Two years ago I wrote a series of articles for this column about the important steps that must be followed in order to ensure good brazing, including the need for proper cleaning of all surfaces to be brazed prior to assembly and actual brazing. Misconceptions still exist in the brazing world about the best way to clean surfaces, and, as shown in Fig. 1, people still ask me if it is necessary to both decrease and then pickle the metal surfaces, or if they can merely use a pickling-acid to both degrease and remove oxides at the same time.
I strongly believe in the expression “Cleanliness is next to Godliness” when I talk or teach about brazing preparation. The faying surfaces MUST be cleaned thoroughly prior to their being assembled for brazing because:
RULE OF BRAZING: The brazing filler metal (BFM)
will NOT bond to, or flow over, oils, dirt, grease,
lubricants, or oxides.
If any of these contaminants are on the faying surfaces during a brazing process, the molten BFM will not respond properly, and braze-failures will be common.
NOTE: Cleaning parts effectively ALWAYS involves two (2) steps:
(1) Degrease first! You must remove all oils, grease, lubricants, etc., via a good, reliable degreasing process, and then:
(2) Oxide removal is second! Surface oxides are always on the metal surfaces underneath those surfaces oils, etc.
Do NOT think that you can use one type of cleaner to accomplish both degreasing and oxide removal! That does NOT yet exist! You must degrease parts first, and THEN go after the oxides on the surface via a separate cleaning solution that can effectively attack oxides!
1. Degreasing. As shown in Table 1, the removal of any oils and lubricants depends on the types of oils/lubricants you are dealing with: namely, are they petroleum/mineral-oil based, or are they aqueous (water) based. Degreasing fluids are NOT necessarily going to be effective at removing ALL types of oils and lubricants. It is very likely that you will need to use a different type of degreasing solution to match the type of oil/lubricant that you are trying to remove. “One size fits all” does not work in this type of cleaning scenario.
As I’ve mentioned in several earlier articles, I’ve visited a number of brazing shops that had experienced greater than normal rejects from their brazing furnaces even when they thought they had cleaned the parts properly prior to brazing. When the process was investigated, it was discovered that they had been using the wrong types of cleaners for the type of oil they were trying to remove: they had been using alcohols and acetones to try to remove the water-based (aqueous) lubricants, which as Table 1 shows, does not work well.
IMPORTANT: Match your degreasing solution
to the type of lubricant you are trying to remove!
NOTE: Synthetic, silicone-based, lubricants – DANGER! These lubricants, if allowed to dry on the surface of parts, can render them non-brazeable. Silicone is not a good actor in brazing, and must be kept away from your brazing areas. A number of shops have found that they need to either burn-off, or machine-off, any of the hardened silicone-based lubricants from the faying surfaces of parts they received from a supplier, since attempts to braze those surfaces did not work even after normal degreasing processes were tried, and they did not realize the difficulty that silicone-based lubricants presented.
2. Oxide Removal. Surface oxides can come from many sources and must be completely removed prior to brazing. All metals can form surface oxides, and these oxides need to be removed from the faying surfaces (surfaces inside the joint) prior to the parts being assembled. This is important because once assembled it will be virtually impossible for any oxides on those faying surfaces to be removed by any furnace atmosphere or vacuum condition. There are two general methods used to remove oxides: (a) acid etching (pickling), or (b) mechanical methods, such as grit blasting, sanding, etc.
A. Acid etching (Pickling). “Pickling” acid-solutions cannot remove oils and lubricants from a metal’s surface! Pickling is used to remove oxides from the metal surface AFTER the part has first been degreased to remove the surface oils/lubricants sitting on top of the oxide layer. A blend of nitric acid (HNO) and hydrofluoric (HF) acid (perhaps about 10% total, the rest being DI water at about 120-140F/60-70C) may be typically used for pickling of stainless steels. Hydrochloric (HCL) is also used for regular steels, and depending on the metals and the amount of oxidation, something as weak as citric acid is often used.
IMPORTANT NOTE: Don’t use the word “Passivate” when referring to pre-braze acid-pickling, since the purpose of the pickling is strictly to remove oxides and any free metallic particles/residues that may be on the surface before brazing so that the surface is as free of oxides as possible. Passivation is a specifically designed acid-process for building up the oxide layer on stainless steel. Yes, it uses acid-solutions just as the pickling process does, but their goals are completely different. Pickling is designed to REMOVE oxides, whereas passivation is designed to THICKEN the oxide layer. Use the correct word for the correct process!
B. Mechanically cleaning metal surfaces. People often use grinding or grit blasting procedures to remove oxides mechanically from the surfaces of metals to be brazed. Unfortunately, all of these processes may involve NON-METALLICS such as aluminum-oxide. Grinding wheels are typically made of aluminum-oxide or cubic-boron-nitride (CBN), both of which are common ingredients in brazing stop-offs! Additionally, if shop personnel use sand-paper or ScotchBrite pads for scrubbing the surface, they are likewise using non-metallics that can actually leave a non-brazeable residue on the surface of the parts being cleaned. Not good! Tumble-deburring, likewise, can leave a surface film from the tumbling media that is used (alumina, stones, walnut shells, corn cobs, etc.).
Please note that the use of these non-metallic materials to clean the surfaces does not mean that they will completely prevent BFM from flowing onto or over those surfaces. It merely means that any residues from those non-metallics will not braze, resulting in voids in those areas where those particles/residues are located. Thus, the void content of the joint will be a bit higher than normal in such areas. This can be prevented by using metallic-materials in your cleaning processes instead of non-metallics.
RECOMMENDATION: Grind, grit blast, or tumble-deburr using only pure metallic material, not oxides! Thus, use a metallic blasting grit, or a stainless steel grinding wheel, or stainless-steel wool pads, or stainless BB’s when tumble-deburring, etc.
3. Rinse Water. And finally, the water that you use in your cleaning operations is very important. I strongly recommend that you use deionized (DI) water in your shop, and not well water or city water. City water can go bad on occasion, whereas having your own deionizer in your shop is an excellent safety measure. Well water may be great for drinking, but it contains minerals (calcium-oxide, magnesium oxides, etc.) that are strong braze-inhibitors.
CONCLUSION: Proper cleaning of components to be brazed is essential prior to their being assembled together for brazing, because once the parts are assembled for brazing, any contaminants still trapped between the faying surfaces of the joint will not be able to be removed, and will then prevent successful brazing, resulting in increased re-work, rejects, scrap, failed parts, delays in shipment, increased costs, and even a bad reputation in the brazing industry for your shop as a result. Therefore, a simple rule to remember for maximum effectiveness when you clean your parts for brazing:
RULE FOR CLEANING: Degrease Degrease first to remove surface oils and lubricants, and then acid-pickle or mechanically clean (using metallic grits, grinding wheels, etc.) to remove the oxides from the surface. Then only use clean DI (or reverse-osmosis, i.e., “RO”) water to be sure surfaces have been washed clean of all residues. Finally, after all this cleaning is done, handle parts with clean gloves, not bare-hands.
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Dan Kay – Tel: (860) 651-5595 – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 46-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Dan can be reached via e-mail at firstname.lastname@example.org, and his website can be visited at http://www.kaybrazing.com/
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