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CNC Machine Shop Finishing Applications: The Passivation of Stainless Steel Machined Parts

[6/16/2015] At Advanced Precision Machining’s (APM) Colorado machine shop, our staff of certified CNC machinists prides themselves in manufacturing the highest quality parts, from blueprint to final inspection. Our state of the art CNC milling machines however, 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. As a part of this process, there are so-called “value added services” offered by machine shops that are often overlooked by customers, and these include part surface finishing applications. The application of a finishing technique, in reality, is a crucial step in milling and machining operations adding additional functionality, protection and a refined look to a workpiece. Recently, APM introduced readers to anodizing as a finish option and blackening (black oxide) of machined parts. In this post, our last entry in the series, we’d like to familiarize you with yet another finishing technique common to CNC machine shops – the post-fabrication method involved with the passivation of stainless steel parts.

So, what is passivation? From a chemistry perspective, passivation refers to metals becoming “passive”, making them less susceptible to the natural environmental effects of air and water, the major contributor to corrosion. In a precision machining operation, passivation involves imparting a shielding outer layer, or micro coating to CNC machine 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 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, testing is often performed by to evaluate the surface of passivated parts - “Did it effectively remove the free iron and optimize the corrosion resistance of the stainless steel part?” Remember, Good passivation can make the difference between satisfactory performance and premature failure, while poor practices can actually cause corrosion.
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 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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