Corrosion? in aluminium plate 1

[drodrig]

Hi there,

We have recived a machined plate from a manufacturer.

It has some funny stains on one surface. Here some pictures

It is aluminium 7570, 45mm thick

These stains are only on one face.

It seems they have roughly milled the top and bottom faces (I guess to get to the 45mm) and then nicely made the pockets, which have no stains.

Is this rust?

One can't feel it with the nails

cheers,

 

[EdStainless]

Did you specify any surface treatment? If not then surface stains like this are par for the course. they might have taken a very light cut off of the rough mill surface.
Do you care about the mechanical properties? Did you restrict what product they could cut this out of? Did it come from rolled plate? Do you know the rolling direction?

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P.E. Metallurgy, consulting work welcomed

 

From a distance this looks like it might be formicary or “ants’ nest” corrosion.
Can you supply a close up photo of one or two of the spots?

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[drodrig]

No surface treatment specified

I care about the mechanical properties. This is a mounting plate holding some load.

I just asked for aluminium 7570. This surface is not important for the use of the plate. But I don't know if it could get worse.

The plate was 50mm. Rolled.

We wanted 45mm so I guess they took 2.5mm on each side

Here a couple of pictures. I can't get them with better quality

This ant nest corrosion looks serious

Apparently happens more often in copper. Here some picture which looks similar:

https://www.researchgate.net/public...orrosion_of_copper_tubing_in_air-conditioning

Do you think this is what we got?

thanks
regards,

 

[saplanti]

I am not an expert on the issue. I guess that these inclusions might be from the manufacture, especially if it was casted inadequately. Sometimes manufacturer melts previously used dirty materials with the same or similar chemical properties and cast it without additional processing to eliminate dirt. Just source how it was manufactured.

In case these are inclusions from manufacture and the plate goes under bending, axial, torsional stresses you may have problems with it.

In case they are stains only strength wise you would not have surprise problems unless it corrodes quickly.

 

[EdStainless]

My strength comment pertains to the strength being different in different directions. This is common in Al but the magnitude surprises some people.
What HT condition is this?
Have you tried to dye pen to see if it is into the surface or is it only discoloration?
My hunch is that it is from the cast/hot work steps and they took 4mm off of one side and just kissed the other. Maybe they needed to because of the shape of the original plate.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[drodrig]

I'll ask if they can get information from the supplier, maybe they had a bad batch

I thought aluminium is "very" isotropic. How different a rolled plate can be?

The material is T6 / T651 (F51)

Do you mean dye penetration inspection? I never did that. If I spray the penentrant I'll see it going into the stains if it is corrosion?

 

[EdStainless]

Whether it is corrosion or surface damage that wasn't fully removed it will hold dye. If it is just staining then it shouldn't hold dye. You might take a flapwheel to a spot and see how deep it is.
One advantage of this HT condition is less variation. The MMPDS indicated that L vs T have about 3-5% different strength, 10-15% different fracture toughness, snd 40% difference in fatigue properties. Don't look at the ST properties, those are usually much worse, and the reason that you are not allowed to cut 'bar' from plate.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[drodrig]

ok
I'll do a penetration test

I've run simulations supposing the material is isotropic, ST properties are important

https://amesweb.info/Materials/Aluminum-7075-Rods-Bars-Shapes-Properties.aspx

Here there is is no information about the ST direction

 

[EdStainless]

You can find an old copy of the MMPDS online free (6th ed I think). It contains great details.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/raw/upload/v1618577695/tips/7050_omu7ao.docx[/url]

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P.E. Metallurgy, consulting work welcomed

 

[drodrig]

Thanks for the info, I see the differences in direction don't occur in the linear (elastic) area

More news about the plate:
I found a flapwheel for the Dremel and worked a bit the surface

Here you can see the result. It is not deep.

I think the bad casting process can be ruled out, but not the corrosion

cheers

 

[saplanti]

This web site has similar image. it could be pitting corrosion or crevice corrosion.

https://fractory.com/aluminium-corrosion/

 

I guess that these inclusions might be from

Don't jump to conclusions about inclusions. The photo evidence could be just an illusion or a delusion, causing confusion.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[TugboatEng]

I agree that crevice corrosion is a possibility. That explains why it's on one surface. If the plate were left flat on a smooth surface the whole face of the plate could become one giant crevice. The moisture source would be the water in the coolant used during machining.

