Parts Cleaning Clinic: Cleaning of Inconel

Q. What cleaning methods are recommended for finishing Inconel 718? Can this material be passivated or utilize the Summa process as 300 and 400 series stainless steel, or is there another way to remove the contaminants in regards to machining parts to size?

A. The Inconel family of metals is a trademarked group of alloys with high nickel and chromium content, generally applied for high temperature or corrosion resistance. Inconel 718 has about 50-percent nickel, 20-percent chromium, 5-percent niobium, 3-percent molybdenum, 1-percent titanium, 0.5-percent aluminum and the balance (around 20 percent) iron. This alloy has good corrosion resistance and high-temperature performance. 

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Following machining, a standard alkaline cleaning process is helpful in removing the machining oils and chips. After this step, a passivation could be done to maintain the corrosion resistance of the material. In general, a standard machining process does little to degrade the corrosion resistance of this material. The primary reason for passivation would be if welding or similar processing is applied. However, if your customer is specifying this material, keeping it in the optimal state for corrosion resistance would be desirable. A passivation process first removes any free iron that may be on the surface from interface with machine tools or other incidental contact. The iron that is alloyed in the Inconel is not a concern for removal. The passivating solution then converts any “free” chromium to chromium oxide, which maximizes the corrosion resistance of this material.

The two primary means of passivating a material like this are similar to 300 series stainless steel. Both the nitric acid and the citric acid passivation processes are suitable. The nitric acid process involves immersion for about 20-30 minutes in a solution of 25-50 percent nitric acid operated at room temperature to about 140°F (60°C). The citric acid process is similar in that it is an immersion process operated in a similar temperature range and time. It generally uses a solution of about 10-percent citric acid. 

I am not familiar with the Summa process. When looking into it, it appears to be a tradename process for electropolishing stainless steel. This process could also be a consideration, although requires additional equipment like rectifiers and racks. As a result, it may be more expensive than a basic passivation process, but could leave the surface with a brighter and smoother finish.

Originally published in the November 2016 issue.