It turns out that NH4OH readily reacts with Al2O3 (I have performed this reaction):
2 NH4OH + Al2O3 + H2O --> 2 Al(OH)3 + N2 + 3 H2
which is actually a two step reaction as the NH4OH directly reacts with Al2O3 liberating active Aluminium that rapidly reacts (like Sodium) in water!
Al2O3 + 2 NH4OH --> 2 Al + N2 + 5 H2O
2 Al + 6 H2O --> 2 Al(OH)3 + 3 H2
So even NH4OH, a weak base, dissolves Aluminium because it reacts with (removes) the Al2O3 and the WATER dissolves the aluminium!
In the case of excess NH4OH, the white clouds of Al(OH)3 eventually vanish with the formation of a clear NH3 complex (perhaps Al(OH)4- ).
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