Advanced Precision Machining

Machine Shop Finishing Applications; The Passivation of Stainless Steel Parts

[March 11, 2021] At Advanced Precision Machining (APM), our Colorado machine shop staff of certified CNC machinists pride themselves in manufacturing the highest quality precision parts, from initial blueprints to final inspection and reporting. While most of our time is dedicated to the shop floor, and offering the best customer service experience in the business, we do like to keep our customers up to date and educated about the latest in precision milling and machining technology, shop news, and our services via APM’s machine shop blog. At this time of year, and with the recent health crisis that’s been gripping our nation hopefully coming to an end, we are transitioning to a new sense of normalcy and scaling up our production in spite of breaking supply chains, labor limitations, and rapidly shifting demand. 

We look forward to another year of continued growth, and as mentioned, one of our goals is to further educate. With this in mind, we want to introduce customers to a lesser-known aspect of machine shop work; the so-called “value-added services” that we offer often overlooked by customers. This includes a specific part surface finishing application that adds to a part’s functionality, protection, and aesthetic quality, depending on what wear a part will have to endure over its lifetime. Back in 2018, the APM Blog familiarized our readers with the electroplating process and how it uses electrodeposition to produce a thin layer of metal onto a precision part. In this post, we’d like to present a primer on yet another surface finishing technique common to many machine shops; the post-fabrication methods involved with the passivation of stainless steel parts.  

APM’s state-of-the-art milling machines are only one piece of a puzzle that has many moving parts. Our machine shop processes most often begin with a customer concept, then move into the design phase with blueprinting and prototyping. What follows is actual production and the eventual on-time delivery of an accurate and aesthetically pleasing component. As a part of this process, towards the end specifically, is surface finishing. Surface finishing techniques take on varying forms such as electroplating, anodizing, blackening, chromating, nickel plating, and many more. The application of any of these methods is a crucial step in milling and machining operations and often imparts that “wow” factor onto a finished precision part. 

So, exactly what is passivation? From a purely chemistry-based perspective, passivation refers to metals becoming “passive”, which means making them less susceptible to the natural environmental effects of air and water, THE major contributor to corrosion. In a machine shop environment, passivation involves imparting a shielding outer layer, or micro coating to machined parts to protect against corrosion, further strengthen them, and improve upon the aesthetic look of finished workpieces. When it comes to stainless steels, the fact that they are impervious to rusting is a common misconception. Milling and machining processes can and do leave behind bits of embedded foreign matter and iron contaminants such as CNC grinding swarf, shop dirt, iron particles from tools, and exposed sulfides, all of which can adhere to a part resulting in premature corrosion and deterioration if not removed. 

The passivation techniques employed by machine shops involve a two-step procedure; a thorough cleaning followed by chemical dissolution in an acid bath to both remove contaminants and impart the formation of a thin, transparent, and protective passive oxide film. This is all performed without affecting the nature of the stainless steel itself. As mentioned, the effect of CNC machining equipment can leave behind an array of contaminants including grease, coolant, dirt, iron particles, and more. At times overlooked by CNC machinists, a thorough cleaning is crucial to the process. Without proper removal, impurities can interfere with passivation deteriorating the surface and rendering the anti-corrosive properties ineffective. The use of a commercial degreaser or grinding off imperfections is recommended. 

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. As a final step, tests are often performed by a machinist to evaluate the surface of passivated parts. The end result is a freshly machined part that is not only aesthetically pleasing but is protected from the elements, and in a condition that is to be expected from the customer. Done correctly, passivation of stainless steel parts creates a perfect balance between proper performance versus unwanted deficiencies. 

Our Colorado-based CNC machinists want to see your job through to its entirety. Do you have questions? Want to learn more about our passivation services? APM’s machine shop is available for all of your inquiries. Contact us today, or for additional information or to request a quote, please visit our website or call 720-797-0693.

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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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