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In article <3174@sunny3.che.clarkson.edu>, kweeder@sunny3.che.clarkson.edu (Jim Kweeder) writes:
> >	Have I denied or disputed that automobiles rust?
> >
> >1.	The corrosion mechanisms which are NOT, in general, responsible
> >	for rust in automobiles.
> 
> But Larry, if cars don't corrode, how do they get rusty??????????
> The very nice chemical reaction you described is still electrochemistry
> and corrosion (and are correct save for the deletion of the necessary
> activation energy for the reaction to go).  You eliminated dissimilar
> metals (with which I would agree) and then eliminated non-dissimilar
> metal mechanisms.  That doesn't leave much left, Larry.

	Okay, now have I denied that automobiles rust by *corrosion*?

	There are several generally recognized modes by which corrosion
occurs, which are sometimes referred to by different names, and which I
will attempt to briefly mention:

1.	General Chemical Attack - This is THE primary mechanism behind
	corrosion in automobiles, and is THE most common cause of any
	corrosion of man-made objects.  As I pointed out in my previous
	article, strictly speaking this is an electrochemical reaction,
	but to think of it as such can often be seriously misleading to
	many people since there are not two readily identifiable electrodes
	or a readily identifiable source of electrical energy.  By the
	expresion "readily identifiable" I refer to being understood
	by people who do not have a good working knowledge of chemistry.

2.	Galvanic Corrosion - This method involves two dissimilar metals;
	while it does occur in automobiles, it is not a significant mode
	of corrosion in this application.

3.	Concentration Cell or Differential Concentration Corrosion - This
	method causes corrosion through potential differences resulting
	from differences in ion concentration (oxygen, hydrogen or metal)
	at different points of contact with the same metal surface.  This
	method is also sometimes referred to as "crevice corrosion", but
	I find that term misleading.  This method of corrosion is more
	common in automobiles than galvanic corrosion, but is is still
	not significant when compared to the incidence of general chemical
	attack.

4.	Erosion Corrosion - Somewhat self-explanatory name, and not at all
	significant as a cause of automobile body rust.

5.	Stress Corrosion - Somewhat self-explanatory name, and not at all
	significant as a cause of automobile body rust.

6.	Pitting Corrosion - A highly localized concentration cell phenomenon
	which causes - what else? - pits in the metal.  Sinces pitting tends
	to form in the direction of gravity, this is not a signifcant cause
	of underbody rust in automobiles, although it may be a minor (by
	comparison to general chemical attack) contributor to rusting
	elsewhere in an automobile.

7.	Intergranular Corrosion - Since this occurs almost exclusively in
	austenitic stainless steels, it is inapplicable to automobile rust
	formation.

8.	Selective Leaching - Only applies to alloys such as brass, and is
	non-applicable as a cause of automobile body rust.

9.	Hydrogen Corrosion - Not at all applicable here!

10.	Fretting Corrosion - Virtually non-existant as a cause of automobile
	body rust.

	Off the top of my head, the above are all of the major corrosion
mechanisms which exist.

	Only the first mechanism - General Chemical Attack - is significant
as a cause of autombile body rust.  Since there is no "opposing electrode"
(like the earth in the case of say, underground tank corrosion) in an
automobile, there is no practicable method of using impressed currents
as a method of either cathodic or anodic protection.  Since the underbody
surfaces of an automobile hardly possess any geometric uniformity, it is
generally impracticable to utilize any cathodic protection methods
involving installation of sacrificial anodes to mitigate this type of
corrosion.  The best that anyone can hope for is the limited cathodic
protection which occurs as a result of factory galvanizing and/or use of
zinc-rich paints.

> >2.	How cathodic and anodic protection schemes are, in general NOT
> >	effective as an "aftermarket" measure for autombiles.
> 
> In general, you are right.

	Now, we're making REAL progress here!  This is the message which
I have been trying to get across!

