[sci.electronics] parasitic anodes for rust prevention ???

wolfgang@mgm.mit.edu (Wolfgang Rupprecht) (06/06/89)

Has anyone tried playing with parasitic anodes for rust prevention of
automobiles?  I have seen several likely gimmics that you "bolt-on"
and forget.  

I was wondering if there is any simple way to make a system that
*really* works for protecting some of the parts that tend to collect
water, such as on the bottom insides of the doors.  I was going to try
a long of cloth-wrapped zinc wire attached to +12v (through a several
k-ohm resistor).  Anyone have any other pointers or insight?

-wolfgang
Wolfgang Rupprecht	ARPA:  wolfgang@mgm.mit.edu (IP 18.82.0.114)
TEL: (703) 768-2640	UUCP:  mit-eddie!mgm.mit.edu!wolfgang

jhs@druco.ATT.COM (J H Shore) (06/09/89)

in article <11854@bloom-beacon.MIT.EDU>, wolfgang@mgm.mit.edu (Wolfgang Rupprecht) says:
> Has anyone tried playing with parasitic anodes for rust prevention of
> automobiles?  I have seen several likely gimmics that you "bolt-on"
> and forget.  
> 
> I was wondering if there is any simple way to make a system that
> *really* works for protecting some of the parts that tend to collect
> water, such as on the bottom insides of the doors.  I was going to try
> a long of cloth-wrapped zinc wire attached to +12v (through a several
> k-ohm resistor).  Anyone have any other pointers or insight?

I would suggest making your parasitic anodes passive, so to speak, 
rather than  using +12vdc.  I suspect that some unanticipated 
results will occur, not the least of which would be relatively
rapid reduction of your zinc wire. Power experts--comments?

Naval ships--and probably merchant marine ones as well--use large 
zinc bars bolted in strategic places both within and external to the
hull to minimize electrolytic action associated with prolonged 
exposure to sea water.  None use an active potential (that I know of).

Again, even if you only use zinc wire, you'll likely have to replace
them fairly frequently. And I suspect you'd have to try out various 
types and locations of the anodes to know how effective it would be
anyway.  Good luck--keep us posted.

larry@kitty.UUCP (Larry Lippman) (06/11/89)

In article <4345@druco.ATT.COM>, jhs@druco.ATT.COM (J H Shore) writes:
> > Has anyone tried playing with parasitic anodes for rust prevention of
> > automobiles?  I have seen several likely gimmics that you "bolt-on"
> > and forget.  

	Corrosion mechanisms and their control is a very complex issue.
Without getting into any specific detail or calculations, my intuitive
opinion is that in an automobile: (1) no significant amount of corrosion
occurs as a result of potential difference between exposed metal areas;
(2) the surface area beneath a vehicle is significantly "small" as compared
to those situations where corrosion control methods are usually implemented;
(3) the physical contours beneath a vehicle are so complex that any passive
or active anode arrangement will be ineffective.
 
> > I was wondering if there is any simple way to make a system that
> > *really* works for protecting some of the parts that tend to collect
> > water, such as on the bottom insides of the doors.  I was going to try
> > a long of cloth-wrapped zinc wire attached to +12v (through a several
> > k-ohm resistor).  Anyone have any other pointers or insight?

	Forget it, and don't waste your time.

> Naval ships--and probably merchant marine ones as well--use large 
> zinc bars bolted in strategic places both within and external to the
> hull to minimize electrolytic action associated with prolonged 
> exposure to sea water.  None use an active potential (that I know of).

	There are shipboard corrosion control systems which apply an active
potential to a corrosion control anode.  An example of such a system that
I have seen is called "Capac", is manufactured by Electrocatalytic Ltd.
of the UK, and is sold on a world-wide basis.  I have seen Capac rectifiers
in sizes between 25 and 600 amperes.

	Also, shipboard corrosion and its control is a totally different
environment from that of an automobile (after all, the ship is continuously
and uniformly immersed in the electrolyte!), and no parallels should be
drawn between shipboard corrosion and the discussion about automobiles.

<>  Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
<>  UUCP   {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
<>  TEL  716/688-1231 | 716/773-1700  {hplabs|utzoo|uunet}!/      \uniquex!larry
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kweeder@sunny3.che.clarkson.edu (Jim Kweeder) (06/12/89)

In article <3220@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:

I need to take issue with some of your intuitive opinions:

>(1) no significant amount of corrosion
>occurs as a result of potential difference between exposed metal areas;

Potential differences are mandatory for corrosion.  What makes corrosion
engineering fun is all the neat and unusual ways for the potential difference
to occur.

