Piting corrosion removal on 2024 T3

[zenith601xl]

The top of the fuslage on our DC4 is coverd in surface corrosion [piting to about 5-15 thou depth].We are using Aluprep and green scotch bright to clean the surface and then alodine.I was trained to physically remove any traces by mechanical means first and then to aluprep/alodine.My question is will the aluprep alone neutralize the piting corrosion or other products like prekote? do the job.Later the surface will be painted and it would be nice if it lasted a few years.Cheers Alan

 

[GrandpaDave]

Suggestion: This is a question best asked of the Materials Engineering folks.

Years ago I had worked with M&P folks on military missile systems (Pershing I's, NO... not Pershing IIs) that had major structural components
that had been damaged by exfoliation corrosion after being in service for years. A process was developed to remove the corrosion and rebuild
the damaged surface with a plasma spray. I can’t say any more about that since my memory ain’t too good these days.

 

[rb1957]

we always use alodine to finish off blends like this.

i'd be more concerned about blending 0.015", how much skin is left ? is the DC4 pressurised ? (i don't think so, but it still sounds like you're removing a bunch of the thickness)

 

[Screwman1]

Same worry here. If you have pitting that deep I would be worried about cracks that have already started at the pit sites. For a repair I would think that you need to remove all of the pitting and get to a smooth clean surface and if you have to take off .015 in places it's going to start affecting your stress allowables.

 

[rb1957]

that brings up a good point ... we normally crack check blends (to ensure there is not remaining crack-like feature).

harping/carping on about the 0.015" ... is the plane painted or polished ? (polishng removes material)

 

[zenith601xl]

Ended up cutting and patching the deeper bits just left with pits 1-4 thou deep approx running the full length of the fuselage top.Still would like to know if the aluprep does neutralize the al oxide and what other alloys may be in the material of the 1940'S within these pits.[DC 4 was used as a freighter and is not pressurized and currently is bear metal neither polished nor painted].We are reluctant to remove/grind the whole top to remove all these pits as such prefer using the aluprep/alodine/scotch bright process only and maybe prekote.[ not worried about the stress risers due to the corrosion pits yet and have looked at these with 10x magnifying glass] Cheers

 

[MikeHalloran]

Wasn't there a big kerfluffle some years ago about Scotch-Brite, used for deburring, effectively reducing the fatigue life of machined parts to zero-ish?



Mike Halloran
Pembroke Pines, FL, USA

 

[WKTaylor]

CAUTION...

IF you are getting generalized pitting corrosion on the skins, most likely the original sheet cladding has been eroded/polished-thru and bare 2024-T3 [was -T4 in the 1940s] material is exposed.

Bare 2024-T3 corrodes severely, especially exterior applications, even when protected by good quality inorganic and organic finishes.

OLD 2024 sheet material is somewhat worse than newer material, since the older versions of these alloys [1940s] tended to have slightly greater tramp material constituents; and the rolling, cladding and heat treatment processes were less consistent than today's processes (even when the raw materials/sheet met spec requirements).

NOTE.
I also strongly agree with others: Typically 10% total material thickness removal is about all any body skin can tolerate without consulting stress. I am sure the DC-4 bodies were NOT pressurized.

NOTE.
The old MIL-jet I work on has these very same body-skin issues [2024-T3 and 7075-T6 clad sheet, some with spotwelded doublers and triplers]. We are experiencing generalized exterior pitting corrosion, EXCO on fay surfaces and gradual skin thinning (due to many years of polishing and sanding). Problem is that our jet is pressurized and stress gets very nervous when significant new blending occurs without very careful [US-NDI or caliper] skin thickness measurements.

It might be time to consider re-skinning. I wouldn't be surprised if Your roll-formed/extruded stiffeners/stringer were also corroding in contact areas with the skin. Finishes applied in the 1940s usully lasted a maximum of 25--30-years when well taken-care-of. IF these areas have never been reworked, Isuspect You have LOTS of hidden damage.

NOTE. The following WWII era video regarding B-29s includes some aspects of war-time fabrication methods. Bare aluminum was anodized or alodined and then primed [zinc-chromate]. Sheet aluminum [webs, formed ribs, stiffeners, body skins, etc] were primarily made from clad aluminum and were generally left un-finished. Joints may/may not have recieved a fay-coat of zinc-chromate primer [primarily bilge and high corrosion open bay areas]. Primer and top-coat paint was avoided where possible due to cost and complexity.

http://www.youtube.com/watch?v=Xui_amVmdqY

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[mfgenggear]

http://www.youtube.com/watch?v=2ETEU9of27E&feature=watch_response

I like this version B-17

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[rmw]

I love the political incorrectness of the era in which both of these movies were made. The B-29 feature states that the wing assembly is so light it can be lifted by a woman. The B-17 piece says that one manufacturing machine was so easy to operate that it could be operated by a woman.

Boy, the price that announcer would have paid today.

Ironically, I think we now know that many times those type tasks are more suited to the abilities of women than men.

rmw