> What I'm saying
> is (1) cathodic protection is an alternative, (2) any corrosion prevention
> method is tricky to apply, (3) that I would *still* just use an organic
> coating (it's the least tricky method, it's cheap, and it works).  I'm
> saying it's worth CONSIDERING.

	I agree with this!  [I say, we're making REAL progess now!]  I have
also "considered" cathodic protection for this problem, and rejected it as
being impracticable.  The point of my articles is to offer this bottom-line
opinion to those readers who do not have the kwowledge and experience to
evaluate the applicable issues themselves. 

> >	How about less than *** 2 *** inches away? [Throwing power of zinc.]
> 
> But Larry, this would be sufficient in my tire well example.  Just a few
> zinc blocks would protect the bottom of the well (which is what I was
> suggesting).  Thanks for proving my point.

	But it ain't that easy to be _effective_.  First, you have to
fasten the zinc to the body, which is going to require drilling holes
in the body.  Second, you are going to have to use a suitable fastener,
which is an invitation to further problems.  Third, if the resultant
connection does not have a damn low resistance, not only will the scheme
NOT work, but it can readily cause further corrosion problems resulting
from the fastening method.

	*I* would not do it - no way, no how, not even on a bet. :-)

> >Anodic protection is tricky,
> >because if the potential goes amok, it cause greatly accelerate corrosion
> >rates over what would occur without protection in the first place.
> 
> Agreed, however, a more severe problem is pH of the electrolyte:  passivation
> of steel doesn't work with acidic electrolytes.

	I hate to say this - since I don't want to provide fuel to use against
my arguments :-) - but the pH of road salt electrolyte solutions tends to
be alkaline, around a pH of 8 like that of seawater.

> >	The above is not - I repeat NOT - a "steel-steel concentration cell".
> >While some concentration gradients may exist in your example, they pale
> >by comparison to what is really going on: a simple chemical reaction as
> >described in some detail in the beginning of this article.
> 
> It is most definitely a concentration cell.  When fresh, the concentration
> of chloride ions at the bottom will be near the solubility of salt in water
> and the concentration at the top will be just about zero.

	But unless the metal traverses both solution layers, there will be
no potential difference across the metal.  While this mechanism of corrosion
is common in boilers and chemical process equipment, the required conditions
simply don't exist in the automobile underbody (except, perhaps, for some
crevices, but this is only a very small part of the problem).

> If I keep the
> cell at a uniform temperature and don't shake it, the gradient will persist
> for weeks.  Further, if there is no gradients, there will be no electric
> potential to activate your reaction (ignoring any defects on the steel
> that may lead to an auto catalytic corrosion reaction).
> 
> >	So what does the above have to do with the price of tea in China?

	There is also a passivity effect (originally discovered by Faraday
in the mid 1800's, that involves the formation of a 30 Angstrom or less
surface film) which is not unlike what you describe, but neither this nor
what you describe is applicable to the comparatively shallow electrolyte
levels which rapidly undergo evaporation, as found in the automobile
environment.

	My original comment about the price of tea still stands.

> >	There is something which you are overlooking.  Magnesium works
> >just fine - when it has: (1) a significant surface area as compared to
> >the area to be protected; and (2) even more important - when it is
> >continuously immersed in the electrolyte!
> 
> And if I find an automotive application that fits these two specifications,
> then I'm set.  You've proven this for me, thanks!

	That's right - you are set!

> >	Tell me about your grandiose plans to eliminate corrosion after
> >you've obtained your degree and worked in the real world of industry for
> >a few years.
> 
> Which degree?  I've already got two of them.

	My apologies.  To me, however, some of your statements sounded like
those of a few book-wise but otherwise unworldly engineering students I have
had the misfortune of employing over the years.  Fortunately, I have only
one student employed as research assistant at the moment (Hi, John), and I
thank my lucky stars he is in a chemistry curriculum rather than in
engineering.  Engineering students are MUCH more dangerous. :-) :-) :-)

<>  Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
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