The pickling topic reminded me that in the food industry etc. there are commercial procedures for passivating metal products--aluminum, titanium, stainless steel, and carbon steel.
Anyone who wants to read more about this process might check out the links within this website:
http://citrisurf.com/whatis.htm?gclid=CMHT9Yji_80CFQ6EaQod8-MBPgIt is interesting that this site points to advantages in using citric acid over nitric acid because the nitric acid is more aggressive toward attacking the metals in the structure that is being treated. And the posts here point out that citric acid in fruit does a good job.
Makes me think that if I want to pickle a knife I will just juice some lemons ["lemons and limes have the most citric acid"]. Cleaning the blade thoroughly with soap and hot water, heating the picking solution a bit, and constantly stirring the solution would probably give a more uniform result and also speed the reaction
And bluing (as on firearms) is a version of passivation: https://en.wikipedia.org/wiki/Bluing_(steel)
For practical purposes, while these processes offer some (small) measure of protection against rust, the experts recommend application of oil to block oxidation even of the treated surfaces. Think of your guns, and you get the idea.*
One lesson to take away is that carbon steel knives still need to be maintained, even after they are "pickled" or blued. A soap and hot water cleaning, wiping dry and allowing the hot blade to finish air drying, then a protective coat applied--the blade will probably last several lifetimes.
Larry
*A side note. On a mountain elk hunt, we endured a long day of just about solid rain. That evening in camp, I wiped down my Weatherby and made sure it was ready for the next day's hunting. I did not find any evidence of rust on the gun. However, other hunters were carrying Remingtons, Winchesters, whatever, and they all had rust staining on their barrels and bolts that evening. I never looked into the alloy used in the Weatherby or the finish they used on it, but whatever the facts--they did make a more rust resistant firearm!