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.
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
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 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.
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 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
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.