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Peter, I thought that ground dry ice was being used as a blasting medium by airframe rebuilders and painters. Jim ------------------------------------------------------------------- Learn About Trimix at http://www.cisatlantic.com/trimix/trimix.html > From: <Gilldiver@ao*.co*> > Date: Sun, 5 Dec 1999 09:36:49 EST > To: cam_banks@ho*.co*, techdiver@aquanaut.com > Subject: Re: sandblasting tanks > > >> 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'. > -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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