ornitz@kodak.UUCP (Barry Ornitz) (06/22/89)
I have been following the debate (?) between Larry and Jim for some days now,
and I believe some additional data is needed. My comments following are based
on my personal experience and on papers presented by B. Husock at the National
Association of Corrosion Engineers Northeast Regional Meeting. Husock was the
VP and chief engineer at Harco Corporation, a company specializing in cathodic
protection systems.
In comparing the efficiency of sacrificial anodes, several factors must be
considered besides the relative activity in the electromotive series. While
quite active, aluminum has never effectively been used as a galvanic anode
material for corrosion protection due to the surface passivation of the
aluminum as the material corrodes. Zinc is a commonly used material; it is
theoretically consumed at a rate of 23.5 pounds/ampere-year. Magnesium is
an even better material (having a higher driving voltage); it is theoretically
consumed at a rate of 8.7 pounds/ampere-year. However, both of these
materials are also consumed in self-corrosion dropping their efficiencies to
26 and 17 pounds/ampere-year for zinc and magnesium respectively. Note that
while magnesium undergoes a greater percentage loss in efficiency, it is still
consumed at a lower rate than zinc for a given current. Of course with a
higher cell voltage, it is easier to get higher currents.
OK, this sounds good - the sacrificial anode will corrode, thereby galvanically
protecting the desired metal. Just bolt on some magnesium blocks as Jim says.
But wait a minute, what about the "throwing power" as Larry calls it (must be
a throwback to electroplating experience)?
What is going on here is the fact that these sacrificial anodes are only
effective in low resistivity environments. Zinc needs an environmental
resistivity of less than 1000 ohm-centimeters to be effective; magnesium is
rarely effective in environments with resistivities over 5000 ohm-centimeters.
If your car were immersed in sea water (15 to 35 ohm-cm), a number of zinc or
magnesium blocks could be used to protect it. You need enough to give 5 to 10
milliamps/square-foot of exposed car.
Now what happens if we are not immersing the car, but have an occasionally
damp surface? Well, it all depends on what the resistivity is. The 1 to 5K
ohm-cm values quoted above are really quite low and are hard to achieve with a
thin layer of slightly damp road dust. So what does this mean? Simply that
the bolt-on anodes may protect the sheet metal for a small distance around them
and no further. How small is small? Read on...
What about active systems, i.e. those with external voltages applied? The same
thing applies - you need the low resistivity environment. In fact, with some
powered systems, corrosion rates can be increased to more than those without
protection if large resistivity gradients exist. This was noted by another
person whose friend had unsuccessfully tried active cathodic protection with
his automobile. A particularly insidious problem occurs where a section of
material to be protected is electrically isolated from its surrounding
protected material (i.e. a chrome trim applied with adhesive, or a painted
body panel bolted on to another painted part like a door panel bolted to a
painted hinge). In this case, if any of the cathodic protection current is
picked up by this isolated part and passed on through the electrolyte back to
other exposed surfaces, corrosion will be enhanced on the isolated part.
So....if galvanic protection does work but it needs a low resistivity
environment, what can you do to protect your auto?
Simple. Cover all exposed surfaces with the sacrificial anode, i.e. hot-dip
galvanize the car. This is Jim's simple answer. But it is really not so
simple. Galvanizing stills needs the low resistivity environment if it is to
protect large scratches, etc. What if you get a scratch through the zinc
perhaps 1/4 inch wide. At the edges of the scratch, the zinc will corrode
sacrificially protecting the underlying steel. But what about the center of
the scratch. Without the low resistivity environment, the center of the scratch
will still rust.
Also consider that the zinc coating is thin and that it is dissolved in place
of the steel. The thin scratch will not rust at first, but as the zinc is
consumed, the scratch widens until it reaches a point where the zinc is no
longer useful. This is what Larry was talking about when he mentioned
throwing power. How far away can the galvanizing protect? Not very far!
