Corrosion Control

Lee H. Erb

Originally published September 1997

(Whether youíre building an SR-71 or a homebuilt with a titanium gear spring, this will be important...)

DONíT MIX TITANIUM AND CADMIUM

A cadmium plated bolt installed in a titanium part will eventually result in titanium "embrittlement." Just recently I found out the mechanism for titanium embrittlement: The cadmium, under pressure and/or heat, will flow (infuse) between the grains of titanium. This weakens the grain boundaries and when the titanium is stressed, a crack will initiate.

Use passivated stainless steel bolts in the AN and NAS series when bolting a titanium part. Examples are AN4C10 and NAS6304U-10. AN4C10 is a 1/4-inch diameter bolt, 1-5/64 (nominally 1 inch) long, 7/16-inch grip, made of a corrosion resistant steel in the 90 ksi tensile range. NAS6304U-10 is a 1/4-inch diameter bolt, ??? long, 10/16 grip, made of A286 corrosion resistant steel and unplated.

Note: The NAS63xx series is A286 cres (cres--corrosion resistant steel, i.e. stainless steel) in the 160 ksi tensile range. The NAS62xx series is alloy steel and the NAS64xx series is 6AL-4V titanium in 160 ksi tensile range at normal temperatures. At 450 deg F it is good for 95 ksi. (Bolt selection is another long subject.)

Even the preparers of the NAS specifications had to learn about titanium embrittlement. When the spec was initially prepared in 1968 there was a cadmium plated titanium bolt defined. The galvanic table indicates that it would act sacrifically as on a steel bolt. It was declared "Inactive for New Design" after December 1991.

Thanks again to Steve Mitchell and Ron Yarborough for sharing their knowledge.


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Revised -- 8 April 1998