Joining brass and steel

i vote brass bolts. I'm thinking of exhaust manifolds where steel touches brass bolts. works well, in fact, better than steel bolts. I have a zillion places where brass plumbing touches steel, brass does fine.

Karl

How about a good epoxy or bonding agent ?

You are probably over thinking it. I've stuck brass locks, hinges, mailboxes and doodads on steel fences and gates. Some have been out there for decades with no real issues.

That being said, I would minimize any direct contact by painting the steel before assembly (using the paint like a nonconducting gasket). Design the joint so it will not chip the paint off and use a steel bolt with a brass nut. Designed like this, the bolts are likely to take the brunt of the corrosion. Keep them cheap and easy to replace.

Paul K. Dickman

Yellow brass will dezincify in contact with plain steel, if the joint is wet. Zinc will leach out of the brass and you'll have a weak copper sponge where the brass once was. Threaded fasteners are the biggest problem with this kind of galvanic corrosion, so brass bolts would be about the worst. The near-contact area between the steel and brass is directly related to the rates of corrosion. You also need direct contact in some area. With two pieces of metal bolted together, you'll have plenty of both.

But the steel *not* in near contact with the brass will rust first, if it isn't protected. If you're covering the steel with POR 15, will it be covered where the brass and steel are otherwise in contact?

It's tempting to suggest using a plastic tube in the bolt hole and plastic washers under the head and nut, and coating both the steel and the brass with a good barrier coating. but that may or may not solve it, unless the barrier is perfect.

The cell to worry about is the zinc (in the brass) and the steel. If you coat the steel (and maybe even the brass) with zinc chromate paint or zinc-filled epoxy paint, the zinc in the paint wll be the sacrificial anode -- for a while, at least. How many years does this thing have to last?

With a very weak electrolyte, like rainwater, galvanic corrosion probably will be slow. But when you bolt the pieces together, you'll likely break through the barrier somewhere and that's where you'll have trouble. A sheet of plastic between the two should help, but in the end, you won't prevent galvanic corrosion unless you have perfect isolation between the two metals.

Good luck. And better luck than I've had with metal on boats. I seem to have a knack for setting up little electrical cells everywhere. d8-)

Test some samples:

I haven't seen significant corrosion on brass + aluminum + stainless home-made pulleys on my TV antenna and remote chimney cleaner.

The chimney brush pulley is off for rearrangement of the mast so I went out and took it apart. It's clean despite several years of rain and wood smoke.

jsw

I saw the solution in a book on aluminum corrosion, bolting stainless to alloy. In this case, you'd either use a brass bolt and nut with the head on the brass side and an insulating washer under the nut or a stainless bolt and nut plus insulating washer with the head on the steel side. They did say paint made a useful insulator between parts, as long as it was really weatherproof. So if you've got a good coating with no holes on the steel part, you're probably good to go as long as you don't get chips or holes in the paint. Another way to go might be cold galvanize on the steel. Potential difference between that and the brass is probably negligible.

Stan

I have a 30 year old stainless pool filter tank with bronze wingnuts on stainless threaded studs holding the cover on. Clorinated water is splashed on it every other week. No visible corrosion so far. Although I do use a little white grease on the threads, and bronze is not the same as brass.

The only corrosion are little pits in the s.s. where it makes contact with the rubber gasket. I think this is where organic leaf debris sometimes gets caught and creates acid conditions.

There is no reason for *galvanic* corrosion to appear with that combination. Brass makes a galvanic cell with steel, chrome, or nickel (as in your stainless) because of the zinc. And if the brass is 85% copper -- in other words, red brass, or plumbinb brass, the zinc generally won't leach out. Bronze can be almost anything, but most bronzes are low in the low-potential alloying metals.

With stainless and chlorine, the biggest problem is stress corrosion. That wouldn't be an issue here but it's hell on sailboat rigging.

Possibly, or it's just holding the chlorine and water.

Very likely the gasket is creating small pockets of stagnant chlorinated water leading to crevice corrosion.

Ed,

How is a brass-zinc connection? I'm no expert but it seems that there would be no zinc leaching from the brass. If it's good, then how about assembling the brass-steel joints with hot dipped steel plates in the joint and use hot dipped steel hardware? Art

In water systems, electromotive force causes ions to flow. Must have plastic between the mix. Otherwise you get holes in the water tank!

Mart>

Interesting article, thanks. That would explain it. The debris helps create pockets of stagnate clorinated water.

Right. After considering all the responses (thanks!) I decided to cut off a 1/4"-20 bolt, made a hole in the brass part, brazed the shank of the bolt in it and drilled a hole in the steel part. I am hoping that this way the brass-steel interface will be at a spot where the corrosion matters least.

There are many other engineering issues to sort out on this project so field testing will have to wait a bit :-)

Michael Koblic, Campbell River, BC

Oops, that would be "stagnant".

That's a good question, but I really can't help with an answer.

In theory, what you're saying sounds right. But brass and zinc have electrical potentials (anodic indices) that are quite far apart (around 0.8 V). The anodic index of brass is quite close to that of copper. Although dezincification is a frequent issue with brass, you still have to consider the potential of the copper.

If brass was a homogeneous material, the anodic indices would indicate that the zinc galvanizing would be strongly sacrificial. But in terms of preventing dezincification, there should be no corrosion or loss of zinc on either side.

You'd have to ask someone who has real experience with it.