Advanced Precision Machining

A Machine Shop Primer On Surface Finishing Applications: Anodizing, Blackening, and Passivation

[February 18, 2019] With the holidays behind us and Spring not too far off now, the ownership and team of certified CNC machinists are hard at work this Winter in our Colorado-based CNC machine shop, and we look forward to another year of continued growth. Although most of our time is consumed with milling and machining high-quality precision parts, while offering the best customer service in the business, our machine shop blog provides a valuable outlet for keeping our customers informed and up-to-date on the latest in precision machining trends, shop news, and our wide range of services. One of our goals is to educate, and with this in mind, we want to introduce readers to a lesser-known aspect of machine shop work; the so-called “value-added services” we offer that are often overlooked by customers. This includes part surface finishing applications that are a crucial step in our operations!

At Advanced Precision Machining, we pride ourselves in manufacturing the highest-quality parts; from design and blueprint, straight through to surface finishing and final inspection. But, did you know that our state of the art CNC milling machines are only one piece in the puzzle that often begins with a prototype and ends with the on-time delivery of an accurate and aesthetically pleasing component? Once the metal chips stop flying and another precision machined part comes out of a CNC mill or other high-speed machining tool, this is close, but far from the end of the metal forming process. For almost all parts, the last crucial step of applying a surface finish has a big impact, and nothing is delivered until final surface finishing is complete.

The processes involved in CNC milling and other machining operations often impart undesirable irregularities onto a part’s surface – that’s the nature of the work. It’s often undetectable to the naked eye, but has the ability to affect the long-term functionality and is aesthetically unpleasing. Therefore, as the last step in all the work we do, surface finishing applications are used to guarantee functionality, protection, a refined look, and other qualities depending on what the part will have to endure over its life. On the shop floor, machinists employ a variety of methods to produce the most accurate and best-looking parts, and they are as varied as the parts they are treating. For purposes of this primer, we’ll provide a brief overview of anodizing, chemical blackening (black oxide), and passivation, but other common applications include surface grinding, electroplating, powder coating, buff polishing, bead blasting, and metal brushing.


Anodizing is mainly utilized on aluminum alloys and imparts a finish that protects the metal by enhancing its naturally occurring surface oxide layer. It’s a conversion coating – not paint. It results in an extremely hard and durable surface, lessening wear on the part and aiding in corrosion resistance. An anodized finish still remains porous however, allowing for the incorporation of color or other treatments into the end product. An anodized part is relatively inexpensive to finish, it’s an environmentally safe process, and they are easy to maintain. The anodizing process involves immersing a part in an acid solution then applying electricity, producing an electrolyte solution. Anodizing is categorized into types based upon the specific acid used, and the most common applications utilize chromic and sulfuric acid.  

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 thickness. It’s cost-effective, requires no application of electric current and is environmentally friendly. It’s also cosmetically appealing, resistant to corrosion and humidity, and protects well against scratching, cracking, chipping and peeling. The two most common methods involved in applying a black oxide finish are hot and cold blackening. Hot black oxidizing or hot oxidizing uses a hot bath of a sodium hydroxide oxidizer to produce a deep black finish. Cold black oxidizing or cold blackening is simpler and safer to use and utilizes a room temperature bath of phosphoric acid and selenium copper. However, it offers less wear resistance and does not produce a pure black oxide coating because of the copper compounds.


The two-step passivation process is reserved typically for stainless steel and involves imparting a micro coating to CNC machine parts that protects against corrosion, adds further strength, and improves the look of finished workpieces. A thorough cleaning is first employed with a commercial degreaser or grinder to remove contaminants such as grease, coolant, dirt, iron particles, etc. Without proper removal, these impurities can interfere with the passivation process and render the anti-corrosive properties ineffective. In the second step, the actual chemical treatment further removes all iron from the surface and imparts the protective “passive” film, eliminating the potential for rust to form. This is accomplished by immersing the part in a bath of nitric or citric acid solution, with sodium dichromate or sodium bicarbonate and water. Which approach to use depends on the grade of stainless steel and prescribed acceptance criteria.

Want to learn more, or have a question about our surface finishing options? Contact the expert CNC machinists at APM’s Colorado machine shop for all of your milling, machining and finishing needs.

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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 and a partner began 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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