Steel or aluminium for bodyshells?

[najaB]

I'm afraid that's a load of rubbish. Aluminium is non-combustible, it won't catch fire under almost all circumstances.

I guess my secondary school chemistry teacher must've been lying when she taught me that Al is the sixth most reactive common metal.

 

[Emblematic]

I guess my secondary school chemistry teacher must've been lying when she taught me that Al is the sixth most reactive common metal.

No, that's correct - however it almost immediately forms a passive oxide film in contact with oxygen which stops further reaction. This makes it very resistant to further reactions - corrosion and combustion. Titanium is even more reactive. Dusts of both metals can be explosive in air, as can many common materials. But the conditions needed to combust these metals in solid form are not going to occur in any conceivable accident.

 

[Liam]

I guess my secondary school chemistry teacher must've been lying when she taught me that Al is the sixth most reactive common metal.

No, she was probably right, though I'm not sure of the actual ranking. Aluminium burns in air, but only in finer forms, such as powders. Many other metals, such as Iron, have similar properties. It's all down to particle size and collision theory.

 

[dubscottie]

The point I was making about carbon fibre/composite fire safety was not about it burning but the potential lethal fumes that could be given off when heated.

 

[Emblematic]

But again, composites have been widely used for train interiors for ages - You'll be hard pressed to find a train without them now, so in a fire, composites will be involved. You could argue that a composite bodyshell would conduct far less heat from an exterior fire to the interior.

 

[Shaw S Hunter]

I understand that whenever possible steel is used for bodyshells as in the case of an accident aluminium can burn if it is set on fire.

You're thinking of magnesium. This is even lighter than aluminium and can be alloyed to make very useful construction materials. However even as an alloy it can burn at relatively low temperature though research has developed alloys which are more fire resistant but at somewhat greater cost. As a result magnesium alloys tend to be used for smaller items where weight saving is still worthwhile; my DSLR has a mag-alloy body.

To get an idea of just how flammable magnesium can be do a websearch for the Honda RA302 F1 car or even the Le Mans crash of 1955.

 

[Liam]

You're thinking of magnesium. This is even lighter than aluminium and can be alloyed to make very useful construction materials. However even as an alloy it can burn at relatively low temperature though research has developed alloys which are more fire resistant but at somewhat greater cost. As a result magnesium alloys tend to be used for smaller items where weight saving is still worthwhile; my DSLR has a mag-alloy body.

To get an idea of just how flammable magnesium can be do a websearch for the Honda RA302 F1 car or even the Le Mans crash of 1955.

They still use magnesium alloys for the wheels in F1 and other motor racing series.

 

[Emblematic]

They still use magnesium alloys for the wheels in F1 and other motor racing series.

Non-combustible magnesium alloys are now available. Unfortunately they weren't around in the 50s and 60s.

 

[edwin_m]

No, she was probably right, though I'm not sure of the actual ranking. Aluminium burns in air, but only in finer forms, such as powders. Many other metals, such as Iron, have similar properties. It's all down to particle size and collision theory.

On checking on Wikipedia I discovered that (contrary to my previous understanding) HMS Sheffield destroyed by a missile in the Falklands was actually built of steel. However it is mentioned that the UK and US abandoned use of aluminium in warships due to concerns about fire.

This probably doesn't have any significant for trains though, as they aren't designed to survive missile impacts, and as mentioned the heat needed to melt either metal would be fatal to any passengers still on board.

 

[Emblematic]

On checking on Wikipedia I discovered that (contrary to my previous understanding) HMS Sheffield destroyed by a missile in the Falklands was actually built of steel. However it is mentioned that the UK and US abandoned use of aluminium in warships due to concerns about fire.

This probably doesn't have any significant for trains though, as they aren't designed to survive missile impacts, and as mentioned the heat needed to melt either metal would be fatal to any passengers still on board.

