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To effectively passivate the free-machining stainless steels, Carpenter has developed the A-A-A (alkaline-acid-alkaline) process that neutralizes trapped acid. This method of passivation can be accomplished in less than 2 hours. Here is the step-by-step procedure:
1st Step : After degreasing, soak the parts for 30 minutes in a 5 percent solution of sodium hydroxide at 160ºF to 180ºF (71º C to 82º C).
2nd Step : Then rinse the part thoroughly in water.
3rd Step : Next, immerse the part for 30 minutes in a 20 percent-by-volume nitric acid solution containing 3 ounces per gallon (22 g/liter) of sodium dichromate at 120ºF to 140ºF (49º C to 60º C).
After removing the part from this bath, flush it with water, then immerse it in the sodium hydroxide solution for another 30 minutes. Rinse the part again with water and dry it, completing the A-A-A method.