Our discussion of black-oxide coatings concludes with a look at after-finishing and testing.
The final step in the black-oxide process is the application of a supplementary coating also referred to as an after-finish. This step will dictate the final appearance as well as improve the functionality of the part. To obtain the best corrosion protection, the after-finish is always applied after multiple rinse steps. The most frequently used after-finish is black oxide and oil. In addition to oil, wax or lacquer can also be used, particularly in applications where the part is to be used indoors and where corrosion protection is desired. Black-oxide coatings on steel fasteners, for example, will not protect them in severe outdoor applications or corrosive environments.
The part geometry and end-use applications will usually determine the after-finish that is used. Factors to be considered are the final application of the part, the protection needed and for how long, the environmental conditions (humidity, vapor, temperature), and the desired final finish and appearance. An oil after-finish will generally be glossy, while a wax after-finish will be more of a matte finish.
Various testing procedures are used to validate the adherence of the black-oxide coatings. The most common is a visual test, which simply involves looking at the component either with the naked eye or under low-power magnification (5-50x). The visual test examines whether or not the metal is a uniform shade of black and if all surfaces are covered. In addition, the test looks for evidence of pitting, intergranular attack or etching. It should be noted that although parts will be black in color, pre-existing scratches, tool marks and other surface defects will be visible after finishing because black oxide does not smooth out or fill in these flaws. The “smut test” is another test performed prior to the application of the after-finish to determine whether or not a black or dark-colored powdery residue is present when the part is rubbed by hand. A humidity test per ASTM D2247 (Standard Practice for Testing Water Resistance of Coatings in 100% relative humidity) is another test that can be used when the components in question will be used outdoors.
Black-oxide coatings are commonly divided into four distinct classes:
- Class 1 – Alkaline oxidizing for wrought iron, cast and malleable irons, common carbon and low-alloy steels
- Class 2 – Alkaline oxidizing for use on certain corrosion-resistant steel alloys tempered at less than 482°C (900°F)
- Class 3 – Fused-salt oxidizing for corrosion-resistant steel alloys that are tempered at 482°C (900°F) or higher
- Class 4 – Alkaline oxidizing for other corrosion-resistant steel alloys
Bluing is classified as a passivation process (rather than a chemical-conversion process such as black oxidizing). Bluing results in a partial protection of the steel from rust. The process gets its name from the blue-black appearance of the resulting surface finish. Bluing can achieved by chemical means, dipping hot parts into certain oils, and by exposing the steel to steam or a wet-nitrogen atmosphere (after the equipment has been purged of air to less that 1% oxygen) while being heated and held in the temperature range of 260-595°C (500-1100°F).
Where appropriate, black-oxide treatments are a simple solution for either cosmetic or physical-property enhancement.