In fact, the main issues which caused the US to revert to steel were to do with structural problems causing shorter than expected service life - fatigue cracking, failed welding and galvanic corrosion were all implicated. Recent reviews have put this down to inappropriate construction techniques at shipyards whose predominant experience was with steel. Other countries, such as Australia, have had better results with aluminum warships.

 

[randyrippley]

On checking on Wikipedia I discovered that (contrary to my previous understanding) HMS Sheffield destroyed by a missile in the Falklands was actually built of steel. However it is mentioned that the UK and US abandoned use of aluminium in warships due to concerns about fire.

This probably doesn't have any significant for trains though, as they aren't designed to survive missile impacts, and as mentioned the heat needed to melt either metal would be fatal to any passengers still on board.

It was the Type-21 Amazon class frigates that extensively used aluminium alloy construction. They suffered badly from cracks, and melted badly when the caught fire during the Falklands war. The Sir Lancelot/Galahad class landing ships used similar construction methods and also showed the same problems

 

[hwl]

On checking on Wikipedia I discovered that (contrary to my previous understanding) HMS Sheffield destroyed by a missile in the Falklands was actually built of steel. However it is mentioned that the UK and US abandoned use of aluminium in warships due to concerns about fire.

This probably doesn't have any significant for trains though, as they aren't designed to survive missile impacts, and as mentioned the heat needed to melt either metal would be fatal to any passengers still on board.

Didn't Sheffield have a reasonable Aluminium top side (Al topside on steel hull) content which proved to be the problem, the quantity of plastic covering including lino on the Al was also an issue.

 

[edwin_m]

Didn't Sheffield have a reasonable Aluminium top side (Al topside on steel hull) content which proved to be the problem, the quantity of plastic covering including lino on the Al was also an issue.

Popular myth, at least according to Wikipedia.

The sinking of Sheffield is sometimes blamed on a superstructure made wholly or partially from aluminium, the melting point and ignition temperature of which are significantly lower than those of steel. However, this is incorrect as Sheffield's superstructure was made entirely of steel.[21] The confusion is related to the US and British Navies abandoning aluminium after several fires in the 1970s involving both the USS Belknap and HMS Amazon, respectively, and also other ships that had aluminium superstructures.[21][a] The sinking of the Type 21 frigates Antelope and Ardent, both of which had aluminium superstructures, probably also had an effect on this belief, though these cases are again incorrect and the presence of aluminium had nothing to do with their loss.[22][23][24]

 

[Mordac]

Cars are (very slowly) moving on from steel to aluminium as well, and then to plastic composites for certain parts.

Aluminium is good for :

Lighter (for a given strength)
Avoiding painting
Avoiding corrosion (although not completely)
Avoiding surface rippling long term

Steel is good for :

First cost
Easier manufacturing, both skills and equipment
Much easier repair
Smaller and one-off production runs

The USA has long favoured stainless steel, particularly since Budd developed their Shotwelding process, and the majority of their vehicles since 1945 have used this, both Main Line and Subway, and are wholly or partially unpainted. Not sure why this approach works for them but not anywhere else.

Corrugated stainless steel is also the material of choice for 'classic' Portuguese coaches and sometimes units. I understand the coaches are a licensed production of French Corail designs.

 

[edwin_m]

Corrugated stainless steel is also the material of choice for 'classic' Portuguese coaches and sometimes units. I understand the coaches are a licensed production of French Corail designs.

Which is odd, because Corail are painted and not corrugated so presumably not stainless steel. Wikipedia does mention stainless steel variants for Portugal.

 

[Mordac]

Which is odd, because Corail are painted and not corrugated so presumably not stainless steel. Wikipedia does mention stainless steel variants for Portugal.

Here's an example (photo not mine):

 

[snowball]

An item on the BBC1 lunchtime news today suggested that in the presence of another material that can retain moisture (as in the Grenfell cladding) aluminium can burn at 300C.

There will be more about it on the South regional "Inside Out" this evening.