Ti/Ti and CRES/CRES fretting corrosion 3

[SkyPunk]

Please correct me if I am on the wrong track. Ti and CRES develop their own oxidized surface somewhat comparable to chromate conversion of Aluminum. Correct? When you put CRES with CRES without paint, or sealant protection there is a tendancy to get repetitive in this process of self protection thus becoming detrimental and therefore corrosion pitting occurs.

For the above corrosion to occur is there a need for reciprocating loads to be applied such as in CRES slip fit inside CRES press fit bushings?

 

[kenvlach]

"Ti and CRES develop their own oxidized surface somewhat comparable to chromate conversion of Aluminum. Correct?"
-- No, aluminum also develops a native oxide surface. Chromate conversion coating gives enhanced corrosion resistance in many applications, especially for heat treatable alloys such as 2024 & 7075 which have intermetallic precipitates which a) interfere with formation of a continuous alumina surface, and b) cause micro galvanic corrosion. A thin chromate coating (Type III per MIL-DTL-5541F) can be used to prevent the formation of a high electrical resistance over time due to thickening of the native oxide or in corrosive applications.

When you put CRES with CRES, frequently the thin surface oxide films are broken and you get cold welding aka galling. If this happens in a reciprocating situation, severe fretting results.

Please give details of the application. Is corrosion involved, or simply mechanics?

 

[sreid]

Frettion Corrosion.

If there are micro movements between two pieces of (usually alike) metals, small particles will be abraided. These combine quickly with oxygen to form oxides which are harder and bigger than the original metallic particles. These abrade even more metalic particles which also oxidize, etc., etc. Since this is very localized, a pit is formed in the metal. Since the fretting occurs at a high stress point, the crack (stress riser) usually leads to sudden complete failure of the joint.

This can sometime happen in an almost unbelievably short time.

 

[SkyPunk]

Thanks for your assistance, and corrections. More to the question. Would this be a significant concern in flight controls involving CRES slip fits inside of CRES press fits w/o protection i.e. dry.

 

[tbuelna]

"Would this be a significant concern in flight controls involving CRES slip fits inside of CRES press fits w/o protection i.e. dry"

I would not recommend a cres/cres interface. It will likely gall. If you must have a cres on cres interface, as a minimum you should apply a DFL barrier, such as moly disulfide. It will also help if the two mating materials differ in hardness by at least Rc 15. Some cres alloys (ie. Nitronic alloys) are less gall prone than others.

 

[RPstress]

Nitronic 60 from High Performance Alloys (

http://www.hpalloy.com)

is claimed to be very good at resisting galling and wear.

Nitronic 30 from AK Steel is claimed to be good for wet abrasion resistance.

We have found in the past that very severe galling can occur with small movements in a very short time in highly loaded joints; this was Custom 455 steel in 15-5PH bushes. (A terrible combination which we really shouldn't have implemented, even in the limited duration of experimental service that we had to cater for.) There was some noticeable corrosion present, though it not serious (this was in an inland atmospheric environment). We did find that Ti 6-4 bushes behaved well with hard precipitation hardening stainless steel pins.

We used to be able to use Cu-Be bushes, which behaved very well and had a high compressive yield. However, this has now become disliked because of the beryllium content. No other Cu alloys seem to have a high enough compressive yield.

In the past we've used precipitation hardening CRES with austenitic CRES in a lightly loaded environment with significant motion and had no problems.

 

[SkyPunk]

Any experience out there with 17-4ph inside a 17-4ph with no protection, no other plating, sea air environment?

 

[RPstress]

I would not risk it, even if the joint was very lightly loaded and the two 17-4 parts were different hardnesses.

If you do have to do it, arrange for it to have a very close eye kept on it.

 

[BrianR]

RPstress, I would have thought there was no problem with Cu-Be as long as the Be content was no more than 2% where it will remain in a stable alloy even if abraded.