Brazing & corrosion

I've been TIG brazing pieces of 3mm sheet steel into a complex professsional ride-on mower deck, to replace rusted panels. I thought I was well-enough aware of the risks of dissimilar metal corrosion, but completely forgot about it as I merrily ran beads of braze over the deck. Fools rush in...

What happens at the exposed junction of steel and silicon bronze (braze), when water and oxygen do their best to cause corrosion? Have I just made an accelerated-corrosion mistake - should I have welded, rather than brazed?

It shouldn't be too bad, especially if it gets a lick of zinc primer and paint to protect the, now bare, steel.

What was the reason for TIG brazing rather than TIG welding in the first place?

Edited By Mark Rand on 28/08/2023 10:26:33

Mark - unfortunately, it's the underside of the deck, so it gets sand-blasted (dusty conditions in summer), and caked with pulped wet grass. Paint doesn't stand a chance...

Brazing preferred because of fillet welds, with different thickness sheet/plate, so - at least at my skill level - undercutting, when welding, is an ever-present risk. Also, the speed of brazing was welcome in the difficult and uncomfortable positions my old body was required to adopt. Torch access always seemed obstructed by another bit of the deck. Using the pedal when standing, kneeling or crouching is always difficult... But using fixed current to braze was OK, whereas it would have caused holes, if welding, as I had to stop and start a lot.

With hindsight, I should probably have used chains of short weld runs, say 1" long, like the original MIG welds.

I'm a bit surprised that TIG brazing isn't more popular - it's a lovely technique, and poses little risk to the base metal, since the arc is always confined to the braze bead.

How about trying sacrificial zinc anodes to slow the corrosion. Have no idea if they would work for this use but shouldn't cost too much. Some more reading *** HERE ***

John

Actually, I think that the 'real-world' background to my question is probably a distraction. I suppose I'm really asking whether a significant electrolytic cell is likely to be set up along the line of the interface on the exposed surface where bronze meets steel. This is a rather different situation from two dissimilar plates with an electrolyte between them, isn't it? I assume that provides enough area for enough current to cause significant corrosion.

Hey Kiwi, Your problem is NOT the braze / iron joint it is the heat affected zone and good old fashioned corrosion ! The heat has removed any paint Etc and the acidic juices are having a hay day ! A regular dose of old oil is the best bet - it will creap in amongst and do what it can - paint will coat the outside but any void or double skin will rot. Good Luck. Noel.

I agree with Noel, coat the insides with something like Waxoyl or old oil, that as Noel says will creep in everywhere and give some protection, I suppose you could also try car underseal.

A perfect job for flux cored wire welding. short tack welds. Vary your amps as you change task

Edited By Ady1 on 28/08/2023 13:11:05

I've bodged brazed exhausts before and never noticed any problems with corrosion at the interface between the two metals - as Noel says, the HAZ alongside a weld is more likely to give trouble.

However spray on stone chip (but not the Shutz stuff) certainly withstands grit blast cleaning. Stone chip stays slightly rubbery which takes the sting out of blade driven projectiles.

Looking in an old book on the corrosion of metals it says that the difference in electro motive force between two mating metals should be kept below 0.25v. In the accompanying table steels have a potential of 0.7-0.78 and phosphor bronze 0.22 (nothing for silicon bronze). So with those figures there is a potential for rusting, and it gets worse when salt is added, as in winter road spray.

Isoflex liquid rubber, as used on flat roof repairs, is also good - coated the underside of my kit car with it. It's not cheap, but worth it.

Rob

I have been doing oxy-acetylene and oxy-propane bronze weld / brazed repairs on mild steel outdoor equipment for over 30 years and have never had the area around the welds corrode, apart from light rust on unpainted areas. A coat of paint or some kind of oil is always a good idea on any exposed steel. Some of the cheaper brand US made mowers had no paint on the deck underside from the factory, so they rusted fast. After bronze welding in mild steel patches, and a dab of spray paint over the welded areas, many have lasted 25 to 30 years without any trouble, usually the cheapy low grade engine wears out before the patched deck rusts out.

My Dad (professional auto mechanic for 50 years) did hundreds of mower decks same way, and many thousands of all sorts of car repairs from rusted oil pans to leaky mild steel exhaust tubes to broken manifolds - with no issues. In his working life today's flux cored MIG sets were either unavailable or very high cost. No doubt the new MIG sets are good, just pointing out they are not the only game in town for successful repairs, and thousands of mechanics got along just fine without them!

I would suggest not overthinking it. If you can paint or oil spray welded repairs, go ahead, if not, don't sweat it, worrying about corrosion. In my opinion corrosion caused by bronze welding in practical repair cases will not be a worry at all desite all the theories.

Many cars and bikes were brazed together at the factory so it can't be that bad. Like Jeff I have used Oxy fuel to braze many things with out electrolytic corrosion being a problem over the last 50 years ! Noel.

Moisture and plant juice isn't the same as seawater. If brazing caused significant corrosion outside of marine applications it would never have become so prevalent.