stainless Steel

Advantages of black finish

• Provides surface protection and rust protection after a post-treatment which increases durability and service life.
• Offers high quality black appearance which is visually appealing
• Dimensional stability is better than other coatings.
• Cost-effective option than electroplating, paint and powder coat.
• Black finish is suitable for large scale and prototyping manufacturing.
• No Hydrogen embrittlement after black finish

disadvantages of black finish

• Black finish can rub off easily
• Labor intensive can increase because of prolonged procedure
• Black finish may deteriorate in hot climate.

Applications of Black Oxide Stainless Steel

• Due to high light absorption of black oxide parts, they are employed in light sensitive medical devices like x-ray
• Electrical black oxide parts like wire stripper, gear for trimmer, cutters are commonly employed.
• Automotive parts like oil filter can, suspension bushing, spart plugs, brakes are black oxide coated
• Mid-temperature coating is employed in turrets, Humvees, shotgun shell magazines in military parts.
• Precision machined parts like bearings, fasteners, and gauges are mostly black oxide coated.

How to Black Oxide Stainless Steel

Black finish on stainless steel parts has few steps of procedure.

• Surface preparation:steel parts are cleaned thoroughly. The purpose is to remove dirt, contaminates, impurity or any oil stains. It can be carried out by chemical or mechanical cleaning techniques.
• Preheating:steel part is heated in a furnace at a temperature 500℃. Temperature and time vary with the desired results and type of material.
• Oxidation:steel undergoes an oxidation where conversion reaction occurs at high temperature of 140℃. This results in the formation of oxide layer of magnetite. This reaction takes place in a controlled oxygen environment.
• Cooling: after the treatment, steel is cooled to room temperature in the air.
• Sealing:The last step is to stabilize the oxide layer with a sealant to provide long lasting protection.

Testing and Inspection

After black finish, the black oxide parts are tested and inspected with visually inspection. This include inspecting the uniformity in color and thickness and to detect any visual defect. Then a Smut test is normally carried out in which black powder residue is wiped off the part by hand rubbing. This determines the adhesion and quality of black finish.

Black Oxide Stainless Steel vs. other Finishes

Black Oxide Stainless Steel vs Bluing

For thin coating of black oxide, bluing is carried out. For heavier coating, black oxide coating term is used. Both are done with the same equipment.

Black Oxide Stainless Steel vs Phosphating

Phosphating has high corrosion resistance. Black oxide is for aesthetic appearance and has less corrosion resistance.

Black Oxide Stainless Steel vs PVD Coatings

Black oxide is cheaper and requires few chemicals to apply CVD is expensive and has superior quality, long service and penetrate deeper in the metal.

Black Oxide Stainless Steel at Tuofa CNC Machining

Tuofa offers high quality surface finish such as electroplating, sand blasting, black oxide coating and many more. Tuofa has advanced tools and equipment and years of experienced staff, engineers and designers for surface finish processes. Highly quality inspection ensures the quality of surface finish of machined parts. For more information please visit: https://www.tuofa-cncmachining.com/

FAQ

Can you Black Oxide all types of stainless steel?

Black oxide works best on stainless steel grades of 200, 300 and 400 series.

How long does Black Oxide last?

Black oxide is durable and long-lasting protection which can stay for years without any prominent sign of damage.