>(2) the surface area beneath a vehicle is significantly "small" as compared
>to those situations where corrosion control methods are usually implemented;

I'm not sure what you mean here.  While area has a large impact on corrosion,
I don't see what you're talking about.

>(3) the physical contours beneath a vehicle are so complex that any passive
>or active anode arrangement will be ineffective.

Huh?  As a matter of fact, cathodic protection is often used on the underside:
it's called galvanized steel.

>> > I was wondering if there is any simple way to make a system that
>> > *really* works for protecting some of the parts that tend to collect
>> > water, such as on the bottom insides of the doors.
>
>	Forget it, and don't waste your time.

No, don't forget it.  If you have a situation where water will consistently
collect, then sacraficial electrodes could be use successfully.  Simply
bolt a piece of zinc or magnesium where it will get wet.  You need to
make sure that you're getting good electrical contact between the steel
and the electrode.  For areas that don't become wet on a reliable basis,
then this idea will only bring mixed results.

>	Also, shipboard corrosion and its control is a totally different
>environment from that of an automobile (after all, the ship is continuously
>and uniformly immersed in the electrolyte!), and no parallels should be
>drawn between shipboard corrosion and the discussion about automobiles.

Hogwash.  Automotive and marine corrosion are parallel.  True, having
the hull immersied in water makes implementation of cathodic protection
(sacrafical anodes or impressed potentials) very straight forward.
However, cathodic protection is very much a viable alternative in
automobiles (eg: zinc coated steel).  The automobile does present some
problems for cathodic schemes, however, if you understand how things
work, then you should be able to apply cathodic protection to certain
automotive corrosion problems.

Jim Kweeder
kweeder@sun.soe.clarkson.edu

larry@kitty.UUCP (Larry Lippman) (06/12/89)

In article <3164@sunny3.che.clarkson.edu>, kweeder@sunny3.che.clarkson.edu (Jim Kweeder) writes:
> I need to take issue with some of your intuitive opinions:

	Be my guest...

> >(1) no significant amount of corrosion
> >occurs as a result of potential difference between exposed metal areas;
> 
> Potential differences are mandatory for corrosion.  What makes corrosion
> engineering fun is all the neat and unusual ways for the potential difference
> to occur.

	First of all, you neatly omitted the phrase "in an automobile" which
preceded and qualified the applicability of my statement to the conditions
found in an automobile.

	Since the topic of this discussion appears to be corrosion mechanisms
which may be mitigated through use of sacrificial anodes and/or anodic
protection (i.e., the use of an external current), it is clear that the
discussion applies to conditions involving galvanic corrosion where a
"corrosion cell" exists.  In order for this type of corrosion to occur,
ALL elements of an electrochemical cell MUST be present:

1.	An anode.

2.	A cathode.

3.	An internal circuit formed by an electrolyte between the anode and
	cathode.

4.	An external circuit, which is a metallic connection between the
	anode and cathode.

	If ANY one of the above elements is missing, then a corrosion cell
does not exist, and galvanic corrosion does not occur.

	A good example of galvanic corrosion is the union of two significantly
dissimilar metals, such as copper and zinc, which would be found where a
copper pipe fitting is connected to galvanized pipe.  In this example,
the anode which corrodes is the zinc (galvanized pipe), the cathode which
does not corrode is the copper, the internal circuit is formed by the
water (i.e., the electrolyte) inside the pipe, and the external circuit
is the junction of the two metals where they are threaded together.

	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.  Since this article will be long enough as it is,
I am not going to digress into crevice or intragranular corrosion mechnisms
which also exist, but which are not readily amenable to anodic or cathodic
protection in this application, anyhow.

	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.

> >(2) the surface area beneath a vehicle is significantly "small" as compared
> >to those situations where corrosion control methods are usually implemented;
> 
> I'm not sure what you mean here.  While area has a large impact on corrosion,
> I don't see what you're talking about.

	I mean two things:

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.

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.

> >(3) the physical contours beneath a vehicle are so complex that any passive
> >or active anode arrangement will be ineffective.
> 
> Huh?  As a matter of fact, cathodic protection is often used on the underside:
> it's called galvanized steel.

	The primary protection mechanisms resulting from galvanizing steel
are: (1) isolation of the steel through cladding with zinc; and (2) the
passivation of the zinc surface through its own carbonate and hydroxide
corrosion products.  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.

