Rustproofing Steel

Kyle Boatright wrote:
Our EAA chapter had a fabric covering seminar today, so of course, we got
off-topic very quickly and delved into the issue of keeping 4130 structures
from corroding, even in a fairly short timeframe after surface rust has been
blasted or sanded away but before priming.

Is there any reason not to use a phosphate conversion coating immediately
after cleaning up the steel? Would this protect it for a week (or a month)
before the steel was primed?

KB



Google turned up this:

http://www.paintcenter.org/rj/feb04q.cfm


Q. Is it appropriate to use both a zinc phosphate conversion coating
(with a non-chromate seal) and an inorganic zinc primer to prepare cold
rolled steel sheet for subsequent top coating with a primer and an
aliphatic isocyanate polyurethane top coat? I have read that the zinc
phosphate conversion coatings create an electrically neutral,
non-conducting barrier layer, in addition to a surface with better
adhesion for the subsequent paint. If so, would that not make the
inorganic zinc primer useless, since there would be no conduction path
to the steel substrate?

A. Thanks for your email. Everything you wrote about zinc phosphate is
correct, and yes, it is an excellent method for preparing cold rolled
steel prior to the application of an epoxy primer and a polyurethane
topcoat. You final conclusion is incorrect. Zinc phosphate is not
intended to provide "cathodic protection" to the substrate, such as a
metallic zinc coating does. Galvanizing, zinc plating and zinc-rich
primers provide cathodic protection and they MUST be able to conduct an
electrical current so that they can function. I think you are confusing
zinc phosphate inorganic conversion coatings with zinc-rich primers.
They are very different creatures.

Zinc and iron phosphates are deposited electrolytically to form an
inhibitive layer over the metal. They "passivate" the substrate so that,
in the presence of moisture and oxygen, galvanic corrosion cannot take
place. You can think of zinc and iron phosphates as functioning like an
insulating blanket, preventing iron ions (Fe2+) from dissolving into the
surrounding electrolyte. Zinc-rich primers (organic or inorganic) are
intended to corrode in place of the expensive substrate, in this case,
steel. Hence zinc is called a "sacrificial" anode in the corrosion cell.

Another major advantage of phosphate coatings is that they provide an
excellent adhesion foundation for paints and coatings. Zinc phosphates
are generally considered to provide better protection than iron
phosphate, but the latter is often quite adequate for its intended purpose.

I hope this answers your question.




John