quote punchbug:
If these are die-cast aluminum, then pitting is all to common for the process. A vacuum draws the molten metal into the die cavity. If there is any leakage in the die cast seal tiny bubbles (pitting) will form. There are many other reason for this as well but it is not relevant to this topic. Long story short it could be corrosion it could be pitting either way it is near impossible to get out.
a little lesson about diecasting.
Vacuum is only used to attempt to evacuate as much air (or gas) as possible from the die cavity area(s). It has nothing to do with the metal flow.
The molten metal is forced into the die at pressures approaching 3000 PSI. Bubbles (or blisters) in the die cast process are typically due gas being trapped in the cavities, most likely to excessive moisture (i.e. water leak, heavy die spray residue) being present during the fill phase. It is possible to see some evidence of what is referred to as "shrinkage voids or porosity" in extremely heavy cross sections of a diecast part, simply because the larger amount of mass causes internal holes to form due to shrinkage during solidification and cooling, but you’d most likely have to cut the part open to find it.
Gravity-pour castings (i.e. sand castings) normally don’t use vacuum evacuation as part of the process. If an aluminum sand casting needs to meet pressure requirements (i.e. leak test spec) then it needs to go through an impregnation process to seal up the pores that are always present,.
Simply put – If a die cast part exhibits surface pitting immediately after casting, it’s likely cause by solder or metal build-up on the cavity or cores of the tooling, or some other fault occurring that is affecting die cooling, die spray, etc..
If it shows up years after the part has been cast, its corrosion.
As for the tank band – Your part is corroded. Period. The black stains are oxidation that has eaten its way into the pores of the material. Unprotected aluminum will oxidize naturally, due to exposure to moisture or contaminants in the air. You may be able to bleach them out, but not likely. If you buff it,the spots will appear to diminish or shrink away, so that might be your best option other than sanding them out or hitting it with glass bead. Either way, it requires some removal of the substrate in order to get down to where the stain has penetrated.
Hope this helps.
Best,
PM T2
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