> >> > I was wondering if there is any simple way to make a system that
> >> > *really* works for protecting some of the parts that tend to collect
> >> > water, such as on the bottom insides of the doors.
> >
> >	Forget it, and don't waste your time.
> 
> No, don't forget it.  If you have a situation where water will consistently
> collect, then sacraficial electrodes could be use successfully.  Simply
> bolt a piece of zinc or magnesium where it will get wet.  You need to
> make sure that you're getting good electrical contact between the steel
> and the electrode.

	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?

> >	Also, shipboard corrosion and its control is a totally different
> >environment from that of an automobile (after all, the ship is continuously
> >and uniformly immersed in the electrolyte!), and no parallels should be
> >drawn between shipboard corrosion and the discussion about automobiles.
> 
> Hogwash.  Automotive and marine corrosion are parallel.  True, having
> the hull immersied in water makes implementation of cathodic protection
> (sacrafical anodes or impressed potentials) very straight forward.

	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.

	In case you haven't guessed yet, the item is a bronze propeller!

> However, cathodic protection is very much a viable alternative in
> automobiles (eg: zinc coated steel).  The automobile does present some
> problems for cathodic schemes, however, if you understand how things
> work, then you should be able to apply cathodic protection to certain
> automotive corrosion problems.

	Since you have taken such vehement issue with my previous article,
you clearly must claim to "understand how things work".  Therefore, please
help me to overcome my obvious ignorance by explaining:

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)?

2.	What are a few "certain automotive corrosion problems", and how
	can cathodic protection be applied?  Please do not talk about the
	trivial case of coating steel with zinc; I believe that most
	Net readers already know about the use of galvanized steel or
	zinc-rich paints in automobile construction.

<>  Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
<>  UUCP   {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
<>  TEL  716/688-1231 | 716/773-1700  {hplabs|utzoo|uunet}!/      \uniquex!larry
<>  FAX  716/741-9635 | 716/773-2488     "Have you hugged your cat today?" 

kweeder@sunny3.che.clarkson.edu (Jim Kweeder) (06/12/89)

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

chang@svax.cs.cornell.edu (Richard Chang) (06/12/89)

In article <4345@druco.ATT.COM> jhs@druco.ATT.COM (J H Shore) writes:
>in article <11854@bloom-beacon.MIT.EDU>, wolfgang@mgm.mit.edu (Wolfgang Rupprecht) says:
>> Has anyone tried playing with parasitic anodes for rust prevention of
>> automobiles?  I have seen several likely gimmics that you "bolt-on"
>> and forget.  
>> 
>
>I would suggest making your parasitic anodes passive, so to speak, 
>rather than  using +12vdc.  
>
>Naval ships--and probably merchant marine ones as well--use large 
>zinc bars bolted in strategic places both within and external to the
>hull to minimize electrolytic action associated with prolonged 
>exposure to sea water.  None use an active potential (that I know of).
>

  My Bentley manual for my '86 VW Golf describes some zinc strips
that are bolted to the front fenders (where the fenders are mounted
to the sides of the engine bay).  These are supposedly used to
prevent rust.  So, at least one manufacturer has tried this and
found it worthwhile to install in production models.  In any case,
isn't galvanized steel just zinc coated steel?  I would suspect that
if he got this active parasitic anode to work, he would end up oxidizing
the zinc coating any galvanized steel he has in his car.

larry@kitty.UUCP (Larry Lippman) (06/13/89)

	I am going to try one more time to get a few points across, and then
I'll probably give up because I've got too much work to do to continue this
argument.

In article <3166@sunny3.che.clarkson.edu>, kweeder@sunny3.che.clarkson.edu (Jim Kweeder) writes:
> >	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. . . .
> 
> In your original article you said potential difference, not dissimilar
> metals. 

	You're right; I said potential difference originally, which apparently
caused you to misinterpret my statement.  Since differential concentration
cells (oxygen, hydrogen or other ionic concentrations) involving the same
metal, in addition to other less common effects do not contribute to corrosion
in the automobile environment, the only effect even worthy of mention is
that of dissimilar metal junctions.

> 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.

	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!

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

	You're right; it is the "other factors" here which preclude the
effective use of cathodic protection in the automobile environment.

> >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.

	I bet I can do it on 10 ml. :-)

	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.

> You greatly underestimate the need for mixing.  I can
> easily see a difference in electrolyte concentration on a car.

	So tell me about it.

> >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.

