Are there safety issues with abrasive blasting magnesium? 5

[KirbyWan]

We use Aluminum Oxide to abrasive blast magnesium parts. Someone said magnesium dust is a fire hazard and we should not do this because it's dangerous. I did a search and could find very little about fire or explosion danger to magnesium in this context. I know magnesium can burn and the powder can be explosive, but it's generally difficult to ignite, so using a cutoff wheel or high speed grinding might be dangerous, but not abrasive blasting as far as I know. Note, I have seen high powered abrasive blasting that shoots sparks off of steel, but I'm working to P&W SPOP 218 in this instance and it has limits on the pressure of this system.

I'm going to use the Pratt and Whitney process as justification of the acceptability of our process, but to be overly cautions, I thought I would put the concerns here to see if someone has the knowledge to verify or falsify my opinion. Has anyone seen a magnesium fire caused by abrasive blasting? Are there other specific limits that should be followed to ensure safety?

Thanks

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[IRstuff]

Magnesium is listed as a combustible powder:

http://www.nclabor.com/osha/etta/indguide/ig43.pdf

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[KirbyWan]

I understand that, but the actual concentration of magnesium powder is negligible. Would this even qualify as magnesium dust? Is there some concentration of the magnesium fines above which this becomes dangerous, either as a concentration per volume of air or per volume of abrasive media? I expect the actual magnesium level in our aluminum oxide powder would be in the PPM range.

Thanks for your response.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[btrueblood]

Are you going to use a cabinet, or in the open air? Would it be possible to use nitrogen gas as the propellant i/o compressed air (You might need to wear scba if in an enclosed space with no ventilation)?

It might be hard to ignite Mg, but once it lights off, it could be lights out for you and anybody nearby. A high school acquaintance and his dad were lost when they were doing some welding too close to a scrap (metal chips) container that held Mg turnings.

 

[IRstuff]

Doesn't seem to me to be worth the risk. Even a flash burn could be painful and possibly scarring.

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[KirbyWan]

We're using a cabinet. Magnesium can burn in a nitrogen atmosphere forming Magnesium Nitride according to Wikipedia. Do you have a link to an article about the accident? I've been looking for instances of Mg fires and they aren't very common in the context of a blasting booth. It seems the bigger danger is when people try to put these fires out with water, which evolves hydrogen which then explodes.

Thanks,

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[KirbyWan]

So we're working to a spec provided by Pratt and Whitney (also the GE SPM approves this process) Both of the specs specifically call out to not blast Titanium due to a fire hazard (or have specific metal removal procedures for Titanium) and make no mention of a danger with Magnesium.

So is there any specific data that indicates there is a danger of fire or explosion in the context of an abrasive blasting booth? I can't accept the "it could be dangerous so ..." The one instance I have found that was vaguely related was a case (see link below) at a cell-phone body fabricator. This was all magnesium dust from machining and polishing operations that were collected with a water dust collector. It seems the water reacted to evolve hydrogen which then exploded, a reaction that would happen very slowly from atmospheric humidity under dry conditions.

http://www.sozogaku.com/fkd/en/cfen/CC1200085.html

Thanks,

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[MikeHalloran]

Call your insurance company. Really.

Somewhere, at a dingy desk somewhere, they have technical people who can be helpful.

Mike Halloran
Pembroke Pines, FL, USA

 

[KENAT]

From a practical point of view, maybe you can consider how much particulate would be required for it to be a hazard. i.e. how often do you need to clean out or what have you the booth to avoid the build up of magnesium particles.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[tbuelna]

This document might be of some help:

http://www.nfpa.org/codes-and-standards/document-information-pages?mode=code&code=480

 

[TVP]

Here are some links on basic finishing techniques that also include discussion of Mg fire hazard, precautions to take, etc.

http://www.nanomag.us/Hydro_Mg_Brochure_Corrosion_and_Finishing_of_Mg.pdf

http://www.magnesium-elektron.com/sites/default/files/Machining Magnesium.pdf

http://www.nanomag.us/Hydro_Mg_Brochure_Machining_Mg.pdf

 

[tbuelna]

This is actually an interesting question. I have seen mag sand castings cleaned using steel shot in a steel wheelabrator cabinet at a foundry. It seemed to be standard practice to produce a uniform surface texture after shake-out, sawing off risers/feeds, and sanding down flash/mismatch. Don't imagine it would be done if it presented a legitimate fire hazard.

