This is a quote from Red Line's website:
Quote:
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The only property which enables aluminum to be used in a cooling system is that it will form protective films under the proper conditions which will prevent the uncontrolled corrosive attack of acids or bases. Poor aluminum corrosion inhibition will cause the dissolution of aluminum at the heat rejection surfaces, weakening the cooling system walls and water pump casing and weakening the head gasket mating surfaces. These corrosion products will then form deposits on the lower temperature surfaces such as in radiator tubes which have very poor heat transfer properties, causing a significant reduction in the cooling ability of the entire system. Red Line WaterWetter® will provide the proper corrosion inhibition for all cooling system metals, including aluminum, cast iron, steel, copper, brass, and lead.
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The black deposits on the aluminium could be aluminium oxide, which is normally white, but perhaps the water wetter turns it black. Aluminium oxide is what normally forms on the surface of aluminium (the 'protective film' in the above quote) and it is very unreactive, preventing corrosion of the metal underneath. Take a close look at the black deposits and see if they are hard or soft. Aluminium oxide is very hard (harder than glass), so if the deposits are soft you know it's something that shouldn't be there...