Copper boilers and Stainless steel bolts

Hi!, my question is; is it OK to use stainless steel bolts instead of GM or PB where they penetrate the boiler shell..

I'm thinking here of the bolts holding the cyñinder etc to the boiler on Minnie. The 3/16" a/f GM or PB has been unavailable for a long time & I don't look forward to machining a stack of bolt heads to 6ba hex.

I Know you can't use brass or steel because of dezincification & corrosion problems but does amybody know of any metalurgical or other reasons not to use stainless?

(The pump body and safety valve use Stainless balls. or should they be PB).

Richard,

Sorry, I don't know much about Boilers ... There are plenty of others here who should advise.

I do know that Stainless and Aluminium don't make good bed-fellows ... Some grades of Alloy Motorcycle Mudguards suffered very badly when mounted with Stainess fasteners.

... It's worth reading the excellent Wikipedia page on Galvanic Corrosion.

MiichaelG.

I would avoid stainless steel screws. The valve balls can easily be replaced if they start to leak but a screw corroding and giving way in a boiler is potentially dangerous.

It is not widely known that "stainless steel" is only rustproof in the presence of air. I once had to replace thousands of stainless steel screws in a boat deck where water had got under the teak decking and the stainless steel screws had rusted away in the wet airless environment.

Russell.

The blunt answer is no, go to a reputable dealer and get either GM or PB bronze bolts Brass would also be OK SS gets what is called osmosis like the hull of a glass fibre boat, you can see that white degeneration on the bolt.  Yes but the balls are hardened.
Bob

Edited By Springbok on 20/08/2013 12:40:39

Thanks all! (I don't log on often it interferes too much with my model making)

some interesting replies here

MiichaelG. the page on galvanic corrosion was an interesting read and was just the sort of thing I was worried about.

I'm not UK based so I can't ask a boiler inspector & I don't know of a ME club locally.

I was hoping that a person with specific boiler info might see the thread and reply.

Guess I'm gping to have to machine all those bolts from much larger stock after all.

One simple solution is to make up some 6BA studding from 3mm or 1/8" bronze and then silver solder on some off the shelf brass nuts. This also has the advantage of better representing the carrot head studs and nuts used on the full size as bolts were not used.

I've seen several Traction engines that use stainless fixings for the cylinder, half way down this page shows some "stud bolts" being made

Russell,

I find your remarks about stainless rather strange, I look after industrial washing machine totally made from 306 and the one thing they dont suffer from is rust, we use alkaline or acid detergents heated to 55 deg C and 82 deg C and rust is not a problem.

Martin P

Thanks Jason thats one thing I'd not thought of. Easier than my solution.

I'd been thinking of either using 5/32 PB hex or 3/16 round & milling square heads above a short round section.

As You've made Minnie perhaps you could clear up one point I'm not sure about. The pùmp is mounted on quite a thick base. Are the fixings in blind threads or do they penetrate the boiler shell as the cyl & motion plate mount. The book is silent re this & from that I am assuming the threaded holes are meant to be blind.

Richard,

You may find this relevant, particularly page 3, paragraph 3

Edited By JasonB on 22/08/2013 19:04:53

Yes I made mine blind.

I think the problem occurs when the stainless steel remains in a wet airless environment. In a washing machine the drums will often be in contact with air as the contents are agitated.

The screws on my boat deck were stainless steel self tappers fixing the teak decking strips to a fibreglass deck. The heads which were exposed to the air (and rain and salt spray) were unaffected but the bottom half of the screw had disintegrated.

Russell.

Russell,

What about the 700 litres of water that constantly sits in pipework and two tanks, surely that would be allowed to rust.

Martin P

Most stupid (sorry incorrect) comment I have seen here in a long time... Osmosis through stainles steel, wtf!

http://en.wikipedia.org/wiki/Osmosis

Frank

I am a chemical engineer having worked for 2 decades in the chemical industry. I know this is a one year old thread but I wanted to clarify something about Stainless Steel, which is a very used material in the industry. I mean it is not only for cooking stuff. Stainless steel main weakness is that it is attacked by hydrochloric acid and sulphuric acid. Other that that it will resist virtually any other chemical including very strong acids such as nitric acid. Crevice corrosion may be caused by the lack of oxygen and can happen in very rare conditions, but not on a boiler. It is not definitely caused by lack or 'air' contact. This kind or corrosion also occurs in other alloys such as aluminium, titanium and copper. My take is that what Russell describes was caused by chlorides. Many modern industrial steam boilers are made out of stainless steel and virtually all heat exchangers are made entirely on stainless steel.

John

I don't know what your boiler is made of but Loctite told me you cant use stainless in copper because of the different rate of expansion.

Stan