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Aluminum Alloys Most Susceptible to Corrosion

07/16/2010 1:12 PM

Hi Everyone,

We use "breatheable" polyurethane films in our products and we test them for Water Vapor Transmission Rate (WVTR). On the right side of the picture, is a cup we have been using for years. They are aluminum alloy and, as you can see, there is no evidence of corrosion.

On the left is one that our shop made. This particular cup was used once. I have never seen Al do this before.

The test procedure is to place ~18g distilled water in each cup, place a sample of the film completely covering the opening and securing the "lid". These are placed in a controlled oven environment of 98ºF and 50% RH for three days. WVTR values (g/m2/day) are determined gravimetrically through successive weighings.

Does anyone have an idea what type of aluminum alloy corrodes in distilled water? (Nobody here seems to know where they got the material to make these.) I measured the volume and weight of the empty cup yielding a density of 2.63 g/mL.

Thanks!

Mike

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Pathfinder Tags: aluminum alloy Corrosion MVTR WVTR
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#1

Re: Aluminum Alloys Most Susceptible to Corrosion

07/16/2010 2:25 PM

Mike,

I'd be more thinking about what might be in the film that chemically attacks the aluminum... and what is pH?

Had the cup on the right been anodized (clear)/ andthe one on the left not?

Milo

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#2
In reply to #1

Re: Aluminum Alloys Most Susceptible to Corrosion

07/16/2010 2:48 PM

Hi Milo,

Thanks for the reply.

I know the ones we made were not anodized. The one on the right may have been. They were made for our CEO by another company whose CEO was returning a favor. I have been unable to track it down. I have not measured the pH, but distilled water has a pH from 5.0 to 7.0.

The same film was used in both types of cups. The film is 0.001" thick with ~0.0008" Pressure-Sensitive Adhesive (PSA) developed and tested for skin contact, so I doubt it is from the film/PSA.

How could I tell if the one on the right has been anodized?

Mike

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#3
In reply to #2

Re: Aluminum Alloys Most Susceptible to Corrosion

07/16/2010 3:07 PM

All of your 5000 and 6000 series aluminum alloys have high corrosion resistance. You can check whether it is anodized by touching the surface with the probes of a multimeter set to read resistance (conductance). If it is anodized, you won't get any reading as the anodize is electrically non-conductive.

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#4
In reply to #3

Re: Aluminum Alloys Most Susceptible to Corrosion

07/16/2010 3:52 PM

Love the multimeter answer RONSETO!

Milo

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#5
In reply to #3

Re: Aluminum Alloys Most Susceptible to Corrosion

07/16/2010 3:54 PM

Hi Ron,

I was all over the net today and did notice your point about the 5000 and 6000 series.

I checked both types of the cups for resistivity and, according to your criteria, the old-style cups are indeed anodized.

We only have 12 cups. I want to make some more so we can test more material at the same time. It appears that after machining, having them anodized would be the way to go, even though we may get no corrosion using a 5000 or 6000 alloy alone.

Also, I imagine that heat from machining could affect the corr. resistant properties as well.

In my web travels today I found that some series machine better than others. This is my design for some new cups:

Height: 1.25"

Body Dia: 2.625"

Rim Dia: 3.25"

Bore Dia: 2.25"

Bore Depth: 1.06"

Do you have any advice on what alloys would be best from a machineability perspective?

Mike

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#6
In reply to #5

Re: Aluminum Alloys Most Susceptible to Corrosion

07/16/2010 4:10 PM

mikerho, 60XX series is rated C on ABC scale for machinability. 2011 T3,T451, T8 is rated A; 2017 and 20124 various temmpers rated B 6061 T6 andT651 rated C

6262 and 7075 various tempers also rated B. I have no 50XX info for machining as bars

None of these grades are rated "a" for corrosion resistance.
Send me your fax by private email and i'll send you analcoa screw machine grade table.

Milo

Glad the anodizing hypothesis worked out.

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#10
In reply to #6

Re: Aluminum Alloys Most Susceptible to Corrosion

07/16/2010 8:04 PM

Hi Milo,

It doesn't show it in the picture, but the original cups had screw-on lids with fairly fine threads for that diameter. The flange outside diameter is ~3" and the threads are probably 12 - 18 per inch (I am home now, can't measure).

Do you think it would take a material of A machineability to achieve this? Although the new cups don't have screw-on lids, I would like to keep the alloys as close to each other as possible.

What I'm thinking is to machine them out of 2011 bar stock (http://www.mcmaster.com/#aluminum/=7zpnw3), followed by anodizing.

What do you think?

Mike

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#11
In reply to #10

Re: Aluminum Alloys Most Susceptible to Corrosion

07/17/2010 1:22 AM

Alloy affects the insulating properties of anodized coatings. Pure aluminum gives excellent resistivity. Random parts from the scrap bin may not. Ask me how I know!

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#13
In reply to #10

Re: Aluminum Alloys Most Susceptible to Corrosion

07/19/2010 9:39 AM

Threading 2011 should be no problem.

Pay close attention to the anodize!

Milo

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#14
In reply to #13

Re: Aluminum Alloys Most Susceptible to Corrosion

07/19/2010 11:03 PM

Thanks Milo!

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#7
In reply to #5

Re: Aluminum Alloys Most Susceptible to Corrosion

07/16/2010 5:16 PM

Anodizing after machining will increase the corrosion resistance.

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#9
In reply to #7

Re: Aluminum Alloys Most Susceptible to Corrosion

07/16/2010 7:42 PM

Thanks Ron, exactly my thoughts.

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#8

Re: Aluminum Alloys Most Susceptible to Corrosion

07/16/2010 7:39 PM

Aluminum-Copper alloys (ie. 2024) while extremely strong and easy to machine, are very suceptible to corosion when exposed to air and water. Possibily the origional was made from a 6000 series Aluminum ( Al-Si) with much better corosion resistance.

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#12

Re: Aluminum Alloys Most Susceptible to Corrosion

07/19/2010 8:18 AM

A good substitute for the aluminum cups is 4 oz. Ball Mason jelly jars, the two peice lid works well for this application.I have used these for years with good results and the price and availibility cannot be beat. Good luck!

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