What part is this?

[afward]

My IA is doing an annual on a '67 M20F (not mine) and found what appears to be a steel doubler on the spar, outboard two(?) ribs from the fuel tank on the right wing.  It's very rusted and needs replacement.  He couldn't find the right part in the IPC so he sent me pictures of the one in the other wing.  Does anyone know what this is?

Inboard side:

Outboard side (sorry for the poor quality):

Thanks!

[A64Pilot]

I do not know this part.

‘However if I were the IA wanting to remove and replace it, I would remove it and just duplicate it as it appears to be a flat plate? Or am I missing something?

This would not be an owner produced part, but simply a repair as simple parts are fabricated all the time in repairs.

Edited July 3, 2021 by A64Pilot

[cliffy]

I don't think a little surface rust is necessarily a cause for replacement 

How deep does the rust go? Can it be brushed clean and an estimate of depth be taken?

Check the regs

Maybe just cleaning and rechromate might be all that is needed. 

[afward]

On the one pictured above I'm pretty sure it's just surface level.  On the other, it's deep.  I'll see if I can get a picture of it.

What blows my mind is it's obviously a steel part attached (with rivets!) to the aluminum spar.  Did the factory really do that?  And if so, why don't we see that problem more often?

[A64Pilot]

Steel is joined to aluminum all the time. Thrush Ag planes have had 4130, then 4340 Steel spar caps huck bolted to the aluminum main spar since the 60’s. Aluminum spar caps are not strong enough or if made to where they are, they are too big.

Often steel used in such applications is Cadmium plated for corrosion resistance, as Cadmium is a heavy metal and environmental concerns are greater now it’s not as widely used as it once was.

On the Ag planes once they have been put in the field for awhile and the steel pieces rust some (The entire fuselage is 4130 tubes)  Osphoric acid (hardware store rust convertor) is used with great success treating rust, it chemically changes iron oxide into Iron phosphate I believe and is very effective at preventing future rust, rust is like cancer, unless all of it is removed it spreads.

‘I more than likely have the name of the acid used incorrect.

I’m not aware of any downsides to rust convertor, nor the FAA’s position if any with it.

[Guest]

2 hours ago, afward said:

On the one pictured above I'm pretty sure it's just surface level.  On the other, it's deep.  I'll see if I can get a picture of it.

What blows my mind is it's obviously a steel part attached (with rivets!) to the aluminum spar.  Did the factory really do that?  And if so, why don't we see that problem more often?

Yours likely have the same combination on your lower spar caps.

Clarence

[afward]

Ok, gotcha. I'll pass all that on to my IA. Thanks everyone!