Simple alcohols such as ethanol can, apparently, corrode aluminum. However, this corrosion reaction seems to be very slow at room temperature and below. Is an ethanol based, non-foaming spray cleaner, propelled by LPG and with an ethanol concentration of, say, about 20%-30%, some (about 2%-3%) alcohol ethoxylate (this acts as a surfactant) and a slightly basic pH (between 7 and 8), really ok to spray on a cold parallel-flow type heat exchanger without fear of any damage? Or should an alcohol-free cleaner be preferred in any case for that job?

My belief is that the corrosion can only happen when the aluminum object is actually submerged in pure alcohol. That little alcohol conveyed by the spray probably evaporates before having the chance to do any damage, but i'd like to understand the surfactant role better, i.e. if any added surfactant agent can act as a corrosion inhibitor.

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    $\begingroup$ "My belief is that the corrosion can only happen when the aluminum object is actually submerged in pure alcohol." And you're right. Just don't keep booze in an aluminium bottle. $\endgroup$ – Karl Jan 4 '19 at 18:15
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    $\begingroup$ I doubt that ethanol is more harmful to aluminium than water. $\endgroup$ – aventurin Jan 4 '19 at 18:30
  • $\begingroup$ I'll say it depends. This guy has tested it, and found nothing: youtube.com/watch?v=xkob6fBNrOg ;-) $\endgroup$ – Karl Jan 4 '19 at 18:42
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    $\begingroup$ And here diva-portal.org/smash/get/diva2:720728/FULLTEXT01.pdf (Master thesis of Jenny Linder at KTH Stockholm) is the reason: As long as there is water present, the infamous ethanol corrosion is inhibited. I take back what I said about the booze above. ;-) $\endgroup$ – Karl Jan 4 '19 at 18:49
  • $\begingroup$ (disclaimer: I just read the abstract. Whoever want's to check out the whole thing is welcome to write it into an answer here. No time here. ;-)) $\endgroup$ – Karl Jan 4 '19 at 23:50

When aluminum metal is exposed to the oxygen of the air at room temperture, it forms a protective air-tight layer of aluminum oxide. So as long as nothing is present that will disrupt the oxide layer on aluminium it is OK for aluminium at 20 oC to be in contact with air.

If an aluminium alkoxide is exposed to water then the aluminium alkoxide will react with the water to form aluminium oxide and the alcohol. So in alcohol which contains some water it is not possible for the aluminium to dissolve in the alcohol.

But if the alcohol is anhydrous then it will be able to enter a scratch in the aluminium and start to react with the metal thus causing corrosion. For titanium that anhydrous methanol is very corrosive.

So as long as some water is present and air is present then at room temperture

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  • $\begingroup$ I'm afraid it's not that easy. Alcohol is probably not reacting vigorously with aluminium metal, but it is definitely altering the (re)formation of protective oxide layer making it less dense at least. OP mentioned there is elevated pH, so that even a weak base can actually react with aluminium. Several government standards (e.g. Russian, I know for sure) forbid using tanks made of aluminum and aluminium alloys for transporting and storing pure alcohols (and booze, too, of course). $\endgroup$ – andselisk Jan 5 '19 at 13:34
  • $\begingroup$ "so that even a weak base can actually react with aluminium" – This was absolutely not what the OP said. $\endgroup$ – SteffX Jan 5 '19 at 13:44
  • $\begingroup$ Read again: "a slightly basic pH (between 7 and 8)" $\endgroup$ – andselisk Jan 5 '19 at 13:46
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    $\begingroup$ @andselisk I have never heard that a pH of 8 could be appreciably corrosive to Al. At pH 10 I would certainly start getting careful, but 7-8? Have you got a source for that? $\endgroup$ – Karl Jan 7 '19 at 9:27
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    $\begingroup$ @Karl Many general textbooks would state that Al is prone to corrosion in both acidic and basic (especially basic) media. Of course, corrosion rates wouldn't be high for the slightly basic solutions, however the corrosion current is never 0 when pH deviates from neutral (see e.g. doi.org/10.1016/0010-938X(96)00037-6). $\endgroup$ – andselisk Jan 7 '19 at 10:53

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