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I thought I would try to deal with dehumidified air from radiators by filling simple troughs of water placed on the top, which in theory would evaporate water as the radiator heated the air. I crafted three simple troughs from folded standard kitchen aluminium foil and sat them inside the two sides of the white-painted radiator, atop of the corrugated metal fins. After a few days where the evaporation rate seemed too low to be meaningful for my air-humidification aspirations, there were black splodges on the inside of the foil that I took to be mould. The water had by this time evaporated so I gave up on the idea and removed the troughs, only to notice numerous and at times significant holes where the aluminium has apparently corroded away. One was a channel of maybe 2 mm wide by 20 mm long.

In a previous revision of this question, there wasn't enough info to go on, so I've repeated the experiment. We have water from the gas boiler leading through copper pipes to a steel radiator painted white, heated to something like 50 °C. The aluminium tray is inserted in the top, and cold tap water from the hot tap in the bathroom is added to about halfway. The radiator cycled hot/cold two times a day. I'm in a heavy water area in the UK (Surrey) with significant calcium content in the water.

Below shows the experiment progression in pictures. Images 1 and 2 show the set up. Image three shows the tray at about 6 days. The black marks turned out to be the holes, and there are white deposits underneath the holes. The last two images show Day 7, the tray removed for observation looking at the back edge from the other side. There were no deposits on the outside of the foil. There were powdery, crusty white deposits on the radiator where the holes were.

Foil tray in radiator

Foil tray in radiator

Six days showing erosion in situ

Seven days, tray removed showing holes

Seven days, tray removed showing holes

I'm guessing the white powder is aluminium oxide, but I don't understand why the corrosion would be so pronounced simply because of 50 °C water temperatures.

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    $\begingroup$ Regular duty aluminum foil (0.001 inch thick) supposedly has about 1 pinhole per square foot; heavy duty Al foil (0.oo15 inches thick) has many fewer. Recycled aluminum is used in cheaper foils and might contain a few atoms of mercury or copper. Chloride ions in tap water are no friends of aluminum, nor is a low pH. Can we get some more information? $\endgroup$ Nov 4, 2021 at 17:29
  • $\begingroup$ Chloride will corrode through Al foil overnight. You could try distilled water ; However, Al baking pans have a fairly long life . $\endgroup$ Nov 4, 2021 at 18:57
  • $\begingroup$ Do the holes correspond to the level of the water? $\endgroup$
    – matt_black
    Nov 18, 2021 at 20:21
  • $\begingroup$ I don't understand why this is closed, at least the focus is clear. What can corrode an aluminum foil containing tap water atop a radiator? Of course some kind of corrosion that I have observed as well. It might be even simply oxide, presumably passivation requires a continuous and solid surface that a thin cheap foil might not provide. Or the chlorine / chloride mentioned above. It is a sensible question with a possible answer. $\endgroup$
    – Alchimista
    Nov 19, 2021 at 9:25
  • $\begingroup$ @matt_black : Yes, the holes seem to be at the water line, created as the waterline receded with the evaporation. $\endgroup$ Nov 19, 2021 at 10:23

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My speculation as the radiators are located by the window, some light induced photolysis may be occurring with metal rich dust particles settling on the surface of the water.

With any formed electron hole ($\ce{ h+)}$ acting on some $\ce{OH-}$ in the water may be transformed into the associated hydroxyl radical, $\ce{.OH}$. In the presence of say a phosphate ion, we could then have the phosphate radical which is acidic.

In this scenario, the attacked Al at the water line may be an aluminum phosphate salt. Other possible ions may similarly be radicalized.

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    $\begingroup$ Thanks for the theory. Radiator is on the side wall away from the window (front wall). It gets little direct sunlight in the winter as the sun is already setting by the time it's that side of the window. It's also in a household environment so would there be much to contribute to 'metal rich dust particles'? $\endgroup$ Dec 19, 2021 at 13:39

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