304 cres bushing in 6061 aluminum fitting

[Boathouse]

We have a situation designed by the OEM of a 304 cres bushing being press fit into a 6061 fitting. It is a dry fit, no plating or passivation called out. As part of a repair, we intend to seal both ends of the bushing/fitting interface effectively keeping moisture out and (from my limited knowledge) eliminating an important piece of the galvanic battery.

Questions have been asked by other people within our organization as to whether or not this is a prudent way to do business. Do any of you guys out there have views either way on this subject?

Thanks in advance for your input.

 

[SWhit]

Not sure what your environment is, but my experience (I work for the US Navy) has been that it is very difficult to prevent moisture intrusion. Generally I try to avoid installing CRES bushings in aluminum without having cad plating between to act as a sacrificial element.

SWhit

 

[tbuelna]

Boathouse,

First of all, 304 cres seems to be a poor choice for a bushing material. Being an austenitic (non-heat treatable) cres alloy it tends to gall and is of relatively low strength (although it can be work hardened). Usually, the purpose of bushing an aluminum part is to improve the wear resistance and strength of the hole. A 304 cres bushing would not help much in this regard. A better material might be A286 Cres or one of the Nitronic alloys.

As far as galvanic compatibility between 304 cres and 6061 Al, there are several considerations. First of all, the 304 cres bushing should be passivated after machining. That is general practice for all cres materials. Second, the aluminum should have some sort of corrosion protection applied after machining, such as anodize or alodine.

My galvanic chart shows 304 cres and 6061 alum as having a galvanic potential of .40 which is generally not considered compatible. It should be .20 or less.

General aerospace practice for this type of situation is to install the bushing "wet" with something like zinc-chromate primer. Swab the mating surfaces, push the parts together, and wipe off any excess. Once the primer dries, it will seal the joint from any moisture intrusion and prevent corrosion.

Good Luck.
Terry

 

[WKTaylor]

Boathouse...

This is a set-up for major corrosion. Suggest You check-out thread338-110291 & thread338-110764 for serious discussion of CRES bushings in 6061.


Regards, Wil Taylor

 

[Boathouse]

Thanks for the useful input guys.

Cheers

Boathouse