 

[WKTaylor]

DR...

Your question has driven me crazy... aluminum alloy 7570 does NOT exist [as far as I can tell].

Is this plate aluminum made from either of these genuine alloys...???

7050 alloy

7075 alloy [I suspect that this is what you are actually referring-to]

It is absolutely necessary to know the 'as-rolled thickness' of the raw-stock and the '-Txxxx temper of the material as-delivered/machined' and the 'material specification' [which it was made per].

From the sparse details you have provided, I am reminded of three very important quotes...

"You deserve to get what you ask for: but if You don’t ask for very much, don’t expect to get very much.” -- Ron xxxxx, xxxxxxx Engineer [summarizing his experiences with critical aircraft fastener manufacturing].

Alternate versions, by other engineers, on Eng-Tips…

"You don't get what you want... You get what you ask for. Ask carefully!” –TheTick, Eng-Tips

"You may not get what you asked for... but [for sure!] you never get more than you ask for!” –EdStainless, Eng-Tips

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[drodrig]

I apologize for driving you crazy with the question.

It is aluminium 7075 T6 / T651 (F51), I mixed the number order up.

45mm thick machined which came from a 50mm rolled plate

 

I don't normally shout, but

From a distance this looks like it might be formicary or “ants’ nest” corrosion.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[WKTaylor]

drodrig…

The part as is see it has a HIGH value in the fully machined state... IE: is complex with high strength requirements.

Hate to tell you... 7075-T6xxx plate is notorious for pitting corrosion, exfoliation corrosion [EXCO] and stress corrosion cracking [SCC].

In military aircraft applications we are prohibited from using parts made from 7075-T6 wrought materials thicker than 0.080-inch [2.03-mm] per MIL-STD-1568, thus...

MIL-STD-1568D
4.6.1.2 Aluminum alloy selection limitations. Wrought product forms of aluminum alloys
2020, 7079, and 7178 in all temper conditions and 7075-T6 shall not be used for structural
applications. Use of 2000-series aluminum alloys in the -T3 and -T4 tempers and 7000-series
aluminum in the -T6 tempers in thicknesses greater than 0.080 inch (2.032 mm) shall not be
used. Suitably clad aluminum alloys or inherently corrosion-resistant alloys shall be used in
exterior skin that is 0.125 inch (3.175 mm) or less in thickness, forms a leading-edge, exhaust
trail area of any source or wheel well area, is spot- or seam-welded, or is the face sheet in
bonded sandwich construction. Non-clad materials may be used for the aileron skins, the flap
shroud skins and the flap shroud closure pocket. To preclude partial aging in heat-treatable
alloys, the bonded sheet shall be in the artificially aged condition prior to bonding. The
references to exterior surfaces and skin herein mean the external surface only and do not
preclude use of material clad only on one side or the removal of cladding from internal
surfaces.

NOW... good news... 7075-T7351 temper plate... which is 'simply' over-age heat-treated -T651 plate [additional temperatures/time]... will maintain high strength and provide a significant improvement in resistance to pitting/EXCO and SCC... (and also) if You add-in **shot peening of all surfaces [to crush/close-up grain-ends]... (and also) thin-film/sealed anodize [MIL-A-8625] (or) chromate conversion coating [CCC] MIL-DTL-5541 Class 1A (and also) corrosion protective epoxy primer [MIL-DTL-23377]… and You will have a huge increase in resistance to all forms corrosion. ** I prefer ceramic bead peening media for aluminum.

Your first step is to make sure Your rolled plate is procured in -T7351 temper... or make sure all parts machined in the -T651 temper plate are quickly over-age heat treated to -T7351 [AHT per AMS2770, MIL-H-6088, etc]. Then apply good corrosion protective finishes.

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[drodrig]

Hi,

Thanks for the detailed answer.

I'll ask about the -T7351

Then I'll check how to do the shot peening and anodize it

I wasn't aware of the problems with thick alu 7075 plates

thanks

 

[drodrig]

I also did the penetrant inspection

There are no traces of the dye in the stains

But the developer doesn't cover the stains as the rest of the material

Here a picture: