[rec.autos.tech] Rust Prevention

ornitz@kodak.UUCP (barry ornitz) (10/20/88)

In article <7200017@silver> commgrp@silver.bacs.indiana.edu writes:
>In article <1450@kodak.UUCP> ornitz@kodak.UUCP (barry ornitz) writes:
>>Similar questions to these were posted to rec.autos several months 
>>ago.  I do not see how electrolytic protection (cathodic protection) 
>>is applicable to automobiles as they are not generally in a conductive
>>or electrolytic environment.
>Such environments exist in coastal areas and  near steel mills, also 
>in the frozen wasteland north of the Ohio River, where city streets 
>are covered with salt whenever there is snow.  It's rumored that car 
>dealers subsidize the application of the stuff.  :-)
>Warshawsky/J.C. Whitney Auto Parts of Chicago used to sell blocks of 
>magnesium to be bolted to the undersides of cars to act as a 
>sacrificial metal to prevent corrosion of the steel.  I don't know how 
>well they worked.  Testing would be simple; suggest it next time a kid 
>needs a science-fair project.
>Frank Reid

Yes, corrosive environments are common as you describe.  However, to be 
effective both the body metal and the sacrificial anode must be electrically 
connected *as well be immersed in the electrolyte*.  I did not state this
explicitly enough in my original posting.

Corrosion will usually occur near where disimilar metals are in contact and
immersed in an electrolyte (galvanic corrosion).  Damp mud with road salt is
suitable as the electrolyte.  The problem with bolting sacrificial metal to
the undersides of cars is that a low resistivity electrolytic path must exist
between the sacrificial anode and the rest of the car.  If your car were
totally immersed in salt water, this would be a good protective system; other-
wise the high resistivity of the surface dirt would prevent the sacrificial
anode from having much effect.  This is why in moist air the zinc galvanizing
on metals can prevent a scratch through the zinc down to bare metal from 
rusting, but cannot protect a large area several inches away from the zinc. In
a highly conductive electrolyte, the zinc will provide protection until it is

To give some better numbers on this, zinc is rarely effective as a sacrificial 
metal in environments with resistivities higher than about 1000 ohm-cm.  
Magnesium can be used effectively up to 5000 ohm-cm.  Sea water, which is 
quite corrosive, has a resistivity of about 15 to 35 ohm-cm, and clean, dry
sand has a resistivity of more than 1E6 ohm-cm.

The effectiveness of cathodic protection can be increased by providing add-
itional potential between the sacrificial anode and the metal to be protected.
This works fine where underground pipes must be protected. The anode is buried
near the pipe to be protected and is connected to the positive side of a power
supply. The negative side of the supply is connected to the pipe.  The soil
conductivity completes the path.

The absence of a low resistivity, uniform electrolytic path surrounding the
automobile is what makes this protection method unsuitable for cars.  

J.C. Whitney sells lots of things of marginal effectiveness, however.  This
*would* be a good science fair project but waiting ten years or more for the
results to be conclusive might delay graduation a bit.  ;-)

Corrosion is enhanced in areas of mechanical stress, a phenomena well known to
the chemical industry.  And before anyone suggests stainless steel for auto
bodies, chlorides (salt) are very corosive to stainless.  The best thing is to
keep the moisture from getting to the metal with proper paints and undercoats.