As you know, 416 stainless is a grade that is sometimes only considered corrosion resistant, not stainless. Some companies say that it cannot be passivated, but we have fairly good success with it. With this grade it is necessary to use the AAA method of passivation (alkaline/acid/alkaline). With the citric acid products on the market you may not have to use the final alkaline treatment, but the pretreatment with alkaline is necessary to remove the sulfur from the surface. (What you are seeing, by the way, may not be flash rust at all, but sulfides on the surface. There are simple tests for this.) There are good products on the market for pretreating the 416ss that are better than just a regular alkaline cleaner. It is also IMPORTANT to be using the correct citric formula to passivate for 416. The wrong formulation of citric can cause similar problems with 416 that are experienced with nitric acid. Contact us if you need help.