Aluminum corrosion from soaking overnight

cosmos_275 · Apr 2, 2019

I'm setting up a part washing tub. I ran some 7075 parts yesterday and thought I'd let them soak in soapy water overnight.. mistake. I come in this morning and the parts are kinda tie dyed looking. It doesn't wipe off, definately corrosion. Two questions: Any way to clean these parts up? Anyone know why this happened? Tub is plastic, water had some palmolive in it. Parts were sitting in stainless grill baskets. I'm wondering if the stainless had something to do with it. Thanks

Robert R · Apr 2, 2019

Aluminum + stainless steel + electrolyte= battery

ripperj · Apr 2, 2019

I had similar happen with 6061, washed the cutting fluid off with hot water and Dawn soap. Before I could blow it dry it left brown stains, nowhere as dark as your picture. I never even set the parts down.

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crossthread · Apr 2, 2019

AL is very reactive. That's why it is used as rocket fuel. If you take off the AL oxide protective coating, all bets are off.

johnfalco · Apr 2, 2019

I've worked with 7075 Al a lot. You can't let that stuff come into contact with steel if there is the slightest form of moisture coming into contact with the two metals. To remove the oxidation I've mostly used scotchbrite pads. I feel your pain, hopefully you don't have too many parts to cleanup.

cosmos_275 · Apr 2, 2019

johnfalco said:
I've worked with 7075 Al a lot. You can't let that stuff come into contact with steel if there is the slightest form of moisture coming into contact with the two metals. To remove the oxidation I've mostly used scotchbrite pads. I feel your pain, hopefully you don't have too many parts to cleanup.

Thanks for the tip. I had them in the tumbler for a while and that was not doing much. Scotch brite is getting it off. Only 25 to do, thanks! Weird, I guess it was the stainless tray.

Wlodek · Apr 2, 2019

Soap solution in water is alkaline and aluminum is easily attacked by basic solutions. This is true of all alloys. A good way to clean aluminum is a weak chromic acid/phosphoric acid solution.

gnorbury · Apr 2, 2019

I'd switch to a plastic basket asap. Stainless loves to corrode aluminum in damp environments.

cosmos_275 · Apr 2, 2019

gnorbury said:
I'd switch to a plastic basket asap. Stainless loves to corrode aluminum in damp environments.

Yes, if only they made plastic grill baskets. I will shop around and use the stainless ones for plastic parts. Thanks for the help all.

DethloffMfg · Apr 2, 2019

This just happened to me; 7075 parts left overnight in plastic container with Simple Green mixture. Most parts were fine, several had definite odd dark corrosion marks as if a drop of something landed on them.

I used aluminum polishing compound and a microfiber cloth and polished the surfaces. The machining was so smooth already that the corrosion came right off and the bright dip etch and anodize shows the material grain, so it fixed the issue with no visible concerns anywhere.

Finegrain · Apr 2, 2019

CosmosK said:
Yes, if only they made plastic grill baskets. I will shop around and use the stainless ones for plastic parts. Thanks for the help all.

They do ... McMaster-Carr

Delw · Apr 3, 2019

feel bad for ya, had that happen back in the 90s, we ran oil in one of our mills. I need to use it for this one alum job. 4 operations on the job. took about a 1/4 of the parts popped them in a 5gal bucket. used the home depot (zep?) purple degreaser. dumped 1/2 cup in there sprayed hose to agitate it. figured I'd let it soak for a few hours, came back they were all dark grey/ black.

only thing I use now is VF-77T soap/compound in my tumbler and once in a while by hand I use 2oz to 3 gallons water in a bucket. never had a problem on any aluminum parts are clean as can be and no water spots. also use it for stainless

cosmos_275 · Apr 3, 2019

Finegrain said:
They do ... McMaster-Carr

Ummm, maybe when I'm rich I'll drop $140 on a plastic basket

I'm thinking more like this: amazon

cosmos_275 · Apr 3, 2019

Delw said:
feel bad for ya, had that happen back in the 90s, we ran oil in one of our mills. I need to use it for this one alum job. 4 operations on the job. took about a 1/4 of the parts popped them in a 5gal bucket. used the home depot (zep?) purple degreaser. dumped 1/2 cup in there sprayed hose to agitate it. figured I'd let it soak for a few hours, came back they were all dark grey/ black.

