More Info on Black Oxide Coatings
More Info on Black Oxide Coatings
Over the years I have answered a number of questions regarding black oxide coatings. I recently received an e-mail reminding me that there is a Mil-Spec Document that discusses four different types of black oxide coatings. The specification is MIL-DTL-13924D. It can be found at http://assist.daps.dla.mil/quicksearch/. There is no charge for this document. A summary table is part of the document and is reproduced below.
Keep in mind that all of the processes listed in the table above require high temperatures. It is critical that you understand how to work with these high temperature processes.
| Applicability to Ferrous Metals | Process and Possible Chemicals | Processing Temperature | Immersion Time |
| Carbon steels, low-alloy steels, wrought irons, cast and malleable irons | Alkaline oxidizing, NaOH, NaNO3, water | 1 or 2 tanks boiling 285–305°F | 5 – 60 min |
| Certain corrosion-resistant steel alloys which are tempered at less than 482°C | Alkaline chromate, NaOH, NaNO3, Na2Cr2O7, water | 250±10°F | 30 – 45 min |
| For corrosion-resistant steel alloys which are tempered at 900°F or higher | Fused salts oxidizing Na2Cr2O7 and/orK2Cr2O7 | Molten bath 750–850°F | 30 min |
| For corrosion-resistant steel alloys | Alkaline oxidizing, proprietary compounds plus water | 250–265°F | 15–30 min |
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