Corrosion? in aluminium plate
Corrosion? in aluminium plate
(OP)
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,
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,
RE: Corrosion? in aluminium plate
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?
= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
RE: Corrosion? in aluminium plate
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."
RE: Corrosion? in aluminium plate
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/publication/39376723_...
Do you think this is what we got?
thanks
regards,
RE: Corrosion? in aluminium plate
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.
RE: Corrosion? in aluminium plate
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
RE: Corrosion? in aluminium plate
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?
RE: Corrosion? in aluminium plate
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
RE: Corrosion? in aluminium plate
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-...
Here there is is no information about the ST direction
RE: Corrosion? in aluminium plate
https://res.cloudinary.com/engineering-com/raw/upload/v1618577695/tips/7050_omu7ao.docx
= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
RE: Corrosion? in aluminium plate
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
RE: Corrosion? in aluminium plate
https://fractory.com/aluminium-corrosion/
RE: Corrosion? in aluminium plate
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."
RE: Corrosion? in aluminium plate
RE: Corrosion? in aluminium plate
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]
RE: Corrosion? in aluminium plate
It is aluminium 7075 T6 / T651 (F51), I mixed the number order up.
45mm thick machined which came from a 50mm rolled plate
RE: Corrosion? in aluminium plate
"Everyone is entitled to their own opinions, but they are not entitled to their own facts."
RE: Corrosion? in aluminium plate
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]
RE: Corrosion? in aluminium plate
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
RE: Corrosion? in aluminium plate
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:
RE: Corrosion? in aluminium plate
I have to CAUTION You... Non-destructive inspection [NDI] of aluminum alloy parts is something You need to be trained for. Have You [or whoever did the job] been trained for Penetrant NDI?
It looks to me like You used the lower sensitivity 'red dye' method [sensitivity level 1 or 2] and may not have etched-cleaned the surface before hand.
IF this were an aircraft part, I would normally specify fluorescent dye-penetrant NDI, sensitivity level 3... which does require a surface etch with a low-PH or high-PH controlled [acidic or caustic] etchant.
Make sure You comply with a NDI standard such as: ASTM E1417 or AMS2647 or MIL-STD-1907 or MIL-STD-6866 [obsolete but useable]... and use penetrant of sensitivity level 3, or equivalent. And then you have to determine what 'accept/reject criteria' to use... 'after blending defect indications to min/max allowed dimensions'.
For USAF we specify aircraft-specific T.O.s with Generic NDI process T.O.s... and accept/reject criteria.
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]
RE: Corrosion? in aluminium plate
RE: Corrosion? in aluminium plate
RE: Corrosion? in aluminium plate
It was made with a class 2 dye
https://www.karldeutsch.de/products/chemical-produ...
KD-CHECK RDP-1
But the surfaces were thoroughly cleaned before
It was a quick test to try to get additional information for this problem
RE: Corrosion? in aluminium plate
Does it mean that the same stains will come to the non corroded surfaces?
If we mill the corroded surface (lets say 0.5mm or even 1mm) and anodize the plate, would that work?
Or maybe it is a too simplistic solution... ?
RE: Corrosion? in aluminium plate
RE: Corrosion? in aluminium plate
Is the 'opposite-side' of the part also machined ['flat']... or was it left 'as-rolled'?
Again...
Per the material certifications ['material certs'] paperwork... that should have been supplied with the raw plate...
What material specification was this plate procured per? Example... AMS-QQ-A-250/12,
What company made it and 'where' was it made.
Also... was the plate machined with coolant/lubricant [fluid] or with cooling airflow or in still air?
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]
RE: Corrosion? in aluminium plate
The opposite side was machined in the same way than the "stained" side. It looks the same. How much they removed from the original block we don't know yet.
Also these other questions we will clarify
cheers
RE: Corrosion? in aluminium plate
RE: Corrosion? in aluminium plate
I think I stand corrected, and have confused formicary corrosion with filiform.
"Everyone is entitled to their own opinions, but they are not entitled to their own facts."
RE: Corrosion? in aluminium plate
Your two surfaces would need to be exceedingly flat to exclude air. Impossible basically.
"Everyone is entitled to their own opinions, but they are not entitled to their own facts."
RE: Corrosion? in aluminium plate
RE: Corrosion? in aluminium plate
"Everyone is entitled to their own opinions, but they are not entitled to their own facts."
RE: Corrosion? in aluminium plate
RE: Corrosion? in aluminium plate
RE: Corrosion? in aluminium plate
I found out some more information
The plate is aluminium 7075 -T651
It was 50mm thick and 2.5mm were removed on each side with a 63mm Garant milling head
The coolant used was liquid. This one (I only found information in German)KMT 20-94
https://www.wieds.de/de/downloads/technische-infor...
The manufacturer claims that there was no lubricant/water left on the top plate for long time. Just during machining
About the material specifications I don't know. I'll investigate further
cheers,
RE: Corrosion? in aluminium plate
Was the side with the corrosion machined first or second?
RE: Corrosion? in aluminium plate
The company told me that the side with the corrosion was machined second.
Is it possible that the coolant got between the milling machine base and our plate?
I am suspicious of them leaving the coolant on the top (corroded face) because they've been waiting for one dimension we gave later