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"Rust color on aluminium, under silicone coating"

June 21, 2016

Q. I have some aluminium coil clamps that we apply a silicone coating to and have done for many years.

The items are vapor De-Greased then aluminium oxide blasted before coating with the silicone and heat cured at 80 °C.

The items look good initially but after a while what looks like red rust spots appear (which is very strange as aluminium does not red rust) anybody have any ideas on this.

60202-1a  60202-1b

I have applied this same coating on aluminium test panels I have and the marks do not appear.

I am awaiting the aluminium grade of the coal clamps as I know sometimes there can be intermetallic particles on 7075 Aluminium which can affect anodized items and was wondering if this may be what is leading to the problems I am seeing.

Paul Rennison

June 2016

thumbs up signHi Paul. I don't have practical experience in this, but based on reading a number of threads here I'll bet that when Robert Probert sees your question he will be telling you to throw away the aluminum oxide grit and get some new stuff which hasn't been used on steel :-)


pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

June 21, 2016

A. Ted Mooney already said it. Your blasting media has been used on iron and been punched into the soft Kleenex-like aluminum -- iron oxide is reddish brown.

robert probert

Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

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