CORROSION NOTES

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Rx/RL Division, USS Dwight D. Eisenhower (CVN-69) FPO New York,
New York 09532

I read with great interest the article ‘Engine
Maintenance’ as I am chemist for the U.S. Navy aboard the USS
Eisenhower. Growing up in Iowa I learned about steam engines and
that is how I became interested in the nuclear power field. In the
reactor and steam plants it is very important that corrosion is
kept to a minimum replacing components or fixing machines that are
clogged with corrosion products can become a nightmare.

I’ll start with the basics. The types of corrosions that
you’ll encounter in a steam engine are galvanic, general,
oxygen and chloride pitting. Galvanic corrosion occurs when you
have two or more different metals in contact with each other in an
electrolytic solution in other words the metals will act like a
battery, and the metals will be slowly eaten away where they meet.
An example of this is a bolt screwed into the boiler, and the
boiler water having a lot of dirt and other contaminates. To
prevent this corrosion, simply keep your boiler clean. Wash the
boiler regularly, thoroughly, and with clean water. While steaming
use a strainer on the injector or water pump, and remember to use
clean water to feed the boiler.

Next is general corrosion. This is more commonly known as
‘rust’. What happens is the oxygen in the air (or water)
will combine with the iron to make nonmagnetic hematite (alp
Fe2O3or red rust) at low temperatures or in
high oxygen solutions. As this is heated to higher temperatures the
alp Fe2O3 will change to
Fe3O4 or alp Fe2O3this
is known as ‘black rust’ predominately found on the inside
of the boiler. This ‘black rust’ is different from ‘red
rust’ in that it adheres to the metal surface while ‘red
rust’ flakes off. In other words, it forms a protective oxide
layer. Here are some tips to help you gain this protective oxide
layer.

a.  In the spring when you fill your boiler for the initial
time, add either hot water (it will have a lower oxygen content) or
add an oxygen scavenging chemical (hydrazine or sodium
sulfiteN2H4 or
Na2So3).

b. After a day or so, steam up your steam engine to drive
out the oxygen in the fill water, and be sure to open the relief
valve or whistle to remove the oxygen from the steam bubble in the
boiler.

c. Keep the engine steamed up for about a week. At the
temperatures that the steam engines run, the formation of
‘black rust’ will take longer.

d. Once this oxide layer is formed, it is good
indefinitely. This procedure should be used immediately after major
work on the boiler i.e., replacing tubes or crown sheets to
increase the new pieces’ lifetime.

Pitting corrosions occur when a drop of water rests on a piece
of metal for a long time. The drop will set up a small area where
there is a difference of oxygen concentration. This difference sets
up an electrical potential (much like galvanic corrosion) and the
metal is slowly oxidized and deposited at the edge of the water
droplet, forming a ‘pit’ in the metal at the center of the
droplet. Chloride pitting should be a problem encountered by
engines with a possible seawater contamination. The one factor
which will determine the amount of corrosion more than anything
else is the water pH as the amount of hydroxyl ions are increased,
the corrosion rate decreases, then increases, so great care must be
exercised when attempting to control pH. My advice is to control pH
when steaming, and control oxygen when shutdown and cold iron. The
11-11 months that a steamer sits idle allows a lot of time for
oxygen to damage the boiler. During the winter when a boiler is
drained and open to the air, general corrosion will have a field
day. To forestall this a person needs to simply lower the oxygen
content. Probably the best method is to drain and dry the boiler of
water, seal the hand holes and place a very low pressure (1#-2#) of
nitrogen on the inside of the boiler and place small bags of
desicants on the inside of the boiler to trap any more water which
may come from frost or condensation.

I hope this information will help some of the engineers out
there to preserve history for our following generations. If anyone
has questions or suggestions, please write to me.

Farm Collector Magazine
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