Bill Aycock wrote:
SNIP
> The effect does exist, and I still want to pursue this thread, to find
> documentation of the effect, if it can be found. It is important that we
> avoid detrimental effects when we can, and this thread has opened some eyes
> (mine, at least) to a problem that had little PR before (at least, to my
> knowledge). If any of you come across a reference, please let me know.
I'd also appreciate more input on this subject!
> I wonder what the effect of "NoAlOx", or similar treatments, has on this
> effect?
>
>From the data I have been able to gather on the anti-oxidant compounds, it
appears that NoAlox, Penetrox, and Ox-Gard compounds for use with aluminum use
zinc particles suspended in a variety of petroleum based vehicles.
Zinc is a pretty good choice for compatability with aluminum in the Galvanic
series. Both appear very near each other, at the active end, on all charts I've
seen.
This is good from a corrosion prevention objective. The closer any two
materials are in the series, the lower the potential that will be developed
when they come in contact with an electrolyte, like any kind of moisture other
than pure de-ionized water.
When lower potentials exist, slower corrosion occurs, because the potential of
the battery that has been created drives the exchange of electrons that results
in the material loss that we call corrosion.
I think I made a post about this stuff 6-8 months ago.
For the sake of newer subscribers or those who missed the previous comments:
Others can hit the delete button now!
Here's how the stuff (Goo) works:
All anti-oxidant compounds consist of a bunch of tiny solid particles suspended
in a petroleum based solution.
The tiny particles are selected to be the most compatible with the mating
materials from a galvanic corrosion perspective.
If we stick the probes of our ohm meter into the paste we should expect to find
that is is not conductive. This is caused by the suspension of the conductive
particles in a non-conductive solution.
The solution, often called the vehicle, exists to provide a way to suspend the
solid particles and allow us to apply them to the mating surfaces. Without the
vehicle, we would just have a bunch of fine dust that would be hard to keep in
place while making the connection.
Initially, the vehicle provides an additional sealing feature to prevent
moisture from getting into the connection.
Eventually, it goes away. This is a totally time and environment dependant
occurence. But, water is a very powerful solvent and plasticizer that usually
finds it way into everything.
Once the vehicle in the compound has been driven out, we have only the tiny
particles left to make the connection. They accomplish this job by being
trapped between the mating surfaces by the mechanical clamping pressure that we
have applied to the joint.
More pressure and more surface area under clamping pressure are better than
small areas and low pressures.
Once the vehicle has left the connection, the only thing maintaining the
connection is the tiny particles that are tightly clamped between the mating
surfaces.
Moisture can now get in there and surround the clamped particle connections,
but it is gonna take quite a while for the corrosion to completely eliminate
the thousands of tiny connections.
The wx is always working at degrading these connections, and will always win
the battle. The only good news is that anti-oxidant paste use and good
preparation and application will make it take many years before the constant
degradation becomes apparent to us.
The tiny particles in the compounds are very small, probably measured in
microns. That is why I've recommended using very fine preparation abrasives in
previous posts.
If we use coarse abrasives to prepare the surfaces we create big scratches or
grooves in the mating surfaces that are deeper than the size of the particles.
This allows the moisture to more readily wick in there and attack our precious
clamped particle connections.
As far as putting a pencil line on an aluminum tube and coating it with the GOO
of choice, eventually the moisture will get in there and have its way,
resulting in the same final failure mode. It will just take a bit longer for
the problem to appear.
> Bill , eating a little Humble Pie on the New Year, and wishing all great
> things to come.
Send me some of that pie! It's the holiday season! Aren't we supposed to get
as much of that stuff as possible before we have to be good boys again?
73, Kurt
K7NV "That's K7 "Nevada" (ex - NI6W)
YagiStress - The Ultimate Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress/
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