Black oxide gives beautiful, durable finish to architectural panels.
Conversion Coatings - Black Oxide, Phosphate, Chromate | Chemical Film | Anoplate, Inc.
Black Oxide | Gallmar Industries LLC
Black Oxide Coating: A Complete Guide to Precision Machined Finish
Amazon.com: 18-8 Stainless Steel Hex Nut, Black Oxide Finish, DIN 934, Metric, M4-0.7 Thread Size, 7 mm Width Across Flats, 3.2 mm Thick (Pack of 100) : Industrial & Scientific