	Did I say that zinc coating would NOT result in cathodic protection?
What I did say is that there are LIMITS to the area of protection to exposed
steel that results from zinc coating.  Galvanizing does wonders for the
cathodic protection of small scores and other damage which you mention;
however, in this situation the surface area of the zinc coating is orders
of magnitude greater than the exposed steel areas to be protected.  As a
somewhat trivial example, one square inch piece of zinc is going to afford
damn little cathodic protection to bare metal say, one foot away.

> >	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.

	I'd really like to see those numbers.

> >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.

	I don't just "think dissimilar metals", thank you.  If you are
going to claim a concentration cell or other mechanism, however, then tell
me about it!

> >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.

	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.

	And now you want to bolt magnesium blocks to the spare tire wells
inside a trunk!  Tell me about how you propose to fasten the magnesium
block to the body metal, the effect of drilling holes in the auto body,
and about how you intend to deal with the problems attendant in maintaining
that junction.  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 Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
<>  UUCP   {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
<>  TEL  716/688-1231 | 716/773-1700  {hplabs|utzoo|uunet}!/      \uniquex!larry
<>  FAX  716/741-9635 | 716/773-2488     "Have you hugged your cat today?" 

seth@poopsie.UUCP (Seth D. Hollub) (06/13/89)

In article <3223@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:
>
>	I am going to try one more time to get a few points across, and then
>I'll probably give up because I've got too much work to do to continue this
>argument.
>
>In article <3166@sunny3.che.clarkson.edu>, kweeder@sunny3.che.clarkson.edu (Jim Kweeder) writes:
>> 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.

You can also throw some salt over your shoulder. (At least it wouldn't be
near the metal).

Click and Clack's (the guys on NPR) looked into supposed add-on cathodic
protectors and the result? About as close to a hoax as one can get in the
American marketplace. (Which is pretty darn close). Maybe Volkswagen has
cathodic protection but it didn't help my VW's rusted hatch. Perhaps Mr.
Kweeder could report on the results of his experiments.

In short, Mr. Kweeder, you may be able to argue theory, but the practical
application suffers.

Of course, keeping the automobile submerged in a 100ml beaker will allow
for that desired current flow. ;-)

"What use is a parade if not for some rain?"



-- 
     "Segments: Just Say No!", "Whadya mean there's no control key?"
seth@vax.ftp.com, ...ftp!poopsie!seth, 18 Rindge Av, Camb. Ma, 02140 USA Earth

kweeder@sunny2.che.clarkson.edu (Jim Kweeder) (06/14/89)

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

wtm@neoucom.UUCP (Bill Mayhew) (06/15/89)

When I was in college, I knew a guy who was a chemical engineering
student that got the bright idea of trying to rig up an active
anode on his Ford Fairmont.  He would up eating the hell out out of
several parts of the unibody where it joined the front subframe
within a couple months of time.

I'd strongly recommend debugging one's technique on a car that one
can afford to mess up before trying it out on the ole Porsche 944.

I live in northeast Ohio which uses more winter road salt than
most other parts of the US.  I've found that the best method of
rust prevention is to frequnently wash the car.  I have a 1983 AMC
Alliance (no laughing please) that's got about 73K miles on it.  It
has received no rust prevention other than what it got from the
manufacturer; to date, it has virtually no corrosion problems.

What I'd like to know is if those cars that have stainless steel
exhaust systems never need to have the pipes replaced.  So far,
I've nver been able to get a tailpipe/muffer to last more than
about 4 years.

Spend a buck a week and go to one of those do-it-yourself car
washes with the power wand and wash out the underside of your car.

Salt is a four-letter word,
Bill

email to:  wtm@impulse.UUCP

larry@kitty.UUCP (Larry Lippman) (06/15/89)

In article <3167@sunny2.che.clarkson.edu>, kweeder@sunny2.che.clarkson.edu (Jim Kweeder) writes:
> Ok, Larry.  You want flames, you got them.

	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.

> >> >	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
> >mechanisms 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.

	Have I denied or disputed that automobiles rust?

	I have been trying to tell you two things:

1.	The corrosion mechanisms which are NOT, in general, responsible
	for rust in automobiles.

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

	So, I'm going to try to get through to you using another approach.

	Most textbooks define ALL corrosion of metal as being electrochemical
in nature.  While strictly speaking this is true, 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.

	Wrong.

	Lets say that I take a 4 by 8 foot sheet of unplated, uncoated steel
and just set it on a couple of tires which are laying flat upon the ground.
For all intents and purposes, there is no electrically conductive path between
this metal plate and the earth.

	The above analogy is not unlike that of an automobile.

