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madman
04-30-2010, 05:34 AM
Is it true that Magnesiumn corrodes form ther Inside out? I faintly recall reading this many years ago. I am not sure though. I have some blocks of this stuff and thought of making some brake caliper hangers from it for my new Motorcycle.

topct
04-30-2010, 07:43 AM
Unless you know exactly which alloy of magnesium you have I would tend to go for some 7075 instead. You could compensate for the weight difference by using thinner sections of the stronger alloy.

JoeLee
04-30-2010, 07:56 AM
It can cause the metal it is in contact with or bolted to to corrode.
Especially if there is moisture and if a voltage is induced. Remember al the junk GM cars of the late 70's and early 80's where the bumpers would corrode right off the car. the bumper mounts where they were bolted to the frame just disolved. I would check the chart of galvanic reaction to see where that alloy stands.

JL.........................

Evan
04-30-2010, 08:10 AM
Do you have a Class D fire extinguisher? Magnesium burns really well, especially when you put water on it. I won't machine magnesium even though I have some. The last thing I want is a metal fire.

topct
04-30-2010, 08:55 AM
This is interesting. It's only 5 pages, the last one being very important.

http://www.magnesium-elektron.com/data/downloads/Machining%20-%20Web%20Brochure.pdf

The stuff is just to strange to consider it for this application. Most of it is used for casting where strength can be gained by making it thicker where needed. Even then I would be surprised to see it being used as a mount for a vehicles brakes.

In fact I think I have heard somewhere where it is illegal for use in certain applications, in and or around brakes for one.

Dr Stan
04-30-2010, 09:34 AM
The answer to his question is no. All metals corrode due to exposure to oxygen (air and/or water). He may have heard of hollow magnesium corroding on the inside due to oxygen exposure.

As to machining mag, I'm with Evan. A class D fire is no fun.

fasto
04-30-2010, 10:43 AM
The O.P. may be thinking about "intergranular corrosion" which I assure you can happen internally, though I don't know if it happens in magnesium.

Some Piper PA-28 and PA-32 airplanes had issues with intergranular corrosion of the extruded aluminum wing spars. Needless to say, it was not a good thing! Topical corrosion prevention does no good against intergranular corrosion, of course, because the electrical circuit is built into the metal.

As I recall, the root of the problem was traced to some kind of a production issue at Alcoa in the mid 70's, of course that was no help to the people who had to replace their wing spars.

ADGO_Racing
04-30-2010, 01:10 PM
Machining Magnesium is not a big deal. We machine center sections and side bells of quick change rears, as well as weld to repair cracks. We have never set the stuff on fire. However it IS POSSIBLE. A class D fire extinguisher is an absolute must have item. We have four of them in the shop, two in the fab section, and two in the machining section. Although I doubt you will have any trouble machining the stuff. We also make wheel nuts for sprint cars out of Magnesium, front steering and rear drive hubs are also a magnesium casting. Also wheel centers for sprint cars and late models are also magnesium. I think if magnesium was as easy to burn as people think, racers would accomplish the task on a regular basis.

Having seen a good number of race cars drag a broken hub on the asphalt for over a mile, with sparks flying from the chassis dragging on the track, I tend to think the fire issue in a shop environment is a little over rated.

garagemark
04-30-2010, 01:44 PM
May be a tad hard to set magnesium on fire, but depending on the quantity of material, a paltry class D extinguisher wouldn't kill it. Especially if there were sufficient piles of shavings lying about. Ever see a landing gear fire? I watched an entire landing gear burn/melt down on a C-5A due to hot brakes (read- stupid pilot). Caught the wheel on fire. They foamed, and foamed, and foamed, and foamed the fire…. And FINALLY put it out. But not before the front left main gear was all but gone, along with a lot of sheet metal and structure in the wheel well.

I have a little coil of it that I tear a piece off and light for the grandson once in a while. It is BRIGHT, SMOKEY, and COOL! But is also short lived and controlled.

I would opt for a good alloy of aluminum if it were me.

Just another $000,000,000.02

CCWKen
04-30-2010, 02:55 PM
It's pretty hard to "light" Magnesium unless it's in powder or thin sheet form. I've got some large pieces of it that took an acetylene torch to get started. The base on my vertical lathe is made out of it. I've drilled it and cut it with with no problems. You just need to take care of swarf and powder and not leave it laying around--About the same as Aluminum dust.

It burns brighter than the sun so don't play with it. But it does make cool blobs of melted dirt and sand. :rolleyes:

topct
04-30-2010, 03:31 PM
I just found some 7075 "magnesium" aluminum lug nuts on the net.

Yes 7075 contains magnesium. But that kind of magnesium won't catch fire.

Is it getting like that other tricky word, billet?

CCWKen
04-30-2010, 04:25 PM
There's only a few of the aluminum alloys that don't contain magnesium. Even 6061 has it. The sad part of using the magnesium alloys is that it must be coated. Well, not must, but if you want to keep looking nice it needs to be coated. It turns a dark grey and looks ugly in pretty short order. You won't have any problems machining it but I would stick to the aluminum alloys just because it's easier to get.

drof34
04-30-2010, 05:07 PM
I once did some machine work on a pair of valve covers off a double A dragster that were made of magnesium. I had no trouble machining them and tried to burn the shavings but couldn't. Magnesium only burns when you don't want it to.

Some time later was doing some welding on steel some 5 or 6 feet away and it (mag) caught fire in the middle of my shop. I scooped it up on a piece of plywood to take outside and when I did it exploded. It exploded because it had burned a hydraulic hose causing oil to get on it.

