Are there any coatings for brass that are more durable than black oxide?
Yes, depending on the use. First, black oxide comes in two flavors. The room temperature version made by a number of companies typically provides excellent protection on ferrous substrates but does not adhere as well to brass. For this reason, it is primarily used as a decorative coating on brass substrates. Hot black oxide is applied at boiling temperatures and provides a more durable coating on brass, but it is difficult to work with and still might not have as much wear resistance as desired.
Other dark coatings for brass that might be hard enough for your uses include black nickel cyanide or a noncyanide black nickel.
Colstar®, a product sold by Metalline Chemical Corp. (a division of John Schneider and Associates, Inc.), is a tin nickel that will not etch. It is not a true black but might do the trick. It claims to have “excellent as-plated hardness,” but I don’t have any independent confirmation of that. Another possibility is a true black nickel. Metalline, Electrochemical Products, Inc. (EPI) and Heatbath, to name those I’m most familiar with, all provide both black oxide and black nickel baths.
Most of the blue conversion coatings for brass are designed for antiquing rather than for durability. However, a blue hexavalent chromium coating might be more durable. I would check with my supplier.
Of course, hexavalent chromium presents its own problems. The current permissible exposure limit (PEL) is 5 µg/m3, meaning that workers’ exposure during an eight-hour workday must remain below this level. As hexavalent chromium limits have become increasingly stringent, there have been numerous attempts to provide the same protection against corrosion (and, in applications seeking a shiny silver appearance, the same reflective attributes) using other materials. The most common replacement is trivalent chromium, which is now used as a plated coating and as a primary material in conversion coatings. For many applications, trivalent chromium is a suitable replacement for hexavalent chromium.
Recent work in nanomaterials also could provide some alternatives. Christopher Schuh, an associate professor in the Department of Materials Science and Engineering at the Massachusetts Institute of Technology (MIT), and former MIT researcher Alan Lund are manipulating nickel and tungsten at the atomic level to create a more environmentally friendly alternative to hexavalent chromium. The combined structure allows the material to be used as a direct substitute.
According to Schuh, “In trials, it has equaled or exceeded hexavalent chromium in reflectivity and in marine environments outlasted chromium by a factor of more than 10.” He adds that “in other cost-related areas, the new process is already better or has the potential to be so - for example, by saving on power costs through greater efficiency, and on labor costs through less finish work in many applications.”1
Schuh explains that it can be quite difficult to get uniform coverage with chromium, especially on parts with complex geometry. The new coating goes down much more evenly, which reduces the need for postplating grinding, machining and buffing. The product is being commercialized through Xtalic Corp., Marlborough, MA, a company founded by Schuh and Lund.
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