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Heatsink material question

Discussion in 'General Electronics Discussion' started by bigone5500, Dec 28, 2014.

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  1. bigone5500


    Apr 9, 2014
    I have some heatsinks that I am concerned about. I originally thought they were aluminum but for some reason I took a magnet and rubbed it across one. I found it to be very slightly magnetic in a strange way. If I take the neodymium magnet and rub it around on the backside of the sink, there is a bit of attraction...not much. I have to rub it around in a circle of sorts.

    I looked at some heatsinks on ebay and saw some that looked like the ones I have. It is coated in something called Iridite. Could this be the reason the heatsink acts magnetic? Could it be that the material is more that aluminum?

    BTW: These heatsinks came off of ~1992 Allen Bradley 1336 variable frequency drives.
  2. Externet


    Aug 24, 2009
    Why is the concern ? Obviously yes, there is other than aluminium material.
  3. bigone5500


    Apr 9, 2014
    My concern is that I won't be able to sell them as aluminum therefore won't be able to get as much for them.
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Some non-magnetic materials can interact with magnets - especially powerful magnets. For example, look on YouTube for videos of neodymium magnets being dropped through copper cylinders.

    I don't know any more than that, but that's something for you to investigage.
  5. bigone5500


    Apr 9, 2014
    That is interesting. I will look into it.

  6. Bluejets


    Oct 5, 2014
    I think you will find the scrap metal people will be more than aware of what it contains.
  7. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Iridite is a chemical coating applied to aluminum to prevent corrosion or improve electrical contact. The heat sinks were probably used with devices that were not insulated from the heat sink if the coating was Type 3 to Mil-C-5541F specifications. See this link.

    As for the "magnetic properties" you observed, this was no doubt the result of eddy currents induced in the aluminum by circular motion of the neodymium magnet you used. I think you can safely claim these are aluminum heat sinks.

    If there are still doubts, you can test for aluminum at your own risk using the following procedure: Use a file to obtain a small quantity of metal shavings from the edge of one of the heat sinks. Fill a small glass container (a Pyrex measuring cup is appropriate) with a saturated sodium hydroxide solution (dissolve ordinary lye crystals in warm water until no more will dissolve; this is an exothermic reaction, which is why a Pyrex container is recommended). Drop the filings into the sodium hydroxide solution. If hydrogen gas evolves, you can be pretty sure the filings are aluminum. Note: hydrogen is flammable, so no open flames, sparks, or arcs. Be careful. Do not substitute brand-name drain cleaners (such as Draino®) for pure lye crystals. Do not breathe the vapors coming off. Best to do this outdoors if you don't have a fume hood.

    Be careful handling the sodium hydroxide as it reacts with organic matter, including skin. Rubber gloves are recommended. It has a "soapy" feeling if you get it on your fingers: wash immediately with large quantities of warm water until the "soapy" feeling goes away. If you spill the crystals or the solution on clothing, rinse immediately under running water. I have lost several pairs of blue jeans because of holes caused by sodium hydroxide solutions that spilled without me noticing exactly where. Dispose of the used solution by pouring it down the drain along with copious amounts of rinse water. The drain will probably benefit from the cleansing action of the sodium hydroxide (lye) solution.

    As a kid, my younger brother and I (with Dad's supervision} made hydrogen gas in 7-Up bottles using water, lye crystals, and pieces of aluminum foil. We attached a deflated balloon to the neck of the bottle, securing it with a piece of string. Some time later (hours IIRC) the balloon was inflated with hydrogen gas and ready to launch into the air. We tied it off above the bottle neck and attached a paper tag with our address, hoping someone would find them and send us a post card telling where they found our balloon, but no one ever did.
    bigone5500, Supercap2F and KrisBlueNZ like this.
  8. bigone5500


    Apr 9, 2014
    Thanks for the reply hevans. I was about 99% sure these were aluminum.As for the iridite, those drives had different mounting configurations. You could mount them inside an enclosure, outside an enclosure, or you could remove the surface mound and have the heatsink protrude out the back of the enclosure.

    On a side note, your avatar photo makes me think of the guy on the Dos Equis commercials 'The Most Interesting Man In The World'.
    Last edited: Dec 30, 2014
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