I'm 76 yrs old but nearly half the skin on my left arm is only 2 yrs old. Once is one too many.

JoeLee
04-30-2010, 05:33 PM
When I was in high school we had magnesium tape in the science lab. It came in short pieces about the size of a twist tie that you would put on your garbage bag. You could lite it with a match.
One day while in the dark auditorium I lit up a piece, you would think someone was welding in there. Got my ass booted out for that stunt.

JL..............

Thruthefence
05-01-2010, 03:13 PM
Many wheels, and indeed brake housings on commercial aircraft are magnesium. And yeah, they will catch fire & burn.

"In fact I think I have heard somewhere where it is illegal for use in certain applications, in and or around brakes for one"

Evan
05-01-2010, 04:59 PM
Magnesium isn't the least bit difficult to light in swarf form. It is so easy that all you need is a stick of lighter flint and a pocket knife. You can buy a fire starter stick anywhere that camping supplies are sold. It is magnesium with a large stick of Misch metal embedded on one edge. To use you shave off some magnesium with your knife and then strike some sparks on the shavings with the knife scraping the Misch metal.

BTW, magnesium is NOT a component of 7075 or other 7000 series alloys. They are alloyed with zinc. Any magnesium in the alloy is restricted to less than .3% to zero. 2024 contains a small amount of magnesium, less than 2%. The regular aluminum alloy series are not alloyed with any significant amount of magnesium. There are aluminum/magnesium alloys but they are identified differently.

gnm109
05-01-2010, 05:41 PM
Do you have a Class D fire extinguisher? Magnesium burns really well, especially when you put water on it. I won't machine magnesium even though I have some. The last thing I want is a metal fire.


All too true. I saw a magnesium fire in a rocket engine turbo pump many years ago at Rocketdyne. It was completely gone in ten minutes.

Magnesium is nasty to work with. You never know what's going to happen.

topct
05-01-2010, 05:55 PM
Magnesium is a component of 7075.

http://www.suppliersonline.com/propertypages/7075.asp#chemistry

I got real curious when someone said they made lug nuts for race cars out of magnesium.

Looked up "magnesium lug nuts".

http://specs.jazzproparts.com/wiki/Lug_nuts

It looks to have become one of those trick words.

Evan
05-01-2010, 08:13 PM
It isn't a component according to the link you posted which agrees with what I said. When an element is list as a "max" value like it is there as ".3% max" that means that is the most that will be tolerated, not desired. In other words the alloy is just fine if it contains none at all. It is very difficult to separate all traces of magnesium from aluminum so the specification allows a trace amount.
wait...

Strike the above. I read one line down at Matweb and took the value for manganese as the value for magnesium. My mistake. Still, the primary alloying element is zinc for 7075 and copper for 2024. What I said about separation is also correct.

boslab
05-02-2010, 02:10 AM
You can get metal to corrode internally quite well, its called intercrystalline corrosion, it occurs between the grains of metal which was probably what your friend was thinking of, mostly the result of poor degassing during manufacture.
As has been pointed out watch the swarf! no water! and are you quite sure its the right material for the job, i would have thought an ali alloy would be ok but i'm certainly no brake expert!
mark

gnm109
05-02-2010, 11:36 AM
You can get metal to corrode internally quite well, its called intercrystalline corrosion, it occurs between the grains of metal which was probably what your friend was thinking of, mostly the result of poor degassing during manufacture.
As has been pointed out watch the swarf! no water! and are you quite sure its the right material for the job, i would have thought an ali alloy would be ok but i'm certainly no brake expert!
mark


Aluminum alloy is just fine for brake calipers on most any car or motorcycle except the most exotic racing vehicles. If one were to machine one's own calipers, the small advantage in reduced weight with the use of magnesium would be more than outstripped by the extra cost and the potential dangers.

Evan
05-02-2010, 02:17 PM
You can get metal to corrode internally quite well, its called intercrystalline corrosion,. . .

Or, in the case of stainless steel, chloride induced stress corrosion cracking.

This looked fine on the outside until I hit it with a hammer.

http://metalshopborealis.ca/pics2/mmss2.jpg

http://metalshopborealis.ca/pics2/mmss3.jpg

darryl
05-02-2010, 05:24 PM
I've wondered about that ever since I first heard of chlorine on stainless problems. Here we have stainless steel sinks and bleach, ammonia- whatever all chemical products that we routinely put in sinks. Ferric chloride is another-

Evan
05-02-2010, 05:30 PM
Ferric chloride eats SS just fine as well as brass, copper and aluminum.

madman
05-03-2010, 06:21 AM
I have a nice sized chunk of it, might just leave it sit and use some 20 series aluminumn, its said to be the most heat resistant and suitable for caliper hangers.

Evan
05-03-2010, 09:37 AM
Good plan Mike. 2000 series is most commonly used in aircraft and is alloyed with copper which does raise the solution temperature. It also machines very nicely and without burrs.

sbmathias
05-03-2010, 03:34 PM
Many, many years ago I set a lathe on fire machining magnesium. Had a VW transmission housing up in a good-sized lathe. To convert old 6volt engines to 12volt (including starter), you had to move to a barely larger diameter flywheel. As such, you had to open up the bell housing in the transmission case.

So, with the bell housing just barely clearing the ways, around and around it went. As slow as we could go, as it was quite unbalanced. Had to feed the tool using only the compound, but that worked OK.

UNTIL

I let the tool pause at the deepest part of the cut. With all those nice magnesium shavings right there, and a little extra friction heat from the stationary cutter, it all went up in a spectacular fire.

Fortunately, it didn't last long, nor was there any damage done to people or equipment. Obviously, it was memorable.