There is some confusion in the brazing industry regarding the correct use of the terms “passivation” and “pickling” when it comes to preparing metal surfaces for brazing. The two terms have completely opposite meanings, which really should be clarified, so that brazing personnel can use these two metallurgical terms properly.
Passivation, as the name implies, is used to describe a process that will make the surface of the metal “passive” to corrosion. This is achieved, for example, on 304L stainless steel (and similar metals) by building up the chromium-oxide layer on the surface of the metal, usually by immersing in a hot nitric-acid solution (20% by vol.) for about 30 minutes or in a hot (150˚F) citric-acid solution (about 10% by weight). Either of these two solutions will help to effectively build up an oxide layer on the surface of the metal to enhance corrosion resistance. This is the opposite of what someone is trying to do when they are preparing surfaces for brazing (i.e. they should be trying to remove the oxide layer, not build it up).
Thus, a pickling solution (which is also an acid solution) is designed to remove oxide layers and scale to thoroughly clean the metal surfaces of any metallic residues from machining as well as any surface scale and oxides. Pickling (acid cleaning) of the surfaces is usually done quickly, namely from seconds to only a few minutes, and is thus designed to do just the opposite of passivation.
It is important for people to know the meanings of the various phrases they use when speaking in order to prevent confusion. By telling someone to “passivate the surface prior to brazing,” you are telling the person to build up the oxide layer on the part before brazing, which is NOT what you want to do! You want to REMOVE the oxide layer, which is what the process of pickling does!
Learn to use the correct words so that people will not be confused by what you say. Pickle the metal surfaces prior to brazing and then passivate the metal surfaces AFTER brazing so as to ensure a corrosion-resistant surface for use in service.
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