The primary issue with using ferrous blast media on magnesium is that it contaminates the surface, and the ferrous residue must be etched off after blasting.

 

[WKTaylor]

All.. . interesting discussion, many good points.

My 2-cents...

1. One thing I am a bit concerned-about, after having re-read several abrasive blasting specs, is that blast cleaning of magnesium parts should, almost universally, be done with glass beads... not abrasive grit such as steel, aluminum oxide, silicon carbide, garnet, etc. During the vacuum retrieval process a large percentage of the removed surface contaminates tend to get recycled to the blasted surface increasing potential for tramp particles embedded in the surface.

2. Glass-beads clean the surface of contaminates, yet embed very-little/no glass dust/particles, into the non-ferrous surfaces. Also, the blasted magnesium surface tends to be smoother/denser/less-pitted than a grit blasted surface. During the vacuum retrieval process the mechanical screening of the glass beads, tends to shed most of the surface contaminates/metal liberated during the blasting process... recycling very little of this tramp debris back to the surface... as abrasive grit tends to do.

3. CAUTIONS.
a. Abrasive grit blasting leaves superficial pits that are sharp/angular… as opposed to the relatively smoother bead blasted surface. A lot of fatigue testing of various alloys clearly shows a degraded fatigue life in the presence of the sharp pitting features due to blasting with abrasive grit [relative to non-blasted surfaces]. However, the smoother glass bead blasted surfaces tend to have a slight improvement in fatigue life for most non-ferrous metals [relative to non-blasted surfaces].
b. Also… abrasive grit is a poor surface preparation for application of corrosion protective coatings [non-ferrous metals]. These softer surfaces often have microscopic grit particles stuck in the angular pits. The combination of microscopic embedded grit particles, plus the angular roughness to the pits tends to inhibit proper application of inorganic base-coats and organic coatings [primer & topcoat]. The irregular surfaces are not as easily filled at the microscopic level by conversion coating or anodic coatings. Also these pitted surfaces are very hard to fill with organic primers and paints. These coatings tend to ‘bridge’ over the pit cavities, leaving micro-voids.

4. General grit-VS –glass bead blasting process summary…

Grit blasted non-ferrous surfaces are prone to premature fatigue cracking and increased potential for reduced/brittle primer/paint film adhesion… leading to the possibility of generalized paint film break-down [stripping-off]... which leads to premature cracking and/or corrosion failures.
On-the-other-hand, glass-bead blasted non-ferrous surfaces tend to retard-fatigue crack initiation; and the smoother/uniform paint finish has improved adhesion due to the relatively pit-free/tight metal surface.

5. Controlling magnesium particles.
Wet grind/drill/mill/bore/etc with corrosion protective liquid machining coolant. The coolant entraps friction heated magnesium particles, which instantly cools/smothers the hot particles. These particles are easily disposed during the clean-up process.

Also, grinding with cooling air on the part, which blows the liberated particles into a water bath, has been used by some mechanics.


Regards, Wil Taylor

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o We believe to be true what we prefer to be true.
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[tbuelna]

KirbyWan-

Pratt & Whitney SPOP 218 is a specific process, "Dry Abrasive Grit Blast (240-320 Aluminum Oxide)". It sounds like you are processing P&W components that require use of this procedure, so I'm assuming your company may have been qualified by P&W to perform the SPOP 218 process. If so, you should contact P&W and ask them to address your safety concerns. Companies like P&W have many M&P experts on staff and I'm sure they would be quite happy to help.