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.