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CNC Machine Shop Finishing Applications: Blackening (Black Oxide) Machined Parts

[May 19, 2015] When it comes to the milling and machining of precision parts, there is one last step in the process that receives less than the lion’s share of attention; surface finishing applications. Used to impart additional functionality, protection, a polished look and more, properly finishing a part on the CNC machine shop floor is a crucial prerequisite before final delivery is made to the customer. These so-called “value added secondary services” are typically outsourced by smaller and mid-sized precision manufacturing facilities, and encompass finishing options such as electroplating, powder coating, anodizing, buff polishing, metal brushing and more. In a recent installment of Advanced Precision Machining’s blog, we introduced readers to anodizing as a finish option. Continuing with this theme, APM’s staff of CNC machinists would like to familiarize you with another commonly utilized technique – that of finishing workpieces by chemical blackening (black oxide).

Black oxide finishing has a wide range of applications for use on ferrous metals such as carbon steel, iron and other powdered metals, and works by creating a metal oxide surface on a part without adding to it dimensionally. In other words, it does not change the part’s thickness, unlike electroplating for example. For machine shops, it’s cost-effective, requires no application of electric current and is more environmentally friendly when compared with other surface treatments. It’s also cosmetically appealing, imparts a barrier that is resistant to corrosion (after oil post-treatment) and humidity, and protects well against scratching, cracking, chipping and peeling. It also reduces light glare and improves adhesion qualities. You’ll often find black oxide coatings used on milling and machining tool parts, motor parts, power tools and threaded components of many types.

The two most common methods involved in applying a black oxide finish are hot and cold blackening. They are chemically speaking, different techniques used in machine shops, but both result in the same performance characteristics. The process itself is a chemical conversion and does not deposit material onto the part being coated. A reaction occurs between the iron present in the metal and the particular oxidizing salts used in the immersion solution, or chemical bath, forming black iron oxide on the parts surface. In both hot and cold methods, precision machine parts take a bath moving between sequential tanks containing separate solutions, usually in order of: an alkaline cleaner, water, the blackening compound (caustic soda), and finally an oil-based sealant.

Hot black oxidizing, or hot oxidizing, uses a hot bath at 285-300 oF containing a sodium hydroxide oxidizer, nitrates and nitrites and is a slower process than cold batching. This is a boiling reaction that produces caustic fumes and steam, so precautions need to be taken, but this process gives a deep black finish and is much more consistent and uniform in appearance. Cold black oxidizing, or cold blackening, uses a room temperature bath solution typically containing phosphoric acid and selenium copper. It simple to use, safer, cost-effective due to energy savings, and is the choice for many machine shops wishing to keep the process in house and do touch-up applications. It offers less wear resistance and does not produce a pure black oxide coating because of the copper compounds. Regardless of which technique is used, it is very difficult to achieve the same protection, appearance and performance qualities from other finishing processes for the same low cost as black oxide, and it has proven to be a go-to finishing option for many precision manufacturing facilities, including APM’s Colorado machine shop.

“APM is dedicated to manufacturing the highest quality precision parts while providing the best customer service experience in the machine shop business”. For additional information or to request a quote, please visit or call 303-776-1910.

About the Author

Gerry Dillon is a co-founder, current owner and certified CNC machinist at Advanced Precision Machining (APM), a full-service machine shop located in Longmont, Colorado. Before making his home in the United States in 2000, Gerry was born and raised on the emerald isle of Ireland and took an interest in milling and machining from an early age, ranking #1 in the Irish National Apprenticeship Program. In 2005, he partnered with his friend and colleague, Kirk Tuesburg, currently APM’s machine shop manager, together launching what’s grown into a leading Colorado machine shop. Gerry brings over 30 years of machining experience to the shop floor, and is certified in all aspects of geometric dimensioning and tolerancing. 


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