Magnesium Burns.... its crying time

[tirwin]

There is a great thread on how to fuse the dash light circuit and another one on fusing the front A/C condenser blower motor. In the case of the latter, there have been a few threads showing the unfortunate results of what happens when the blower motor seizes. The first years that had factory A/C didn't have an inline fuse next to the blower motor. This was added sometime in the late '80s.

There is a lot of good info spread around in various threads. Another one is the various ways people have replaced the ceramic bullet fuses with ATO fuse panels. That's a good opportunity to split out a couple of high amp draw A/C components that are on a single circuit today. And of course we have people like timmy2 that make replacement wiring harnesses.

It would be nice to have a summary thread of such things that consolidated a lot of the good 'best practices' for convenience.

[J-Mac]

Originally Posted by ischmitz

I don't think you are "adding to the puzzle" but have identified another clue to understand what happened here. The battery got hot because it was supplying a monster-amount of current once the positive wire shorted to GND. What isn't clear (yet) is whether the fire came first and melted the insulation of the positive wire running to the alternator or whether the wire shorting due to bad insulation was the source of the heat that started the fire. In any case your observation is consistent with what the fire department says (this doesn't look like a fuel fire). That pretty much leaves an electrical fire.

I believe the harness running to the alternator shorted to the top of the engine in a way that super-heated the positive or the ground wire to the alternator. This caused the insulation to melt and eventually catch fire. From there the alternator shroud burned and the rest was history. It would also explain why the fire came back after being put out. The battery kept heating the wire and the red-hot wire kept melting and evaporating plastic fumes that fueled the fire.

I still don't believe the magnesium actually burned. If that would have been the case the fire would have been so hot the entire garage and the car would have been consumed. The fire was hot enough to melt the metal but not hot enough to ignite it.

Once you have seen magnesium wire burn you'll know the difference. As others said it's as bright as a welding arc and looking into it will damage your eyes. A magnesium engine block burning would be like looking into the sun.

Ingo

So a battery cut out could have prevented, at least, the second fire. I'm just wondering if fitting a cut out switch jumps to top of the list of 'best practices'. Mine is a '75 with an '86 motor and wiring is not stock.

[ischmitz]

Race cars usually have a battery master switch accessible from the outside

[J-Mac]

Originally Posted by ischmitz

Race cars usually have a battery master switch accessible from the outside

Mine is a street car only. I guess my question should be modified to include 'for street only driven older 911s should a cut out switch be fitted etc.....

[Trackrash]

Originally Posted by ischmitz

I still don't believe the magnesium actually burned. If that would have been the case the fire would have been so hot the entire garage and the car would have been consumed. The fire was hot enough to melt the metal but not hot enough to ignite it.

Once you have seen magnesium wire burn you'll know the difference. As others said it's as bright as a welding arc and looking into it will damage your eyes. A magnesium engine block burning would be like looking into the sun.

Ingo

Not true, at least from my experience. We used to burn VW cases in fire rings at the beach on a regular basis, (in my youth). When a large block of mag burns it is much different than a small piece of wire, like in your science class.

We would throw the VW case on the bed of coals and after a while it would start to glow and smolder.

Then we would spray water on it. The result was almost nuclear with a blinding fire ball rising into the air. The city has since outlawed this practice.

[Indescikov]

Originally Posted by J-Mac

So a battery cut out could have prevented, at least, the second fire. I'm just wondering if fitting a cut out switch jumps to top of the list of 'best practices'. Mine is a '75 with an '86 motor and wiring is not stock.

I believe a current limiter would have prevented all of this.
It is standard practice for Part 25 aircraft, they will absorb a large current surge but will blow if the current remains. They are 325A for a 250A starter/generator. They locate them so as to isolate any direct battery connections, just like the battery connection from thI e front batteries to the starter motor.

This said, once the car is returned from the insurance company, we plan of dissecting the wreckage carefully to see if there is any evidence of where it started.
I have the exact same car and it is a big concern

[chris_seven]

It is interesting that Magnesium has an Auto-Igntion Temperature that is around 500 degC but a melting point of around 650degC

Traditionally magnesium in a foundry environment was covered by Chloride Salts but now gases such as Sulphur Hexafluoride are used.

