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> Finally sandblasted it with 30 mesh sand at 100psi from a home compressor, > and got a nice finish not unlike the "bead blasted" finish now available on > Luxfer aluminum 80's. > > So, what's the consensus here? Shall I condemn my tank, get it re-hydroed, > or just continue on as usual? Don't think I took off much aluminum, > relative to the tank thinkness, nor did I get the tank hot. I'm inclined to > go on using the tank, but open to other opinions. Thanks. What you did by any of these blast or abrasion methods was remove all of the anodize on the surface. Now the tanks will show a surface corrosion in very little time. You could have then re-anodized but it will cost you. At best get a good aluminum primer and top coat on as soon as posable. Also step up your visual inspection of the tank surface for any pitting under the paint. If there are any deep pits - check with the tank manufacturer for the max allowed, scrap the tank. As for the surface morphology of the metal, the sand blast has forced silicon (and anything else that was mixed in the sand) inclusions into the surface of the aluminum which will reduce the fatigue life. If this was an aircraft, you would have just scrapped the part. For an aluminum tank there shouldn't be a problem as the fatigue loading is low (lets face it we just do not cycle a tank much more then 100-150 time a year unless you are doing training). In the future use a nonacid chemical stripper or plastic media (I recommend type 5). Here is why: Chemical strippers: Most strippers use methylene chloride as the active chemical. Methylene chloride is one of the best strippers ever used but it has been declared a Hazardous Air Pollutant (HAP) by the EPA and as such is being restricted. You just may not be able to get it much longer. As an alternative the stripper manufacturers are changing to Benzoic Acid as the base. Benzoic acid may/can cause hydrogen embritlement and stress corrosion in high strength steels. Therefor it should never be used on steel tanks. So unless you can remember that the new acid strippers ARE NOT TO BE USED ON STEEL TANKS, do not use them at all. Media Blast The aircraft industry has been forced away from using methylene chloride by the EPA for the past 2-3 years. The current restriction is 55 gallons of HAP containing stripper per air frame (that's for a piper or a 747!) for use on parts that cannot presently be stripped by other methods. The alternative for aircraft is Plastic Media Blast (PMB). The Type 5 media currently used will not remove anodize and by adjustment of media flow and air pressure can be used on aircraft skins as thin as 0.02". So, now that you have done it, check for corrosion and do it differently in the future. Peter Johnson Senior Manufacturing Engineer Sikorsky Aircraft -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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