🚀 go-pugleaf

In article <3227@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:

>	I must be blessed.  Unfortunately, I am the type of person who does
>not easily say "the hell with it" and give up - so you've still got my
>attention and my time.

I am most honored.

>	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.

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

In general, you are right.  In general, NO corrosion prevention method
is effective.  A preventitive method must be choosen and applied with
care in all situations.  You are saying that cathodic protection
would not work period.  YOU ARE WRONG.  Your examples are all very
good, but you can't prove something can't happen by providing
examples of where it didn't work.  For every negative example, I can
provide a positive example.  So, get off it.

>. . .it may mislead an otherwise
>intelligent but inexperienced soul - in this case, YOU - into believing that
>with a sacrificial anode here and some impressed potential there one can stop
>all of the corrosion in the world in one fell swoop.

NO, NO, NO.  At no time did I simply say, "duh, bolt-up some zinc and
everything would be just ducky."  Look again, Larry.  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.

The fact is, Larry, is that I can stop all of the world's corrosion given
enough zinc and potential units.  However, I never said it would be a good
idea to do this.  What I am saying is that it *is* an alternative to 
consider.

 [A good example deleted.]

>	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.  BTW, I would use magnesium:
it's more electronegative and would give greater throwing power.

>	But do you realize something?  If you clad the entire surface with
>zinc, you no longer have an iron-zinc couple which is exposed to the
>electrolyte.  So, what you really have is NO LONGER CATHODIC PROTECTION,
>afterall.  You will not have cathodic protection in any area other than
>where the zinc surface is broken.

I've already agreed with this.  However, what I'm trying to get across
is that it IS cathodic protection since it is certain you will have
defects in the coating.  If someone were moron enough to use a noble coating,
the car would rust away in just a few years.  The cathodic mechanism is not
just a nice after-thought or side benefit, IT IS ESSENTIAL.

>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.

>	What you fail to realize is that while cathodic and anodic protection
>schemes are indeed effective, they have limited usefulness.

I never disputed this.  What I'm saying is that all protection schemes have
limited usefulness and there is *no* reason to single out cathodic protection.

>	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.  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?

See above.

>	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!

>	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.  As for my gradiose plans, I
simply intend to apply my knowledge in a careful manner to best exploit
the situation at hand.  That's all any engineer can hope to do.  To
accomplish this, I must be willing to consider ALL alternatives
objectively and select the best one based on my knowledge and judgement. 
You, Larry, have failed to do this.  While you've nicely proven why
sacraficial anodes would be a poor idea in numerous situations, you have
gone on to conclude that it is of no use on automobiles.  I'm calling
you on this and have provided an example where it would be useful.  I
will conceed (and have) that it is not the best alternative in that
situation, but it is a good one.  Your blanket statement that cathodic
protection has no place in automobiles is very poor engineering.

Jim Kweeder
kweeder@sun.soe.clarkson.edu