Identifying White Precipitate in Saline Solution 7

[KimWonGun]

Over several months I kept a piece of aluminum coated with polyurethane in a sea salt water solution. Upon inspection I noticed a soft, white substance that formed as clusters at the interface between the polyurethane and aluminum. Attached is an image of the piece; I apologize for the poor quality of the picture.

Does anyone know what the white substance most likely is?

 

[zdas04]

What does it tast like? Is it soft and crumbly? It is probably a precipitate of a solution that is locally super-saturated and may even be NaCl. Problem is that there are so many solutes in sea water that it could be any of a hundred things.

David

 

[KimWonGun]

I am hesitant to taste something that may be toxic.

It is indeed soft and crumbly.

 

[Ron]

Agree with zdas04...could be most anything. Not likely toxic...taste might tell you something.

Photo is not good as you noted, but it looks like filiform corrosion is occurring between the coating and the aluminum. Perhaps another photo would help. Filiform corrosion will look like little worms crawling under the surface of the coating.

The crumbly stuff is probably a combination of crystalizing salts from supersaturation as zdas04 noted and oxides/chlorides of aluminum.

 

[CoryPad]

Likely aluminum oxide/hydroxide (corrosion products) formed and may have some chlorides (sodium, calcium, magnesium) trapped in there also.

 

[rb1957]

i hope the suggestion to taste it was (no pun intended) "tongue in cheek" ... i suspect that it is aluminium oxide (maybe hydro-oxide) ... i think there are standard chemical tests to verify

i thought this was relevent ...

http://apps.risd.edu/envirohealth_msds/AluminiumOxide.pdf

maybe this thread ...
thread367-289644

 

[KimWonGun]

Many thanks for the feedback.

Does the attached image provide additional insight?

 

[zdas04]

rb1957,
The suggestion to taste it was not intended as a joke. I have regularly tasted things that look like salt build-up to see if they were salty. I don't sprinkle it on a steak, but I do wet my finger, touch the questionable material and put my finger on my tongue. There are materials in the world that could hurt you in that dose, but I've been lucky I suppose. I've frequently been able to redirect an investigation based on that "test". Just as frequently I've had a nasty taste in my mouth for a few minutes.

KimWonGun,
The second picture was a lot prettier, but I can't tell much from it.

David

 

[KimWonGun]

Unfortunately the depth of the white powder only allows me to focus on a small part of the picture using a microscope. Attached is another image which focuses on the interface rather than just the powder.

Does this offer any further insight?

 

[KimWonGun]

Did the additional image from the earlier post provide any more insights?

 

[Ron]

KimWonGun...sometimes too much magnification doesn't let one see the whole picture. Do you have a photo that is not magnified? Looking at the surface area in context might be more helpful to allow us to help you.


zdas04...same here. Been lucky too, I guess, but chances of toxicity are rather remote...

 

[MintJulep]

KimWonGun,

You've been given some pretty good guesses thus far. But based on some photos and non-specific knowledge of the chemistry involved that's all you'll be able to get.

If you need to KNOW, scrape some off, put it in a little plastic envelope and send it off to a lab from spectroscopic analysis. Then you'll know.

 

[KimWonGun]

I appreciate all the valuable feedback.

Since my earlier images were too blurry to be much help, permit me to offer one last image made with a different camera for any further insights anyone has into identifying the white powder. I am grateful for the suggestions already provided.

Investing in the considerable cost of spectroscopic analysis may be required later; my current objective is just to report the most likely candidates for further internal discussion.

 

[vpl]

I go with CoryPad's explanation -- aluminum oxide with other salts mixed in.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[btrueblood]

I'd agree with CoryPad and Vpl, but...not knowing the alloy of aluminum, there could be more zinc oxide and/or magnesium oxide in the white powder than alumina. We're all pretty much guessing, most metallic salts are white powders in dry form. It's most likely an oxide, because the chloride salts are easily dissolved, and thus less likely to precipitate. But, if you won't taste it, or hold a sample of it in a flame, we can't really help beyond the guesses you have so far...

 

[KimWonGun]

The aluminum is 6061. Would that then suggest more magnesium oxide?

 

[Ron]

Aluminum oxide, zinc oxide, and magnesium oxide are all going to present in the same way. You'll have to have it tested to determine. I would not expect 6061 to have any great quantity of magnesium oxide...mostly aluminum oxide.

 

[btrueblood]

What Ron said. Especially that you will need to have it tested. MgO might preferentially form and then precipitate (over Al), more likely Zn would oxidize first...but it would depend...on lots of things.