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In article <3221@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:

>In article <3164@sunny3.che.clarkson.edu>, kweeder@sunny3.che.clarkson.edu 
(Jim Kweeder) writes:

[A pretty good description of galvanic corrosion omitted.]

>	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, and which are amenable to anodic
>or cathodic protection. . . .
>	Stated another way, two pieces of metal, galvanized or bare, fastened
>together with zinc-plated or bare steel fasteners or by welding, will not
>BETWEEN THEMSELVES undergo galvanic corrosion in the environment of an
>automobile body.

In your original article you said potential difference, not dissimilar
metals.  You don't need dissimilar metals to form galvanic cells or
haven't you noticed that cars do rust.  Different steel alloys, different
heat treatments, differing amounts of cold work all cause changes in the
electromotive potential.  Thus, it's very easy to set-up a steel-steel
galvanic cell.

Also, any corrosion is amenable to cathodic protection (although other
factors may make the implementation difficult).

>1.	An autombile is not a "large" enough object for significant potential
>	differences to exist between one end and the other, with one cause
>	of such potential difference being, say, differential oxygen
>	or other ion concentrations in clinging surface water.

Excuse me, but I can set-up a pretty nasty concentration gradient in a
100 ml beaker.  You greatly underestimate the need for mixing.  I can
easily see a difference in electrolyte concentration on a car.

>2.	The possible cathode area beneath an automobile is insignificant
>	when compared to the anode area (i.e., the body steel).  In order
>	for significant galvanic corrosion to occur, the cathode area
>	must approach or exceed the magnitude of the anode area.

True enough.  However, your still hung on the idea of needing dissimilar
metals.

>The mechanism of cathodic protection ONLY comes into
>play where there is a DEFECT IN THE ZINC CLADDING, resulting in exposure
>of bare steel, at which point the surrounding zinc coating will function as
>a sacrificial anode for COMPARATIVELY SMALL AREAS OF EXPOSED STEEL.

Too bad you haven't had the opportunity to work in an automotive press shop
(I have).  Pressing often leaves scores and other damage on parts.  Thus,
if zinc didn't provide cathodic protection, then the coating would be
ineffective.

>	Oh, really?  It's that simple?  How many square inches of zinc or
>magnesium are necessary to protect say, one square foot of body metal?
>And will there even BE any protection through such a seemingly simple method?

Yep, just that simple.  If the original poster wants some numbers, I'll
be glad to open my corrosion engineering book and work them up for him.

>	Do you have any idea what important item ships have that automobiles
>don't have?  This particular item is essentially the SOLE cause of galvanic
>corrosion of steel in ship hulls.  This item also functions as a clearly
>identifiable cathode.

Yes, I have some familarity with steel hulled ships and boats.  My parents
were considering purchasing a used boat with a steel hull.  However, the
boat had laid for a couple years with the impressed current system turned
off; so, the survey revealed cosiderable corrosion damage.  Was the corrosion
around the stern?  No, the stern had magnesium blocks.  The damage was mostly
along keel where there are no dissimilar metals but apparently potential
differences.  The boat was not purchased.

>1.	Where and what are the cathodes which form the galvanic corrosion
>	cells in an automobile?  What is the ratio of surface area of
>	these cathodes to the body metal (the anode)?

As I pointed out earlier, there are numerous ways to create a potential
difference, dissimilar metals being only one method.  I'll forgo an
attempt at a complete listing since it would be incomplete.  This is the
whole problem with corrosion engineering:  being able to predict or identify
galvanic cells.  If you just think dissimilar metals, you'll miss quite
a bit.

>2.	What are a few "certain automotive corrosion problems", and how
>	can cathodic protection be applied?

Well, here's a quick example.  Say that you have a trunk leak you can't fix.
Thus, water will collect at the bottom of the spare tire well.  To prevent
rusting out, you can do three things:  (1) apply a protective coating (ie
paint), (2) provide drainage to minimize electrolyte accumulation, or (3)
attach a few small magnesium blocks around the bottom of the well.  In reality,
I would suggest a combination of methods to cover contingencies.

I do not intent the tone of this or my previous posting to be vehement (and my
apologies if it does).  Further, you do have a fair amount of knowledge in
corrosion.  However, corrosion engineering is a field where a little bit of
knowledge is dangerous and one must constantly remind him/her self that
something may be easily overlooked.  Corrosion has many tricks and thinking
along narrow lines could be deadly.  I'd suggest you find a good corrosion
book and fill in the gaps in your knowledge.

Jim Kweeder
kweeder@sun.soe.clarkson.edu