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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