Anodised aluminium and corrosion?

I'm thinking about making a couple of glass covered solar evaporators, to purify well water. There are plenty of designs around, generally
fabricated out of a fiberglass base and silicon sealants to carry the dirty water. These seem to have a minimum life span of about 10 years.
Is there any reason why I should not use black anodised aluminium as the water carrying base? The temperature can exceed 80C on a hot day.
Would there be a problem using seawater, in the long term?
A couple of hours googling has not turned up much information.
thanks
Martin
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Here's one: http://www.alzscot.org/pages/info/aluminium.htm
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On Fri, 12 Oct 2007 21:40:24 +0100, in sci.engr.metallurgy "Grimly

Ah, that old chestnut...somewhat inconclusive
How about if I asked "What condition would a sheet of 2mm black anodised aluminium be in, after 10 years in seawater at 85C for 4 hours a day, minimum temperature about 0C for less than 100 hours per year" . That is sort of worst case
Typing that into google is not that helpful
Martin
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How pure is this alumium ? How thick is the coating ? Maybe it doesn't need to be anodised ?
"Anodized coatings have a much lower thermal conductivity and coefficient of linear expansion than aluminum. As a result, they will crack if exposed to temperatures above 80C, although they will not peel"
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In addition (and i don't know what the ph of seawater is but with regard to anodisation:
"The coating stays stable for pH-values between 4 and 9 for pH-values between 4 and 9. The coating stays stable for pH-values between 4 and 9."
If you google "corrosion of aluminium in seawater" there is plenty of stuff comes up, including articles about sacrificial metals, it is also clear that there are many factors that could affect it, including how pure the aluminium is, if it's an alloy what is it alloyed with, how thick the anodisation is and how it is anodised, also temperature and wether it is completely submerged or not etc etc.
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On Sat, 13 Oct 2007 07:45:52 +0100, in sci.engr.metallurgy "Grimly

Thanks, you've given me enough information to avoid using anodised aluminium
Martin
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Martin-
I have followed this thread from the begining & I not totally sure of the design but I do have a fair amount if experience with metal in corrosive & semi-corrosive environments.
Stainless steel & aluminum can be a problem corrosive / marine environments.
You mention well water & then talk about sea water. Well water might have pH problems & sea water of course is a great electolyte.
The stainless / aluminum combo is only a problem if the bare metals contact each other AND the electrolyte.
(An aside, I clamped a sheet of bare aluminum .06" x 12" x 18" to a SS pool ladder pole......the sheet was Swiss cheese in a week)
If you properly prep the holes (maybe drill then anodize & then use SS pop rivets) with special thick goopy primer
YEARS ago in aerospace with a MIL-SPEC zinc chromate primer but I do not know it is is still used (environmental issues)
but here is something I found (but have never used)
http://www.vannay.com/eck.html
here is some great info on aluminum corrosion
http://www.key-to-metals.com/Article14.htm
bottom line use a 1000, 3000, 5000 or 6000 series alloy stay away from 2000 & 7000
use a Type III "hard anodize" its much thicker but check with you plater.
cheers Bob
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On Sat, 13 Oct 2007 15:08:47 -0000, in sci.engr.metallurgy BobK207

Thanks for that,
FYI this is what I am trying to avoid : http://www.epsea.org/const.html They seem to need a lot of expensive food grade silicon gloop to stop the fiberglass outgassing when the solar still is empty.
This is why I thought hard anodising would be a suitable alternative, and quicker to fabricate
Many people around here (Cadiz,spain) have well water , but it is generally very brackish, quite undrinkable, and closer to the coast the well water can be quite salty.
Martin
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Probably a silly question, but why can't you use plastic ?
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On Sat, 13 Oct 2007 18:03:57 +0100, in sci.engr.metallurgy "Grimly
snip

I know even less about plastics, than I do about metal, and I'm only making a few, so a mould could be very expensive (?), and I think the 80C+ could be a problem
(Could you suggest a plastics newsgroup that I could annoy?)
Martin
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wrote:

PP has a melting point of 80C so that would be no good but there are many other types of plastic, for example i beleive most HDPE has a melting point of around 130C (at least 110C), i believe there are plastics with melting points of 180C, possibly higher.
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>"Anodized coatings have a much lower thermal conductivity and >coefficient of linear expansion than aluminum. As a result, they will crack if >exposed to temperatures above 80C, although they will not peel"
That's true for Hard Anodized coating, because they are much thicker than other anodized coatings. But in this application where the coating is protected from direct abrasion, a Type II (Sulfuric) anodize would work just fine. Make sure it's >>sulfuric<< anodize, not chromic. Also, all anodize coatings have to be sealed. Standard sealing with Type I (Chromic) and Type II (Sulfuric) anodize is with either a dichromate, nickel acetate, or cobalt acetate, solution. Since this is for water for human consumption, avoid these sealers. Specify "Boiling wtaer seal". Since you are apparently located in Spain, I would specify ISO 8079, "Anodic Treatment of Aluminum Alloys - Sulfuric Acid Process - Dyed Coating"

Zinc Chromate is being rapidy phased out and is currently banned in EU This has been a problem for aerospace companies, and many are switching to zinc molybdate and other substitutes, but these will also eventually be banned.

Type III is great for abrasion, but has the conductivity and CTE problems mentioned above

"Food Grade" is very important, not just for the goop, but for any coating that comes in contact with drinking water.

Hard anodizing (Type III) is actually harder to fabricate than Type II

Temperature is not a problem. There are lots of food-grade plastics with a heat-deflection temperature that exceeds 80C (176F)

Polycarbonate (Lexan) has HDT of 127C minimum PolyethyleneTerepthalate PET (Rynite) has a HDT of 220C min. Liquid Crystal Polymer (Xydar, Vectra) has a HDT of over 311C
These are all food-grade plastics
HTH
--
Harry Andreas
Engineering raconteur
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wrote:

I would agree with the above and i think the op certainly has plenty of info now, as you say it is important to use the right type of anodising and to seal it (this is more important as the coating thickness is increased i believe). As for the plastics and the previous comment about moulding, i'm sure the poster could quite easily find something suitable ready made that would stand over 100C.
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On Mon, 15 Oct 2007 21:58:22 +0100, in sci.engr.metallurgy "Grimly

many thanks for the information
Martin
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Martin Griffith wrote:

If you have steel and aluminum in contact within a water bath, you are asking for galvanic corrosion. As an example, if you use stainless steel fasteners in your aluminum base, and the stainless steel fasteners are in the water, you will probably lose some aluminum to galvanic corrosion.
In theory, anodizing prevents that galvanic corrosion. In practice, the anodized coating often gets a scratch. Now the aluminum is losing all of its material in one localized area around the scratch. In that case, the aluminum can get pits in a hurry. The increased conductivity of salt water exacerbates that problem.
If you can use aluminum or plastic fasteners and associated hardware, you might avoid that problem.
--
Dave
dvt at psu dot edu
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On Fri, 12 Oct 2007 17:27:20 -0400, in sci.engr.metallurgy dvt

Thanks, I've understand that (a failed degree,fortunately, in textile chemistry 40 years ago,but many beers) and there will be no dissimilar metals in contact. But in principle there should be little corrosion?
Martin
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