Many people say that
corrosion ‘is alive’ and you
have to kill it to keep metal from continuing to rust. The significance
is that
once rust is formed, there is mobile oxygen in the metal, and the oxygen
can move deeper into the metal causing further rust. Techniques to kill
rust all rely on dissolving or converting rust into black oxide; then
installing a coating that inhibits the formation of more rust.
When iron or steel starts to rust, it will puff up and expose clean
metal to the open air, allowing rust to continue to the depths of the
metal. If your piping system is already rusting and you want to clean
it or at least stop the rusting, you have few alternatives. Until now.
There are a few products
on the market which fall into the category of organic rust converters.
These products contain acid to convert rust
to black oxide and polymers which bond to rust. Instructions say to remove
all loose rust, paint on the product, and let it dry. We've tested these
products and it did convert the surface to black oxide, with further
testing though, the surface began to rust again with the introduction
of an electrolyte, the resulting surface is black and rough. Additionally,
the converted rust isn’t durable, any damage or flaking exposes
new metal to oxygen and electrolytes, so this is not a recommended treatment
for piping systems and vessels.
Another remediation method is to remove loose rust and coat the metal
with a product like WD-40. This fills pores with a non-corrosive substance
and prevents additional rusting as long as you keep the coating film
in place. This is sufficient to slow the progress of rust but is not
a permanent cure.
The most crude is
sand blasting or bead blasting. Sand blasting will remove some good
metal and will work-harden the surface, so glass bead
blasting is used for more often. Immediately after any blasting, metal
is clean and exposed, so it is essential that some form of rust proofing
gets applied as soon as possible.
Sand paper and steel wool will also remove rust, but they don't get
into tiny crevices.
Another technique for removing rust is etching with Phosphoric Acid.
Phosphoric Acid has a unique property of dissolving iron oxide quickly
while etching iron very slowly. The problem with this method is that
the acid will attack bare metal and will start the process of hydrogen
embrittlement. Another unique advantage of Phosphoric Acid is that
it leaves a fine coating of iron phosphate behind. Iron phosphate
prevents
rust. However, the iron phosphate coating is not very thick and not
durable; additional protection is still required.
Polyguard
has the best product on the market for corrosion remediation (under
insulation normally) that is not inexpensive to purchase, however
it is inexpensive in its application.
A corroded system
must first be UT tested to determine if there is enough ‘good
metal” wall thickness to withstand the operating pressures of
the system.
Once this is determined, remediation is simple; you only have to
remove any loose scale with a power brush or hand brush. It is
not necessary
to take the surface down to bare metal.
RG-2400
is then applied to the metal surface (20-30 mils thick), the system
is then re-insulated, and a good vapor barrier applied.
If mechanical
protection or USDA approved jacketing is required, it is also installed
over the vapor barrier. With the advent of Zero Perm vapor barriers
like Polyguard’s Fabwrap 20, Alumaguard, or VaporGuard, there
is little chance that an electrolyte will enter the system. (see
our web page on
water buffering).
The monies saved by not having to glass bead blast the metal surface
down to new metal easily surpasses the cost of the surface conversion
gel.
Though RG-2400 is usually applied to below ambient systems, it
can usually be applied while the system is in operation – AS LONG AS THERE
IS NO CONDENSATION FORMING ON THE METAL SURFACE.
If you want to analyze further savings, consider that the installation
time for RG-2400 is approximately 10% of that of blasting,
priming, and painting! How much savings is realized by your
plant being
back in production
in a tenth the time?
When evaluating the cost of RG-2400 protection on your system,
compare it to ALL the costs involved in other forms of
remediation, including
plant shut down time.
