Brazing flux - Is residue corrosive to carbon steel?

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Brazing flux - Is  residue corrosive to carbon steel?I never thought of this before, but... I need  to braze up a carbon steel panel that is peppered with rust holes. The repair area's back side is not accessible because of a second panel located there.I have the repair area white-blasted clean, so getting a  bond is not a concern.Since the bare metal behind the finished repair will be inaccessible -to rinse or to otherwise clean- I'm concernedif the residue of trapped borax-based flux could corrode the base metal.Possibly the same way the residue from acidic solder fluxes can continue to corrode for a long time?Any thoughts?OldIron2, could you slip your chemist's hat on for a minute?Thanks allLast edited by denrep; 09-15-2010 at 04:25 PM.
Reply:The different brazing fluxes all vary a bit, but most, as you stated, have Borax or Boric acid as a base. Some have small amounts of Fluorides (I think the more reactive ones used for stainless steel) and I suppose other things I don't know about. You might be able to get useful information about the composition of the particular one you want to use by looking up its MSDS.Soldering flux usually has zinc chloride which breaks down, even at room temperature, to give HCl which is corrosive. The borates are stable, just serving to dissolve the metal oxides when hot but not undergoing oxidation-reduction type reactions. They aren't hygroscopic, either. I have seen flux from brazing left on iron objects for years without causing problems, other than when it does get knocked off, the paint over it comes off too. In short, I'd say the flux used for copper based brazing should not be a problem; that from some silver solders might be a bit worse. One caveat; I live where the R. Humidity isn't very high; in Florida or on  N. Carolina's coast, that might make things behave a bit differently.Can't Alfred tell just by sniffing or tasting it?
Reply:I've never had a problem w/ the white fluxes I've used for a lot of years causing oxidation. It just always appeared to be a form of glass after the brazing. Have Alfred do a computer model w/ accelerated time lapse on it and see what he comes up with.                                               MikeOl' Stonebreaker  "Experience is the name everyone gives to their mistakes"Hobart G-213 portableMiller 175 migMiller thunderbolt ac/dc stick Victor O/A setupMakita chop saw
Reply:Thanks men.Poor Alfred, always the guinea pig. For the accelerated test, Alfred said he'd mix some in with his oatmeal and see how his zinc-capped tooth and iron gut react. I'm gonna go with it. I'll post the pics when I do.Thanks again
Replyenrep, The traditional brazing fluxes for bronze brazing are as stated mostly borax and boric acid. There are no flourides or "salts" in it. The fluxes are not corrosive to ferrous material and often the fluxes are left inside of tubing joints ( i.e. bicycles ). As maybe an added level of comfort, remember bronze brazing flux usually comes in steel cans.....-Aaron
Reply:I think Oldiron2's comment covers it well with regards to RH. I live in a seaside environment and have done a few vehicle repairs over the years on various biodegradable cars I've owned. I always tried to remove all flux riisidue but inevitably, not always successfully. After some months, the painted surface would "erupt" over any residual flux entrapments and when cleaned there would be a white powdery substance there that could be cleaned off and then the area repainted. I suspect it erupted as  a result of moisture getting to the flux residue and allowing it to powder up.Residual flux also used to show up in the body soldered joints of locally assembled cars here too, although that is a more corrosive flux. But if not washed completely then rust wasn't far away. Also in the early days of assembly here, one manufacturer managed to use a hygroscopic seam sealer in mounting front mudguards and the ensuing corrosion was a major worry.We have an underseal product here called Goldseal which is lanolin based and has a grease like consistency and stays that way. It also creeps quite well. I've seen a panel removed after an accident repair and even after a couple of years it was still like grease on the back of the panel. I've seen it rubbed into a rust bubble on the underside of a A pillar in a Nissan van and it seemed to stop the rust from developing any further in the next couple of years that I drove it. I wonder if you could get something like that in behind the rust afflicted panel after you've repaired it. Fisholene (fish oil based) is also very good but does dry off with time, leaving a solid film. There was a Valvoline Tectyl at one time here which was pretty good until the local applicator discovered he could lower his costs by incorporating kerosine to make it go further. Kind of negated the benefits of rust proofing. This was all in the days prior to the phosphate dipping on the assembly line.I have an '84 Golf  (Rabbit) and on a hot day, it will still drip a little of it's rust inhibitor out of the hatch. It was one of the first vehicles to have a six year anti rust perforation warranty on it from new.The flux I used for tinning the water pump casting listed a fluoride on its label. I also know of reported instances of fluoride or fluorine problems in plants here using silver solders, in situations like adding carbide tips to circular saw blade blanks.There are also rust encapsulating two pot paints available and they only need the thickness of a brushed coating to be effective. Evidently if they go on too thick, they're less effective."One of the things we have to be thankful for is that we don't get as much government as we pay for." (Charles Kettering)Mitch 180 (NZ)Lincoln SAM-400-220 + ?-400 Fordson Major + 2 x Tractapac Humber 80 + Procut 40 PlasmaMiller Spectrum 375
Reply:Yes, the brazing fluxes are pretty ionic, which when combined with water vapor can cause a lot of corrosion.  They ARE hard to get off once they cooled down!  But if you do not want something rusting a little, try to get them all  off and washed down.
Reply:Back when I used to do bodywork with brazing {before migs became popular) I saw paint fail over the bronze so we usually covered it with some filler before painting.  I don't remember any major problem with corrosion from the back side although we tried to undercoat the backside after brazing whenever possible."The reason we are here is that we are not all there"SA 200Idealarc TM 300 300MM 200MM 25130a SpoolgunPrecision Tig 375Invertec V350 ProSC-32 CS 12 Wire FeederOxweld/Purox O/AArcAirHypertherm Powermax 85LN25
Reply:Just read in Alcoa Aluminum- Brazing.  Flux doesn't cause corrosion unless exposed to moisture. I presume this the reasoning flux in packaged in steel cans.   I have been working on some stool made of stainless and brass tubing.  Upon final assembly there will be no way to get all the flux out of the joints.
Reply:The residue flux is hydroscopic. What appears to be glass will eventually absorb moisture from the air and turn into a corrosive paste. The water alone will do the corroding and the paste will provide some help.Try to get it out with hot water at least.Last edited by AyAy; 10-04-2010 at 08:21 AM.Orion pulse-arc welderStick an O/A too
Reply:From what I read here, it seems the answer to original question is, could be yes and maybe no. My personal experience using 20 Mule Team Borax is, no signs of rusting or corrosion caused by flux residue after many years.
Reply:Originally Posted by Uncle TomFrom what I read here, it seems the answer to original question is, could be yes and maybe no. My personal experience using 20 Mule Team Borax is, no signs of rusting or corrosion caused by flux residue after many years.