Fig. 1 Pickling and Passivation have two different purposes
In last month’s column we explored the definition of the word “brazing”, so that the meaning of that important term could be more fully understood and differentiated from the terms “soldering” and “welding”. In this month’s column, I would like to take a look at some additional words that are often misused in the world of brazing, namely, the words passivation and defect. These words have been mentioned and discussed in this column in years past, but it’s time to bring them out again for a new generation of brazing personnel.
I still hear people use these metallurgical terms improperly when they are describing certain criteria related to braze-prep or braze-inspection; so let’s look at them again to understand them more correctly.
In a number of brazing shops I visit there still seems to be some confusion 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”, illustrated in Fig. 1, have completely opposite meanings, and thus, these two terms need to be clarified so that brazing personnel can use these two metallurgical terms correctly.
Passivation, as the name implies, is used to describe a process that will make the surface of the metal more “passive”, i.e., passive/resistant 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 volume) solution for about 30-minutes, or in a hot (150F) citric-acid solution (about 10% by weight). Either of these two solutions can help to effectively build-up (i.e., make thicker) the oxide layer on the surface of the metal, to enhance its corrosion resistance. This is just the opposite of what someone would be trying to do when they are “prepping” (preparing) surfaces for brazing, in which it would be desirable to remove the oxide layer, not build it up! During braze-prep, oxides must be removed because brazing filler metals (BFMs) do not like to bond to, or flow over, any surface contaminants such as oxides, or oils, dirt, etc. If any of those surface-contaminants remain on the surface, it can seriously affect the brazeability of that component by preventing the flow of the brazing filler metal (BFM) on that surface.
Thus, the acidic solution used to clean the surface of the parts prior to brazing should be called a “pickling” solution (i.e., an acid solution), 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 actually telling that person to build up the oxide layer on the part prior to brazing, which is NOT what you want to do! Your actual desire is to REMOVE oxides, free-iron particles, etc. from that surface, so that no rusting, or pitting corrosion, etc., can happen.Thus you really should say: “Pickle the surface prior to brazing….” because acid pickling does remove oxides and metallic residues very effectively!
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.
Don’t just take my word for this. Here’s an interesting very brief video about this:
As you can see from this video, the KCI-TV interview confirms that cleaning of the metal surface is a “pickling” operation, and the “passivation” treatment builds up the surface oxides on the metal surface to improve corrosion-resistance.
Can passivation be used 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 not be done with an acid bath containing nitric-acid, since the nitric acid in the passivation-solution can 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 with a nitric-acid bath 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.
All brazed assemblies should be inspected after they have been brazed, usually beginning with visual inspection, in order to verify that the brazing filler metal (BFM) has properly melted and flowed, resulting in a nice looking, complete meniscus (fillet) around the outside of the joint. Additional non-destructive testing (NDT) may then be used, if needed, to determine if there are any internal imperfections that could affect the final performance of the brazed parts in service.
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. But, that person was WRONG to use the word “defect” when describing that anomaly/imperfection!
When anyone in the world of brazing uses the term “defect”, it should only be used to refer to something that is indeed “defective”, i.e., it will NOT perform properly, as brazed, due to that particular imperfection. Thus, if it will not perform correctly due to that particular imperfection, then yes, that particular imperfection can be properly called a “defect”. If actual “defects” are uncovered during inspection (as shown in Fig. 2), then that part should be set-aside for either repair, or it should be scrapped! No one should ever ship defective parts to their customers, period!