Will AZ91E Magnesium Alloy Corrode in Air?

[kurketom]

Hi I am doing a research on what material will suitable for using in solarcar suspension part and magnesium alloy seems to be a pretty good choice because of it's weight /strength ratio.

But seems it have problem in corrosion resistance. Since it's a structural part we are not planning on applying paint or coating. I read some articles online saying AZ91E have "excellent corrosion resistance". Is this mean it won't corrode in air? If it won't then why all the coating treatments like painting, coating...Also is there any other magnesium alloy won't corrode too? Also will magnesium alloy stand in Thx

 

[Enthalpy]

1) Don't use magnesium, it's bad stuff and poorly available. Good alloys of aluminium, titanium, or iron are better at equal mass.

Only carbon fiber is better than steel.

2) Corrosion depends completely on the context. Air in the lab differs from rain which differs from a river which differs from the Ocean. Radically.

3) You can have surface treatment. Easy, cheap. Do it.

4) A carburizing steel is weldable, available as rod and tube. Heat-treated without C it provides 700-800 MPa yield strength yet stays tough (check 25CrMo4, 17CrNiMo6...). Aluminium 7022 is weldable as well. Or PH13-8, also PH17-4, excellent for a suspension but expensive. Or even consider S700MC (available as sheet, not rod): fully weldable, 700MPa YTS.

 

[enigma]

Look into anodizing aluminum or titanium, depending on your requirements. It's cheap to do.

 

[Enthalpy]

By the way, I'd love to have magnesium alloy available! Some parts would be better of magnesium than plastic or aluminium, for instance to avoid complicated aluminium shapes for the same mass and strength.

Problem with magnesium is the same as titanium:
- Very poor availability! Presently, designers use it only as cast parts because extrusions and laminations are so difficult to get in the proper shape, size and alloy.
- These metals don't make progress! Steel improves at a quick pace, aluminium less so. For magnesium and titanium, we still have the same old stuff (AZ91, TiAl6V4) as for 50 years. Anything else is unavailable.

I wanted to machine boxes of magnesium for the electronic boards of my satellite. Nothing difficult: the shape was essentially a stub. I needed something metallic, that doesn't rust in a room and protects the electronics from falling objects. Unavailable, I had to use aluminium.

I wanted to replace an aluminium part (7022 or 2024), 0.6m*2.4m*0.3m with many inner holes to enlight it, hence in two parts glued together, by one single plain magnesium part: unavailable.

So if one company worldwide wanted to offer a line of laminated and possibly extruded material, of good alloys (...better than plastic!) and available ex stock, and let it know, maybe they could live from it. Even better, of course, if high-perf alloys were developed, but this needs more investments.

 

[kurketom]

Thx for the reply! Seems like magnesium is a dead end... I will keep digging

 

[etudiant]

Magnesium actually works pretty well if you respect its properties.
For instance, the old Volkswagen engine block was made from magnesium, which was the key to allowing the engine to be air cooled.
However, the US Navy found magnesium to be a corrosion nightmare for carrier based aircraft and thin section magnesium material burns fiercely when ignited by a crash or accident, so it has become orphaned.

 

[kurketom]

Wow engine block? Isn't magnesium is very easy to ignited? How can they withstand the temp. of the explosion in cylinder? And is it this mean it is strong enough to be a structural part? also we are pretty amateur, we don't know any special treatment but i do saw ppl say body shop can do coating on them. Will it be enough in term of "respect their proporties"? Or we need to do other preparation? Thx for replying btw!

 

[etudiant]

Ignition depends on heat input vs heat diffusing away. The engine block diffused the heat away fast enough that the material never came close to ignition temperature.
Magnesium is an excellent structural material, strength and stiffness not dissimilar to aluminum. The US Navy A5J Vigilante strike/recon aircraft used it extensively.
The properties issue is harder, the coatings have to be compatible, corrosion from galvanic or salt water is problematic, the material has to be machined with precautions lest the turnings catch fire, etc. That said, for suspension parts such a struts or axles the material should work fine. Your biggest problem may be finding someone who is geared to work with this metal.

 

[kurketom]

Oh, thanks so much for the info! But just one more question: if we are using magnesium as part, there probably will have magnesium dust produce at the joint and powder will ignite easily. Is there anything needed to take notice like more lubricant or other precaution?

 

[etudiant]

Magnesium joint parts need lubrication just like any other metal, else the surfaces gall and seize. Powder is not that easily achieved and you don't have to worry about that.

In general, magnesium is not some unattainium material, it is just a nice light aluminum alternative, with some quirks that have to be factored in.
For most prototype uses, the limitations are immaterial. It is only when you need to factor in long term durability and/or mass market exposure, where stupid is a fact of life, that you need to be careful.

 

[kurketom]

Thx a lot for the help!

 

[davidyanni10]

Just wait until we get FCC magnesium ;)