It is surprising to hear people misuse several common metallurgical terms when they are trying to describe braze-prep or braze-inspection criteria. Two of these misused terms that I’ll talk about in this article are “Passivation” and “Defect”.
1. Passivation vs. Pickling
There is some confusion in the brazing industry regarding the correct use of the term “passivation” (when “pickling” is actually meant) when it comes to preparing metal surfaces for brazing. The two terms, “passivation” and “pickling”, have completely opposite meanings, and thus, these two terms need to 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”, i.e., 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 (20% by vol.) solution 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 (i.e., make thicker) the oxide layer on the surface of the metal, to enhance corrosion resistance, which is just 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, the acidic solution used to clean the surface of the parts prior to brazing should be called a pickling solution (which is an acid solution also), since pickling solutions are designed to remove oxide layers and scale, to thoroughly clean the surface of metals of any metallic residues from machining, etc., as well as any surface scale and oxides. Pickling (acid cleaning) of the surfaces does not require the parts being cleaned to remain in the hot acid solutions for a long time, but instead is usually done quite quickly, namely from seconds to only a few minutes.
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!
So, learn to use the correct words, so that people will not be confused by what you say. Thus, you pickle the metal surfaces prior to brazing, and you passivate the metal surfaces AFTER brazing. Don’t use the term “passivate” incorrectly.
Passivation Following Brazing with Either Silver-Based, or Copper-BFMs?
Passivation of stainless steel components that have been brazed with a copper-based or silver-based brazing filler metal (BFM) should never be done after brazing has occurred, if the acid bath contains nitric-acid, since the nitric acid in the passivation-solution will rapidly, and completely, dissolve away all the copper and silver in the BFMs, and thus, can completely destroy the integrity of any such brazed joint.
Passivation of stainless steel components can be done when the stainless is brazed using nickel-based BFMs, since nickel-based BFMs will not be hurt by such a nitric-acid process.
Someone once told me: “Because the part had only one defect in the braze joint, the part could be released to the customer because of the minor nature of that defect”. What that person meant was that the braze-joint had some kind of minor imperfection, anomaly, flaw, void, etc., which wouldn’t affect the serviceability or performance of the part when put into service by the customer. WRONG!
When anyone in the world of brazing uses the term “defect”, it must be used only to refer to something that is indeed “defective”, i.e., it will NOT perform properly, if at all! No one should be shipping defective parts to their customers, period!
As defined in the American Welding Society’s book: AWS A3.0 “Standard Welding Terms and Definitions (Including Terms for Brazing……)”:
Defect. A discontinuity or discontinuities that by nature or accumulated effect render a part or product unable to meet minimum applicable acceptance standards or specifications. The term designates rejectability.
A simple phrase I like to use is: Defect = Reject
If a part has a defect, it is therefore (by definition) to be rejected, and must be reworked before it can be released to the customer, or it must be scrapped if the defect cannot be fully repaired.
Learn the REAL meaning of terms used in world of brazing, so that people won’t misunderstand you, or what you intent for them to do. It is very important..