It is, however, possible to melt Magnesium without it burning as the oxide film produced during the early stages of heating is reasonably plastic and provides a degree of protection from the atmosphere.

When you have a reasonably solid lump of Magnesium being heated gently on a bonfire it will start to soften and melt before it burns.

Chucking a bucket of water onto the surface breaks the oxide film due to rapid contraction and the evolution of steam, once the protective film is broken - Bingo an instant huge fire.

Good luck with a fire extinguisher when this happens.

https://www.youtube.com/watch?v=wZt1YnFMfLQ

[ischmitz]

Keep in mind pure magnesium as a solid, power, or strip acts completely different due to the different surface areas. And yes, highly flammable especially when in powder or strips.

However, the magnesium used as structural material in automotive applications (engine cases, wheels etc.) is an alloy with traces of other metals. That changes its properties such as flammability completely. Google is your friend here

[daepp]

We too, when we were young, dumb, and full of .... used to drop VW cases on fires out in the hi-desert near the dry lake beds. We had one burn so long and so bright that when we needed to leave we had to bury it to get it out. Dumb I know but...

[Bill Douglas]

I think magnesium is the bit that makes fireworks burn bright. I remember school science class where we burnt strips of it.

[Discseven]

Inde... I posted earlier in pain over this. Now am wondering what your plan to do?

[Indescikov]

Originally Posted by Discseven

Inde... I posted earlier in pain over this. Now am wondering what your plan to do?

I believe the person who can best answer that is Gary.
His plan is to buy back the car from the insurance company and
"Phoenix" it.
Waiting on the insurance company...from what I heard today, he should know something next week

[Discseven]

^^^ Phoenix'ing... great!

[dicklague]

Originally Posted by Trackrash

Not true, at least from my experience. We used to burn VW cases in fire rings at the beach on a regular basis, (in my youth). When a large block of mag burns it is much different than a small piece of wire, like in your science class.

We would throw the VW case on the bed of coals and after a while it would start to glow and smolder.

Then we would spray water on it. The result was almost nuclear with a blinding fire ball rising into the air. The city has since outlawed this practice.

Burning VW cases? What fun!! What kind of wasted youth did you have????

[Trackrash]

Originally Posted by dicklague

Burning VW cases? What fun!! What kind of wasted youth did you have????

Then there were the balloons filled with oxygen and acetylene......

[MARISOL78sc]

Guess it would look something like this.....

https://www.youtube.com/watch?v=nhY0xzKcPoE

[Indescikov]

Originally Posted by chris_seven

It is interesting that Magnesium has an Auto-Igntion Temperature that is around 500 degC but a melting point of around 650degC

Traditionally magnesium in a foundry environment was covered by Chloride Salts but now gases such as Sulphur Hexafluoride are used.

It is, however, possible to melt Magnesium without it burning as the oxide film produced during the early stages of heating is reasonably plastic and provides a degree of protection from the atmosphere.

When you have a reasonably solid lump of Magnesium being heated gently on a bonfire it will start to soften and melt before it burns.

Chucking a bucket of water onto the surface breaks the oxide film due to rapid contraction and the evolution of steam, once the protective film is broken - Bingo an instant huge fire.

Good luck with a fire extinguisher when this happens.

https://www.youtube.com/watch?v=wZt1YnFMfLQ

OK, to add fuel to this blog.
Here are the two largest pieces that remained at the site of the fire.
They weigh absolutely nothing and are very porous
Look oxidized and burnt, not melted
Thoughts?

[Trackrash]

Magnesium Oxide.

[SCadaddle]

Originally Posted by Trackrash

Then there were the balloons filled with oxygen and acetylene......

In several years past it wasn't unusual to see someone burning a magnesium VW engine case in the infield of the Daytona 24 at night.

As for the balloons, I knew a kid that tried that. Filled up a bunch of them then put them in his pick up truck cab with him during a cold, dry, winter night. They rubbed together and exploded. Blew out every piece of glass in the truck, blew the back of the cab into the bed and blew up his guts internally, not to mention his ears. He survived it, somehow.

[J-Mac]

One on the left is definitely a tiny giraffe head. The one on the right is magnesium oxide.