Xref: utzoo sci.electronics:6569 rec.autos.tech:8568
Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!csd4.milw.wisc.edu!leah!rpi!sunny3.che.clarkson.edu!kweeder
From: kweeder@sunny3.che.clarkson.edu (Jim Kweeder)
Newsgroups: sci.electronics,rec.autos.tech
Subject: Re: parasitic anodes for rust prevention ???
Summary: Misinformation Busters :-)
Keywords: corrosion, causes, misconceptions
Message-ID: <3164@sunny3.che.clarkson.edu>
Date: 11 Jun 89 17:53:38 GMT
References: <11854@bloom-beacon.MIT.EDU> <4345@druco.ATT.COM> <3220@kitty.UUCP>
Reply-To: kweeder@sun.soe.clarkson.edu (Jim Kweeder)
Distribution: usa
Organization: Clarkson University, Potsdam, NY
Lines: 52

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