To achieve the best adherence, the metal must be clean. So, the first steps in the coating process are cleaning, rinsing and thoroughly drying the metal surface. If rust or scale is present, additional steps (such as acid pickling or alkaline de-scaling) should be done. Then the surface may need to be neutralized in preparation for oxidizing. Any other types of coating – such as zinc, nickel, chromium, cadmium plating or phosphates – must be stripped prior to the black-oxide process because the oxide layer will not be consistent in thickness, will vary in color and/or will flake off if attempted over dirty or plated parts.
When considering use of a black-oxide coating, the design engineer must evaluate: the material to be coated, the application (end-use) environment and length of time the protection will be required, post-oxidation finishing operations, and storage conditions prior to use (humidity, temperature, etc.)
A variety of black-oxide-coating chemicals can be used. One such family of product is shown in Tables 1 and 2.
Table 1. Black-oxide coatings for ferrous metals
| Product Name | Form | For Use on | Description |
| Pentrate® Ultra | Powder | Steel | Black oxide used for the deepest black finish on steel. Produces a uniform black-oxide finish with no rub off or dimensional change. Low drag-out reduces operating costs. Contains rectifiers to remove nonferrous contamination. This premium black-oxide product has incorporated inhibitors for the control of colloidal iron buildup in the operating bath. Pentrate Ultra meets or exceeds all government specifications, including MIL-DTL-13924D for Class-1 black-oxide coatings. |
| Pentrate® Ultra Liquid | Liquid | Steel | Liquid version of Pentrate Ultra |
| PX®-5 | Powder | Stainless steel, cast and malleable iron | A black oxide with a unique balance of alkali and oxidizers that operates at 121-127°C (250-260°F). Meets the requirements of MIL-DTL-13924D for Class-4 black-oxide coatings. |
| PX®-5-L | Liquid | Stainless steel, cast and malleable iron | Liquid version of PX-5 |
| Kwik-Blak® LT | Liquid | Steel, cast or malleable iron (not for use on stainless steel) | A room-temperature black-oxide process that saves energy and time. Produces a deep black finish when used at 10-15% by volume. Bath can be filtered for extended service life. |
|
Pentrate® Touch Up |
Liquid | Steel | Used to repair scratches or imperfections on previously blackened steel parts. |
Table 2. Black-oxide coatings for nonferrous metals[1]
| Product Name | Form | For Use On | Description |
| Durablack Liquid | Liquid | Copper and copper-bearing alloys | A highly concentrated alkaline product used to blacken copper. Operates at 104°C (220°F). Alloys of copper can be blackened when Durablack Pre-Dip L is used. |
| Durablack Pre-Dip L | Liquid | Copper-bearing alloys | Conditioning step needed prior to blackening copper-bearing alloys in Durablack Liquid. Completely compatible with Durablack Liquid. No rinsing is required between stages. |
| Zinol® 1436 | Powder | Zinc plate and zinc die castings | A lustrous black conversion coating providing an attractive appearance and excellent corrosion resistance when used in conjunction with a protective seal. |
| Pentrate® EE-2 | Powder | Silver solder | Used to blend soldered joints with the black-oxidized steel surfaces. Operates at 82-88°C (180-190°F). |
References
- Cleveland Black Oxide (www.clevelandblackoxide.com)
- Heatbath Corporation/Park Metallurgical (www.heatbath.com)
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