🚀 go-pugleaf

Ok, Larry.  You want flames, you got them.

Note:  if you're tired of this or if you're not interested enough in corrosion
to get the straight word, hit 'n' now and avoid the following:

In article <3223@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:

 [Quoting Larry from an earlier article]

>> >	The important point to bear in mind is that there is no significant
>> >presence of dissimilar metallic junctions beneath an automobile which lend
>> >themselves to the formation of galvanic corrosion cells which affect any
>> >significant area of the automobile body
 [Material Deleted]
>	I already mentioned this in my previous two articles, in addition
>to the above.  The point to bear in mind is that these other corrosion
>machanisms are NOT SIGNIFICANT in the automobile environment!

If we read these statements closely enough, we will find that automobiles
never corrode.  Isn't that nice.  However, Larry, there are a number of
upstate NY rust buckets in the parking lot of this building.  How did
they get that way?  Gemlins?  Divine intervention?  Sorry, Larry, but
you can't have it *both* ways.  I'm afraid that you're going to find
out if you bother to look at any texts or design literature that
concentration cells, and other non-dissimilar metal corrosion mechanisms
are recognized as a problem in mild steel.

>	So what?  It's not going to be done with either the same chemistry
>or metallurgy found in an automobile environment, so it's a non-applicable
>example.

OH, you want a specific example.  Ok, first I buzz down to your house with
my nibblers, cut a hunk of sheet metal out of your fender, bring it back
here and put it upright in a largish beaker.  Next, I pour about 75 g of
10 mesh calcium chloride/sodium chloride (aka road salt) into the
bottom.  Finally, I carefully pour some water (tap or distilled) into
the beaker.  Viola, one steel-steel concentration cell using materials
common on our nation's highways.  Happy?

 [Quoting me]
>> You greatly underestimate the need for mixing.  I can
>> easily see a difference in electrolyte concentration on a car.
>
>	So tell me about it.

Well, without going into the gorey details, the fact is that plain
diffusion is very slllooowww.  If I take a perfectly still beaker of
liquid (at a uniform temperature to avoid bouyancy convection) and add a
second liquid extremely carefully, and keep it where the temperature
will remain uniform and as little vibrations as possible will shake it, it
will take weeks, if not months, for the solution to come to chemical
equilibrium.  Yes, this experiment has been performed several times. 
Mixing is such an important aspect of chemical engineering that books
are devoted to it.  It is not a trivial operation. 

 [My description of putting magnesium in your trunk deleted]

>	Oh, balls.  I am trying to get across a message of whether or not it
>is PRACTICABLE and REASONABLE to implement any type of cathodic protection
>in an automobile environment.

You really should proof read, Larry.  Galvanizing is cathodic
protection.  So, that's example one.  What I proposed is all very
reasonable and practical.  I wouldn't say it's the best idea (I would
just indulge in some Rust-Oleum), but it would work.  Further,
galvanizing works only because it is cathodic protection.  If you were
stupid enough to use a more noble metal for a coating, the first scratch
in the coating would quickly corrode.  Unfortunately, you get much the
same result from paint.  It's not nearly as bad since we aren't
plastering the small scratch with a hugh cathode, but the paint is
permeable and does render the scratch slightly anodic. 

>Then tell me about what will happen after the magnesium gets wet the
>first time, and a solid layer of magnesium oxide forms on its surface. 

Larry, this is your best example of leaping without looking.  Magnesium
is frequently used for sacraficial anodes on ships and inside water heaters.
Do you think elves come along and scrape off the oxide?  Wrong, the oxide
layer is not sufficient to prevent further corrosion of the magnesium
(this is why we don't use aluminum as a sacrifical anode, it does form
an excellent oxide layer).

So, Larry, sorry I had to bust your bubble; but, I just can't let your
brand of dis-information go by.

Jim Kweeder
kweeder@sun.soe.clarkson.edu