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Black oxide coating

Dr. Dmitri Kopeliovich

Black oxide is a conversion oxide coating formed on the metal surface as a result of a chemical reaction of the metal atoms with an oxidizing agent (air, aqueous solution, molten salts).

Conversion coating is a film of a chemical compound formed in the reaction of the substrate substance with another substance. This reaction distinguishes conversion coating from a conventional coating applied on the substrate surface without changing its chemical state.
Examples of conversion coating are Anodizing (electrochemical process of growing oxide film on the surface of anodically connected metal in an acidic electrolyte solution) and Phosphating (coating consisting of an insoluble crystalline metal-phosphate salt formed in a chemical reaction between the substrate metal and a phosphoric acid solution).

Black oxide coating is used mostly for Carbon steels, Alloy steels, Tool and die steels and Cast irons. Stainless steels and Copper alloys may also be black oxide coated.

Black oxide coating of steel is composed of magnetite - ferrous-ferric oxide Fe₃O₄ (FeO*Fe₂O₃), which is formed as a result of oxidation of iron by Oxygen of air or from oxidizing ions (in aqueous solutions or molten salts).

Hot alkaline black oxide process

Hot alkaline process is a formation of black oxide coating on the surface of the part immersed into a boiling aqueous alkaline solution of oxidizing salts.
Hot alkaline process is the most popular method of black oxide coating.

The temperature of hot alkaline solution should be in the range 275-295ºF (135-146ºC). The boiling temperature is determined by the salts content. At the temperatures above 295ºF (146ºC) the obtained oxide coating turns red and rusty. The temperatures below 275ºF (135ºC) indicate low concentration of salts resulting in too slow oxidizing process. Boiling temperature is adjusted by either addition of salts (temperature increase) or dilution of the solution (temperature decrease).

Typical compositions of hot alkaline solutions:

Sodium hydroxide (NaOH) 80-90 oz/gal (600-700 g/l) + sodium nitrite (NaNO₂) 27-33 oz/gal (200-250 g/l)
Sodium hydroxide (NaOH) 80-90 oz/gal (600-700 g/l) + sodium nitrate (NaNO₃) 53-33 oz/gal (400-250 g/l)
Sodium hydroxide (NaOH) 67 oz/gal (500 g/l) + sodium nitrate (NaNO₃) 67 oz/gal (500 g/l)
Sodium hydroxide (NaOH) 87-100 oz/gal (650-750 g/l) + sodium nitrate (NaNO₃) 13 oz/gal (100 g/l)+ sodium nitrite (NaNO₂) 13 oz/gal (100 g/l)
Potassium hydroxide (KOH) 133-160 oz/gal (1000-1200 g/l) + potassium nitrate (KNO₃) 20-33 oz/gal (150-250 g/l)

Stages of hot alkaline process

Cleaning. The part is mechanically cleaned and degreased in an alkaline solution.
Water rinsing.
Pickling (acid cleaning). Oxide films and rust stains are dissolved in acid.
Water rinsing.
Black oxidizing in the boiling alkaline bath. The part is immersed into the bath for 5-40 minutes.
Water rinsing by pressurized water stream.
Drying.
After-finish treatment. The oxide surface is sealed by oil, wax or lacquer. After-finish treatment not only imparts required cosmetic appearance (matte, glossy, lustrous) but also improves corrosion resistance and anti-friction properties of the part.

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Cold black oxide process

Cold black oxide is a process of non-conversion black finish coating deposited from phosphoric acid solutions containing selenium and copper compounds.

Cold black oxide coating is not really oxide. It is softer than conventional black oxide coating. The color of cold black oxide coating is not consistent and may be rubbed off from the surface. Cold black oxide coating is used as a simpler and less hazardous substitute of hot black oxide process.

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Molten salts black oxide process

Molten salts process involves molten oxidizing salts, Oxygen of which reacts with iron atoms forming a film of black oxide.

A mixture of sodium nitrate (NaNO₃) and potassium nitrate (KNO₃) in equal quantities is used for the molten baths preparation. Melting point of such eutectic composition is about 485ºF (250ºC).

Typical operating temperatures of the molten salt baths are 550-650ºF (288-343ºC). Stainless steels may be treated at the melt temperature 900ºF (482ºC).

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Benefits of black oxide coating

Decorative black color. Required surface appearance (shiny, glossy or matte) may be obtained. The color is stable at increased temperatures.
Minor dimensional changes. Thickness of black oxide coating does not exceed 3*10^-5 inch (0.75 µm), half of which is added to the part dimension and the second half penetrates into the part depth.
Strong adhesion between the coating and the substrate. Black oxide coating does not peel from the surface.
Good lubrication characteristics. Black oxide improves anti-galling properties. In addition to this after-finish oil/wax decreases coefficient of friction of the black oxide coated parts.
Low light reflection. Reduced light glare of moving parts, surgical instruments and hand tools coated with black oxide causes less eye fatigue.
No hydrogen embrittlement. Since Hydrogen is not evolved in the chemical process of black oxide coating, no hydrogen diffusion into the metal occurs.
Slight improvement of corrosion protection. Black oxide coating provides some corrosion protection for internally used parts (not in corrosive environments).
Cost effectiveness. Black oxide coating is less expensive than other types of coatings.

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Applications of black oxide coating

Fasteners;
Machine and cutting tools;
Machine parts (bearings, shafts, springs, etc.);
Medical tools and equipment;
Gauges;
Automotive parts;
Hand tools;
Sport equipment.

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