only thing I use now is VF-77T soap/compound in my tumbler and once in a while by hand I use 2oz to 3 gallons water in a bucket. never had a problem on any aluminum parts are clean as can be and no water spots. also use it for stainless

I've been running ceramic rocks and dawn/palmolive in the tumbler and I haven't had issues, although, they aren't usually in there for more than a couple hours. I will check out the VF-77T. It seems the steel rack was the culprit. I will test it on some scrap overnight.

Guest · Apr 3, 2019

7075 and water moisture do not mix well. It will even corrode if left in a humid environment.

hanermo · Apr 3, 2019

Really ?
Alu boats last forever in salt water and damp moisture laden air inside.

The alu oxide surface forms very fast in air, seconds, and then deepens in colour over time (months to years).
But the alu oxide has almost zero depth and never corrodes aka rusts, as such, on it´s own.

20 year old sailboats with alu masts and hulls have 100% thickness and integrity.

Mostly, only copper bare metal and alu in seawater corrode together significantly.
SS and steels and brass with alu don´t generally corrode to a significant degree.

You get surface rust and dirt, but no deep pitting, absent electrical current like power cord leaks.
Professional boatbuilders are divided, but generally alu is hard to paint for boat hulls and best not painted.
Unpainted lasts forever.

Dualkit said:
7075 and water moisture do not mix well. It will even corrode if left in a humid environment.

Finegrain · Apr 3, 2019

hanermo said:
Really ?
Alu boats last forever in salt water and damp moisture laden air inside.

The alu oxide surface forms very fast in air, seconds, and then deepens in colour over time (months to years).
But the alu oxide has almost zero depth and never corrodes aka rusts, as such, on it´s own.

20 year old sailboats with alu masts and hulls have 100% thickness and integrity.

Mostly, only copper bare metal and alu in seawater corrode together significantly.
SS and steels and brass with alu don´t generally corrode to a significant degree.

You get surface rust and dirt, but no deep pitting, absent electrical current like power cord leaks.
Professional boatbuilders are divided, but generally alu is hard to paint for boat hulls and best not painted.
Unpainted lasts forever.

Al boats are not made of 7075. 5000-series more likely. Chalk and cheese

Regards.

Mike

jz79 · Apr 3, 2019

edit: my response was to Hanermo, Mike beat me to it

I think you're confusing different alloys, 7xxx is known to have very weak corrosion resistance, even after proper anodizing it is generally weaker than anodizing done on less alloyed aluminum grades, this directly translates to the natural oxide it develops

and since when did boat builders started to use 7075 on their boats? there are other grades which are made to resist salt corrosion, but 7075 is most definitely not one of those

aluminum and stainless steel is also generally known to be galvanically incompatible, it is even advised to use zinc plated hardware with aluminum instead of stainless, in real world application though it generally doesn't matter, but if you look up galvanic compatibility of different metals, you will see that aluminum and stainless on that chart are pretty far apart, the further apart they are, the worse the more active one will try to protect the more noble one

and one other note, they use magnesium anodes on boats to stop aluminum from corroding, or corroding less

MrSteve · Apr 3, 2019

hanermo said:
Really ?
Alu boats last forever in salt water and damp moisture laden air inside.

Surprisingly, yes!
7075, while not as corrosion prone as 2000 series aluminum, e.g. 2011, 2014 & 2024, isn’t particularly corrosion resistant. For best corrosion resistance 1100, 3003, 5052 & 6061 etc. or Alclad 2024 sheet (w/ a thin protective pure alum cladding) would be your friends.

SAG 180 · Apr 3, 2019

I do some etched markings on 2000 series aluminium and rinse them in a bucket of tap water to remove the acid: just in a bucket of water you can see bubbles of hydrogen coming off as the aluminium corrodes in just tap water.

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