Aluminium Firebox

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Hi Bill

He explains why in chapter one of Building Simple Model Steam Engines: "I don't recommend the use of galvanised or zinc-coated steel, as the coating will react with any adjacent copper or brass and cause corrosion. The same applies to aluminium."

Brian

Also the ignition temperature of aluminium in air is given by an internet source as 550C while the hottest part of a methanol flame is given as 1910C

Internet sources admittedly, but personal experience does suggest aluminium can burn quite easily.

I have seen a bonfire made up of Honda 50 light alloy crankcases - it used to be a 'party piece' at bike rallies.

See http://nuclearpowerradiation.tpub.com/hdbk1081/Table-2-Melting-Boiling-And-Ignition-Temperatures-Of-Pure-Metals-In-Solid-Form-33.htm and http://www.derose.net/steve/resources/engtables/flametemp.html

Edited By Nick Clarke 3 on 14/03/2019 17:11:51

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Yes Bill, it is in book 1.

Brian

Edit:  Not as bad as some authors, at least both books are still available.  My pet hate is "full details are in my earlier (out of print, hard to find and frighteningly expensive) book" (or article in the case of LBSC!).

Edited By Brian G on 14/03/2019 17:23:17

Various other problems with Aluminium in addition to Brian's corrosion & compatibility point.

Aluminium is hard to solder and weld. Pretty much has to be riveted.

Great care would have to be taken to keep it cool because it has a low melting point:

Zinc - 420C
Magnesium - 650C
Aluminium - 660C
Brass - 970C
Copper 1080C
Steel - 1370C
Tungsten - 3400C

An ordinary coal fire burns at about 600C, hotter if a blower is used.

Under the right conditions, Zinc, Magnesium & Aluminium will burn violently, which can be useful. Thermite is a mixture of rust and aluminium powder and it reaches 2500C - hotter than most gas flames apart from Oxy-acetylene. As the reaction produces a stream of molten iron it's used for welding.

Dave

Aluminium / steel, especially stainless-steel, couples are very unhappy partnerships, but it's the steel rather than the aluminium that corrodes.

I don't know what happens with aluminium/copper, but I think in some situations it will be the copper that is eaten.

This point is also pertinent to anyone building miniature railways with aluminium rail. It may seem best to use stainless-steel fastenings and I know some have done this. However, whilst they are good with mild-steel, the combination risks aluminium-alloy rail-ends corroding rapidly, leaving the stainless-steel screws all smugly bright and shiny.

I found this from experience at work, having spent some twenty years trying to tell my far more highly "edumificated" betters why their nice anodised-aluminium test-pieces emerged from a few days in a laboratory tank holding only tap-water containing ordinary swimming-pool treatments, with white "measles", especially around the stainless-steel screws!

I am not convinced by the fire risk. The high-Mg proportion aluminium alloys will burn in the right conditions, but the allow we are most likely to encounter is bog-standard HE30 (or whatever it's called now!), which is Al with a trace of copper. That doesn't so readily burn at all. In a simple model steam-boiler it's more likely just to melt if it runs dry.

YET... aluminium alloy IS used commercially for pressure-vessels, and for the heat-exchangers in modern domestic hot-water boilers (though a friend in the trade says they don't last long). If you read the dreaded Pressure Equipment Regulations - the original lawyers' version, more than the DTI guidance-books - you find its authors only ever knew of aluminium or stainless-steel alloys. I think the difference relevant here between commerce and we amateur engineers is that the trade has far greater resources of exotic alloys designed for the intended application classes, their relevant fabrication methods, and sophisticated testing facilities.

Molten aluminium and water is highly explosive!

Any system made of dis-similar metals and containing an electrolyte ( 'water' is, essentially ,a battery.

It will deconstruct itself fairly quickly without intense care and attention .

Think 1960's cars , engine components, bodywork.

Old VW Bug transmission casings worked even better: Magnesium!

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I once received a phone call from an aeronautical engineer complaining about our supplying him with an aluminium greenhouse equipped with galvanised and stainless steel fittings. I agreed that there should be electrolytic corrosion, but that I had never encountered it in 20 years, and that I had seen 30+ year old buildings without any problems. The only reason I could think of was that the 6063 aluminium formed an oxide skin before this happened. Are there any metallurgists out there that can come up with a better explanation?

Brian

So are all molten metals with very few exceptions including Mercury and specialist low melting point alloys like Cerrosafe.

Tim

It doesn't take too much heat for aluminium to loose it's strength, here's a couple of examples of using aluminium containers for displacers in hot air motors, I'm a slow learner, well actually the second one was put in in a hurry for a demonstration run, and it did demonstrate the reason not to use aluminium after running about half an hour there was a clatter, then stop, the motor now has a stainless steel displacer can.

Ian S C