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,

 

[WKTaylor]

drodrig…

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]

 

[swall]

I am not familiar with the term "formicary corrosion", but the corrosion in this case appears to be filiform corrosion. Filiform corrosion is common in aluminum that has been painted and the worm track corrosion occurs under the coating. In lieu of a paint coating, I can envision filiform corrosion happening to a aluminum article that has been left clean surface down with an aqueous based residue on the surface. A trip to the machining source would be in order here.

 

[TugboatEng]

This explains the corrosion on only one surface.

 

[drodrig]

No trained people for the penetrant testing.

It was made with a class 2 dye

https://www.karldeutsch.de/products/chemical-products/penetrants/red-and-white-test/?lang=en

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

 

[drodrig]

About the corrosion only on one face and not on the other and not on the machined pockets is very strange.

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... ?

 

[TugboatEng]

The solution is trivial. Don't set the plate on very smooth and flat surfaces for extended periods of time.

 

[WKTaylor]

A few important follow-on questions...

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]

 

[drodrig]

Why not laying it on flat/smooth surfaces??

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

 

[TugboatEng]

Laying it on a flat smooth surface excludes oxygen and prevents the aluminum from forming it's protective oxide layer, crevice corrosion.

 

swall,
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."

 

Laying it on a flat smooth surface excludes oxygen and prevents the aluminum from forming it's protective oxide layer, crevice corrosion.

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."

 

[TugboatEng]

That's exactly right. This part has been milled to be exceedingly flat and was likely camped to the exceedingly flat cast iron carriage of the milling machine all while being drenched in wet coolant. The icing on the cake is that the surface is freshly machined and hasn't even had a chance to passivate itself.

 

I won't argue.

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

 

[TugboatEng]

One last piece to add to the puzzle is that this piece is quite large and complex and likely spent several days clamped on the machine which would give plenty of time for this small amount of corrosion to occur.

 

[3DDave]

How long would any anti-corrosives in the coolant last after being squeezed to a thin film without reacting and being depleted?

 

[drodrig]

Hi again,

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-informationen/TI-KMT-20-94.pdf

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,

 

[TugboatEng]

It wasn't water left on the top plate that causes this issue, it's water between the plate and the table.

Was the side with the corrosion machined first or second?

 

[drodrig]

Hi,

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