Anodizing 7075 & 7050 Rivet - Need opinion

[CR100]

I figure this is the best place to ask ths question,

We are designing an aluminum fan drive hub (7075) and would like to use a 7050 solid rivet. Our concern is with corrosion as this is an on road applicatin (winter salt). We were going to anodize the hub first and then leave the rivets as is.

Does anyone have an insight on what type of problems we would run into.

Note we were going to use 7050 plate for the drive hub but it was unavailable at the time.

 

[TVP]

Aluminum alloy 7050 in the T73 temper is considered to have high resistance to stress corrosion cracking according to specifications like NASA-STD-P025, so general corrosion should be the primary concern. If the rivet is uncoated during the vehicle phase, there will likely be some small amount of white corrosion products after 3-5 years, but probably nothing substantial, detrimental, etc., as long as there are no iron/steel parts in the joint area, and as long as the assembly is periodically "cleaned" by rain or other water without heavy buildup of mud, salt, etc.

 

[palomc]

If you are going to machine the hub from 7075 T6 you should use some care. I just checked in MMPDS table 3.1.2.3.1(a) for rolled plate and T6 gets a "D" stress-corrosion rating in the ST direction which means practically any sustained stress at all will cause cracking in that direction. If it is 7075 T7651, the problem is much reduced.

In any case, with 7075 be careful how you machine the part, make sure the plate through-thickness direction (ST) is the direction that carries negligible load in the finished part. Also, will the hub mount onto a steel shaft? In my industry steel fasteners are cadmium plated for installation into aluminum and steel-aluminum interfaces are protected. However the difference in galvanic potential between the aluminums and the carbon- and alloy-steels (not the stainless steels) are small enough that you can consider it if the loads and criticality of the part aren't too high.

There is always the possibility that corrosion at the steel-aluminum interface could seize the hub onto the shaft, though. It would be better if you cold seal that interface somehow to keep the electrolytes out.

 

[RPstress]

Different types of Al alloys can corrode one another galvanically, but it's not usually considered a problem within alloy families. 7050 and 7075 are close enough that it shouldn't be a problem. It might pay to wet assemble the riveted joint even so.

Anodising the hub before assembly might imply you're drilling the holes before anodising. Anodising probably won't be that effective down into the hole if so. Wet assembly is the usual treatment. Priming and painting the assembly is also recommended.

I would second palomc's observations about 7075-T6; best avoided. T7651 is probably ok, but if there's any chance of sustained stress T7351 is even better. These tempers give you half a chance of controlling toughness, too. In T6 7075 has an essentially uncontrolled toughness. In fact it's not that good in any temper.

If the alloy metallurgy meets the spec for 7175 it's a lot better. (Quite a lot of 7175 is 7075 that happens to meet the 7175 composition spec, or at least it was 10 years ago.)

Having said that I've seen a 7075-T6 fitting used successfully on an airliner flap vane. It's been flying for over thirty years without any systemic material problem appearing.