	Now, lets throw some water containing sodium chloride on top of the
plate, and look at the chemistry (the sodium and chlorine ions do not
participate here):

	2Fe + 2H2O + O2 --> Fe+2 + 4OH- --> 2Fe(OH)2

	What we have here is ferrous hydroxide, which precipitates, but
which upon exposure to oxygen and water is unstable and undergoes the
following reaction to form ferric hydroxde:

	2Fe(OH)2 + H2O + 1/2O2 --> 2 Fe(OH)3

	Ferric hydroxide, when it dries, forms what is commonly known as
rust.

	Now, the $64 Question is: from a PRACTICABLE standpoint, how can we
stop this rust?

	Well, we can do a number of things, including but not limited to:

1.	Painting the steel, other otherwise coating it with a substance
	which is impervious to water.

2.	Electroplating the steel with chromium, nickel, zinc, etc.

3.	Hot dipping the steel with zinc (galvanizing), tin, lead, etc.

4.	Cladding the steel with thin metal which is pressed in place using
	rollers.

5.	Using more exotic deposition processus such as flame spraying,
	vacuum deposition, sputtering, etc.

6.	Using a chemical conversion process, such as treatment with
	phosphoric acid (called phospatizing), which is sometimes used as
	a precusor for automotive body metal finishing.

	From a PRACTICABLE standpoint, that's about it.

	Now, I just know what YOU are getting ready to shout: "Sacrificial
anode to the rescue!"  "Impressed potential to the rescue!"  Okay, well let's
consider these alternatives.

	In order to apply cathodic protection that provides a reasonable
degree of corrosion retardation, it will be necessary to have a current
density of at least 5 mA per square foot.  So, let's take a zinc bar that
is say, .1 in thick, 1 inch wide and 8 feet long and run it down the
middle of this steel plate, and fasten it with a low-impedance connection
to the plate.

	Now, we have just reduced the steel surface area of 4,608 in^2 by
96 in^2 occupied by the sacrificial anode.  Okay, so how far away from
this one inch strip do you think we will have a current density of at
least 5 mA/in^2?  2 feet away (to the edge of the plate)?  1 foot away?
6 inches away?

	How about less than *** 2 *** inches away?  Now, I admit that for
the benefit of this article (you're killing my spare time doing even this
much) I have not done the calculations that involve the half-cell potential,
cell plate surface area, electrolyte conductivity, steel sheet resistivity,
and current/potential gradients, but from past experience I can tell you
that the answer is well under 2 inches.

	So, in order to have any reasonable chance at achieving cathodic
protection using sacrificial anodes, you are going to have to install a
zinc strip no farther than every 2 inches down the length of this sheet.
That's going to require at least SIXTEEN strips of zinc for this sheet,
with the strips themselves occupying ONE-THIRD of the surface area of
this steel sheet.

	Okay, so the above scheme of cathodic protection may work, but
be honest now, do you think it is PRACTICABLE?  Hell, you just may as
well clad the entire surface with zinc and be done with it!

	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.

	Do you understand, yet?

	Okay, now let's cover the issue of impressed current from a power
supply for cathodic protection.  Well, we have a similar problem with the
sacrificial anode example above: how to get enough current density.  In
the case of an impressed current cathodic protection system, the anode
will have to be _insulated_ from the cathode (i.e., the steel sheet), which
will significantly complicate the installation.  While we can now increase
the anode spacing and reduce the surface area, we are not under these
conditions of a plate with a THIN electrolyte film going to be able to
decrease anode surface area beyond a factor or 2 or 3.  This still is going
to require a significant amount of cathodic protection anode surface.
And now we need a power supply.

	Do you think the above is practicable for an automobile?

	Okay, now we have one more dragon to slay: anodic protection to
passivate the surface of the steel using an impressed current.  This is
much more difficult to achieve since not only will we will still require
auxiliary electrodes (in this scheme called cathodes), but we will require
VERY precise potential control using one or more reference electrodes
with electronic potentiostatic control.  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.

	Do you really think the above is practicable for an automobile?

	What you fail to realize is that while cathodic and anodic protection
schemes are indeed effective, they have limited usefulness.  These methods
are excellent for buried pipelines, buried tanks, above ground tanks, and
shipboard use - but they are generally difficult if not impracticable to
implement for other applications.  The successful applications all have one
thing in common which is NOT found in an automobile: the object to be
protected is ALWAYS in contact with an electrolyte which covers most if
not all of the surface to be protected!

> 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?

	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.

> 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. 

