[RedDevil]

Originally Posted by Allch Chcar

Wait, Ethanol with water is less corrosive to mild metals than Pure Ethanol? Do you have some information on that behavior that you can share with us?

Sure. In the Wikipedia article on ethanol blends (very on topic btw!) is a hE15 section that refers to that. It even has a link reference to its scientific source, which sadly cannot be accessed. It has the wholemarks of truth, except the proof.

The experiments show that water in fuel ethanol inhibits dry corrosion.
At 10,000 ppm water in the E50 experiments by JARI and 3,500 ppm water in the E20 experiments by TU Darmstadt the alcoholate/alkoxide corrosion stopped. In the fuel ethanol this resembles 20,000 ppm or 2 volume% in the case of JARI and 5 x 3500 = 17,500 ppm of 1.75 volume% in the case of TU Darmstadt.

The observations are in line with the fact that hydrous ethanol is known for being less corrosive than anhydrous ethanol.
The reaction mechanism will be the same at lower-mid blends. When enough water is present in the fuel, the aluminum will react preferably with water to produce aluminum oxide, repairing the protective aluminum oxide layer, which is why the corrosion stops. The aluminum alcoholate/alkoxide does not make a tight oxide layer, which is why the corrosion continues.
In other words, water is essential to repair the holes in the oxide layer.
Based on the Japanese/German results, a minimum of 2 vol.% or 2.52% m/m water is currently proposed in the revision of the hydrous ethanol specification for blending in petrol at E10+ levels.

Water injection has additional positive effects on the engine performance (thermodynamic efficiency) and reduces overall CO2 emissions.

(Wikipedia on hE15)

Another reason lies in the particular properties of ethanol.
Just like water it has dipolar polarisation, and even just like water it does not really fit nicely onto other molecules of the same kind. That is why water actually expands when it freezes; it is forced to form bridges that don't match nicely with their dipolar properties.
But where the ethanol and water molecules dont line up nicely with their own kind, they do with each other; so well in fact that the resulting volume of the mixture is distinctly less than the total volume of the same stuff separated.

A 96% ethanol and 4% water mixture will contain ball like clusters of molecules consisting of a single water molecule surrounded by ethanol molecules, facing the water molecule with their O-H part. The other end of the ethanol (the -CH2-CH3 part) sticks outward and is relatively inert.

10 PM over here. Time to polarize the guts