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This confirms that nitric oxide could be formed by electrical arcing. Despite the small concen-
tration, it still exceeds that normally found in air by several orders of magnitude. Nevertheless, even
if such nitric oxide is present, it must react with moisture for nitric acid to form. With humidity
controlled between 30 and 6O%, available evidence suggests that the current waveguide system was
not exposed to excessively moist conditions.

4. RECOMMENDATIONS

Silver is attacked by nitric acid and will be corroded by reducing acids in the presence of oxidizing
agents [2]. Nitric acid is a strong oxidizing agent. It oxidizes all metals except gold, platinum,
rhodium and iridium [3]. In strong acid solutions, the hydrogen is continuously evolving as bubbles
from the corroding metal and this process continues until either all the metal or acid is consumed.
If the waveguide system is operated in conditions in which moist air is not absorbed during
operation and if the system is purged of any nitric oxide after operation, the corrosion can be
eliminated. It would also be best to pass the air through a dryer prior to introducing it into the
waveguide to guarantee moisture levels are minimized.
For improved protection against nitric acid formation, if arcing does occur, it is best to electroplate
with noble metals other than silver, since it is attacked by nitric acid. The noble metals have
extremely high corrosion stability and do not rely on the formation of an oxide coating. Their high
cost and low strength limits their use to thin films and liners on other structural materials [4]. They
are economical for numerous corrosion applications. Platinum is resistant to nitric acid at all
temperatures and concentrations [5].
Electrodeposited platinum is reasonably dense and generally adheres well. Mechanical and physi-
cal properties depend greatly on plating conditions and thin coatings are used for corrosion and
wear resistant electrical contacts [5]. Gold is very good in dilute nitric acid and strong sulfuric acid.
Rhodium electroplates well and is used for critical valve parts and other applications where total
resistance to an aggressive environment is necessary [2]. A 37% rhodium 63% nickel alloy has
better resistance to general corrosion than 14 carat yellow gold [5]. Rhodium finds most of its
applications as an element in platinum to which it imparts added corrosion resistance to many
acids. In general, electrodeposition has been employed for thin rhodium coatings. A rhodium
thickness of 5 x 1OP6-2O x loP6 inch over silver minimizes tarnishing [5]. In this case, these metals
can be used for protection against nitric acid if formed due to arcing.
Quality control during waveguide manufacturing must guarantee that no nitrate ions are on the
waveguide. In bright dipping, a small amount of metal is corroded but the part has a shiny finish
as opposed to a dull oxide coating. Speed of operation and uniformity are the essentials of bright
dipping. The acid acts very quickly and long exposure time will result in more corrosion. After
dipping the parts should be very quickly rinsed in cold water and then hot water and dried [I].
Pure, clean water, e.g. distilled water, is undoubtedly the best for making solutions. It is very
difficult for small amounts of silver nitrate to dissolve in water that has impurities in it. However,
in distilled water the silver nitrate will perfectly dissolve to a clear solution [I]. Water taken from
wells is sometimes found unfit for the best results in plating, if it contains lime or is strongly
mineralized with iron, sulfur or magnesium.
Acknowledgement-The authors are grateful to Ms Rachel Adams of the Mechanical Engineering Department of Loyola
Marymount University for typing the paper.

REFERENCES

1. Hawkins, H. J., The Polishing and Plating of Merals, Lindsay Publication, Bradley, IL, 1987, pp. 98-100.
2. National Association of Corrosion Engineers, (N.A.C.E.) Corrosion Basicx, An Introduction. N.A.C.E. Publication.
Texas, 1984, pp. 590.
3. Waser, J., Trueblood, K. N. and Knobler. C. M., Chem One, McGraw Hill, New York, 1976, pp. 80.
4. Butler, G. and Ison, H. C. K., Corrosion and irs Preuenlion in Waters, Reinhold Publishing Corp., New York, 1966, pp.
101.
5. Metals Handbook Committee. Metals Handbook, Vol. 1. 8th edn, American Society For Metals, Metals Park, OH, 1961,
pp. 805, 1178, I179