	So what does the above have to do with the price of tea in China?

> You really should proof read, Larry.  Galvanizing is cathodic
> protection.  So, that's example one.
> Further,
> galvanizing works only because it is cathodic protection.

	I told you once, I told you twice, and I'm telling you again
that the primary effect of galvanizing is NOT cathodic protection, but
it is physical isolation through cladding.

> 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),

	Finally something I agree with!

> >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).

	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!  Neither of these two events
are going to occur when bolting a magnesium block to a tire well in a
trunk, but they DO occur in your example of water heaters and ships.
Furthermore, another reason why I mentioned the oxide coating is that it
makes it difficult to achieve a low resistance electrical connection to
the metal to be protected.

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

	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.

<>  Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
<>  UUCP   {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
<>  TEL  716/688-1231 | 716/773-1700  {hplabs|utzoo|uunet}!/      \uniquex!larry
<>  FAX  716/741-9635 | 716/773-2488     "Have you hugged your cat today?" 

kweeder@sunny3.che.clarkson.edu (Jim Kweeder) (06/15/89)

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

larry@kitty.UUCP (Larry Lippman) (06/17/89)

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.
<>  UUCP   {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
<>  TEL  716/688-1231 | 716/773-1700  {hplabs|utzoo|uunet}!/      \uniquex!larry
<>  FAX  716/741-9635 | 716/773-2488     "Have you hugged your cat today?" 

kweeder@sunny1.che.clarkson.edu (Jim Kweeder) (06/19/89)

I'm glad to see we've got this down to a dull roar.

In article <3235@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:

>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.

Larry, this is the whole heart of corrosion engineering.  Since it is
virtually impossible to prevent steel in coming in contact with water
and air, corrosion engineering is to prevent the formation of the
suitable conditions for this reaction to go.  ALL of the various
mechanism you listed below are stil general chemical attack.  The
only difference is that a particular reason has been fingered for
supplying the activation energy (galvanic couples, pits, stress, etc.).
It may not be easy to identify the electrodes, but one must if there is
any hope in controlling corrosion.

Thus, corrosion is corrosion.  Anything is a variation on the same theme.

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

Yes, this is a significant corrosion mechanism is *some* cars.  Did you
every see a body panel corrode along the character lines and folds
(good example:  GM Suburban)?

>6.	Pitting Corrosion - A highly localized concentration cell phenomenon
>	which causes - what else? - pits in the metal.

This is also significant mechanism in body panels where either defects or
corrosion is left during manufacture.  Since I've worked in the body
industry, I've seen this.

>	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. 

Well, we not making progress, just getting things clearer.

The bottom line is that other methods are better, not just practical.  There
are situations where cathodic protection is practical (and I supplied one).
The reason it's not the best choice is that (1) paint is cheap and easy
to apply, (2) since we can tolerate some corrosion, we can accept paint
which supplies most imperfect protection, (3) the area we are considering
is easy to get to so application and maintainence is no problem.

However, (3) points to situations where paint would be dangerous.  Since
a flaw in paint worsens corrosion, we shouldn't use paint where there
isn't a back-up protection mechanism or where we can't easily find and
correct the flaw.  At this point cathodic protection comes into play
(and is used extensively as galvanized coatings).  I still see no reason
to completely reject the use of sacrafical anodes.

>	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.

Well, drilling holes is not a problem unless it's an outer class A body
panel (your car already has thousands of holes in it).  Next, a steel
nut and bolt should work very nicely.  The keep the connection clean, I
would use a conductive grease between the parts.

These problems are not insurmountable.  I agree that in some situations,
these concerns would probably make cathodic protection the sub-optimal
choice, but they are by no means barriers.

>	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.

Every hear of acid rain? :-)

>	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).

Well, crevise corrson is not a small part of the problem.  Crevise corrosion
occurs in the joints in the automotive unibody.  If these joints rust apart,
the car is junk.  Further, corrosion begets more corrosion; thus, once a 
good start is made in the crevice, this will spread the corrosion into the
homogeneous areas.

The point I've been making is that it is *very* easy to set-up concentration
gradients.  Once the corrosion is started, it will continue very easily in
conditions that would be unable to start corrosion on vigin steel.  Since
corrosion engineering is preventing your reaction from getting started,
this is a valid concern.

>Engineering students are MUCH more dangerous. :-) :-) :-)

Well fortunately: (1) I *am* an engineer, (2) you supplied smileys :).

Enough of this,

Jim Kweeder
kweeder@sun.soe.clarkson.edu