[s-cars] Thermal properties of anodizing Aluminum

[Trevor Frank]

Most of what I did know about anodizing I have forgoten, in
semiconductor we really studied this layer and the different processes
for a number of reasons.  I believe most important is the fact that the
thermal conductivity of aluminum oxide is worse than pure aluminum.
Hard anodize is nothing more that a really thick layer of aluminum oxide
which is basically a ceramic, it's thermal conductivity is about the
same as a stainless steel, not so good, from what I remember.  The
saving grace on having something anodized is that you stablize the oxide
layer more or less and if you are smart you have it black anodized.
Many heat sink manufacturers will do this to make sure their product
looks nice for a long time.  I have heard the debate that the
improvement in the emmisivity due to the black coating offsets the loss
in thermal conductivity.  Of course all of this is subject to surface
area and temperature.  The higher the temps the more emissivity is a
driver, at our temps it isn't such a big deal but every bit helps.  At a
very low surface area to mass ratio we wouldn't even worry but with our
high surface area to mass it is an interesting question.  Personally I
feel it is more academic than anything else, the long path length, like
24 inches at least, though the intercooler has plenty of time to get any
heat out that you could.  As the delta T goes down you lose any driver
to lower the temp, hey this is why an air to air is more efficient.

The other question to ponder is, do you ever want to weld on this thing
again.  To weld a piece that has been anodized means that you have to
make damn sure you cut through all of the anodized layer before you
start welding.  It being a ceramic has a significatly higher transition
point than aluminum and will really crap up your weld, for lack of a
better term.



-----Original Message-----
From: mlp qwest [mailto:mlped at qwest.net]
Sent: Wednesday, February 12, 2003 10:11 AM
To: s-car-list at audifans.com
Subject: [s-cars] Thermal properties of anodizing Aluminum


Anyone on the list have any particular knowledge or expertise on the
thermal effects of anodizing aluminum?

I gather there are at least three (3) major types or subcategories
anodizing, sometimes described or referred to as:

1.	Type 1 - "Chromic Acid" ~> usually leaves very thin deposit

2.	Type 2 - "Sulfuric Acid" ~>  ditto

3.	Type III aka "Hard Coating" ~> a much thicker process, which can
be done
in a way to add  very different characteristics to aluminum.  Type III
anodizing can, and is used to significantly inhibit aluminums otherwise
high thermal heat exchange capabilities, as for example in coating
aluminum pistons to reject heat etc.

Anyone know how processes 1 or 2 affect aluminums heat transfer
capabilities? ' I'd like to know what, if any kinds of anodizing are
done to ENHANCE heat transfer either by conductivity, or for convection
transfer purposes.

Thanks
Mikee "Wondering if there could be a Purple Anodized Radiator in some
one's future" P.


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