Galvanizing has its own problems, of course. Hot-dip galvanizing is not very
practical for complete automobiles. Zinc plating produces such thin coatings
that they would hardly offer any benefit for corrosion protection in autos.
Also have you ever tried to paint galvanized rain gutters and gotten the paint
to stick without peeling? The zinc surface must be passivated for the paint
to adhere properly; this is not always such an easy job.
What about the "cold galvanizing" paints? Actually these fall into two kinds:
inorganic and organic. Only the organic zinc-rich paints are commonly
available to the general public. Rustoleum offers one and so do many other
companies. These paints are basically fine zinc powder in an organic binder
such as alkyd or epoxy. The resistivity problem is still present here. The zinc
particles must make good electrical contact with the base metal and each other
to be effective. Sadly, there isn't enough zinc in most of these paints to be
very good. I have used one of the better organic zinc paints made by Devcon.
A one pint can weighed four pounds and didn't cover very much but it still
cost plenty at the time. Once dry, the painted surface was electrically
conductive; because of this it worked quite well in my application (radio
tower repairs). Forget the spray can "cold galvanizing" paints. There isn't
nearly as much zinc in these as in the brush-on varieties. Still, these
organic-binder, zinc-rich paints are better than ordinary paint for corrosion
protection when properly applied.
The better zinc-rich paint is the so-called inorganic zinc. Once again zinc
powder is used, but the binder chemistry is based on ethyl silicate. Applied
over a bare steel surface in the presence of moisture, the binder forms iron
and zinc silicates to hold the matrix together. These paints offer probably
the next best thing to hot-dip galvanizing in corrosion protection. Special
solvents are needed with these paints along with the right protective clothing
for the painter (although considering the isocyanates and aromatics in normal
auto lacquers I would feel safer with the inorganic zinc). These paints are
very expensive and have limited shelf and pot lives. They also require that
the base surface be exceptionally clean and roughened. Sand blasting with
coarse sand is recommended - not exactly the best foundation for a mirror auto
finish. Porter is one company among many who make these paints.
To summarize since this has been rather long...
Bolt-on sacrificial anodes are a waste on money in automotive applications
because of the lack of a very low resistivity environment. Likewise, active
cathodic protection schemes fail for the same reason on automobiles. On boats,
the situation is quite different.
Galvanizing (hot-dip) is good for things that can be galvanized; unfortunately
automobiles usually do not fall into this category. Throwing power is still
a problem unless the environment is low in resistivity.
Cold galvanizing paints are a help but not a panacea. Inorganic zinc paints
are better than the more common organic binder zinc paints.
Other suggestions....
Have the auto undercoated as soon as possible. Make sure drain holes in doors,
air vents, etc. are open and do not allow water to collect. Repair road dings
and parking lot scratches promptly. Remove or passivate (using phosphoric
acid based treatments) any rust before repainting. Rinse road salt off as soon
as possible. Keep the car clean and waxed. Chlorides attack stainless steels
so even the DeLorean owners need to do these things too. ;-(
Additional references....
If anyone is still interested in cathodic protection, both active and passive,
Harco Corporation used to offer reprints of several of the papers mentioned
above for a few dollars each. My copies are rather old, so it might be worth
contacting Harco for more up to date information; my information says their
home office is in Medina, OH. I am sure all USENET readers know how to go
to their local library and use the Thomas Register to get the full address and
telephone number. ;-) I would suggest "Fundamentals of Cathodic Protection",
"Cathodic Protection, One Way to Prevent Underground Corrosion", and
"Corrosion, Cathodic Protection, and Common Sense" - all by B. Husock.
As an official disclaimer, mention of any companies in the above posting should
not be taken as an endorsement by Kodak. I have personally used Devcon-Z
paint in the past and have been satisfied. Your mileage may vary! The Harco
papers were referenced in an AIChE course on corrosion and materials of
construction. [Yes Jim, _I_ am an engineer too; but many of my friends are
chemists. ;-) ]
Barry Ornitz
ornitz@kodak.com
...rutgers!rochester!kodak!ornitz