Black Oxide Again

Does black oxide offer significant corrosion protection?


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Q: In an earlier issue of PRODUCTS FINISHING, you briefly discussed black oxide finish. I am new to this industry and would like to know more about the black oxide process. Does black oxide offer significant corrosion protection? J. K

 

A: The black oxide process is used for finishing steel. When applied properly and oiled or waxed after application, an attractive black finish is obtained. The high--temperature black oxide process is essentially a chemical conversion of the steel surface and does not change surface dimensions significantly. The finish gives a modest amount of corrosion resistance and can tolerate temperatures up to approximately 450°C.

There are two main processes used for black oxide application. The classical method involves high temperature and a highly alkaline solution. The solution contains sodium hydroxide, sodium nitrate, sodium nitrite, wetting agents, and other additives. The baths are used at a temperature of 280–295°F.

The second black oxide process is operated at room temperature. It does not function in the same way as the high-temperature process. It gives you a black finish but tends not to have any of the properties, except for the color, of the high-temperature black oxide. The solutions used are proprietary and contain phosphoric acid plus selenium and copper compounds. The finish obtained using this process is not a chemical conversion but a deposit of a black selenium/copper/iron compound.

Additional information about black oxide processes can be found in the 2008 edition of the PRODUCTS FINISHING DIRECTORY AND TECHNOLOGY GUIDE, a supplement to this month’s issue, as well as in the 2006 edition of the Metal Finishing Guidebook and Directory (www.metalfinishing.com). You can also search the PRODUCTS FINISHING database at www.pfonline.com for information. A standard, AMS2485, is available from www.sae.org. 

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