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.
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.
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
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?
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 80°C, although they will not peel"
In addition (and i don't know what the ph of seawater is but with regard to
"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.
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
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)
here is some great info on aluminum corrosion
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
Thanks for that,
FYI this is what I am trying to avoid :
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.
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?)
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.
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 >>sulfuricYEARS ago in aerospace with a MIL-SPEC zinc chromate primer but I do
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
Type III is great for abrasion, but has the conductivity and CTE problems
"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
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
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.