You need to check pourbaix diagram for Al

As suggested by Azmi Kamardin, a Pourbaix diagram for aluminum will tell you a great deal.  However, most Pourbaix diagrams are just for pure Al in water and do not consider the effects alloying elements or other ions dissolved in the water and the diagrams generally only exist for a temperature of 25C.

This means that for environments other than deionized water at 25C, you will need to perform an electrochemical test.  The simplest would be a potentiodynamic sweep to determine the open circuit potential of your aluminum in the environment in question.

However, to address your first question based upon pure aluminum at standard conditions, it is a function of pH.  The Pourbaix Diagram for aluminum for acidic conditions where Al2O3 is not formed is a potential that ranges from approximately -1.7V to -1.8 vs SHE as the concentration of Al3+ changes from 10^0 to 10^-6.

As for how is this value calculated, it is simply a calculation of the Al to Al3+ reaction using thermodynamic free energy values.

If you are not familiar with this field, you should probably take a look at the book titled "Atlas of Electrochemical Equilibria in Aqueous Solutions" by Marcel Pourbaix.  The front section describes the method of performing the calculations and the rest of the book shows diagrams for most elements.

Prof Smith is absolutely correct. If you want to know the actual potential, you have to do the measurement yourself. Use Pourbaix diagram as a guideline only.

Thanks for your answers, I have Al substrate in contant with ethanol,can i use Pourbaix diagram ?

In contact with ethanol? Pure ethanol or solution?  

Basically, Pourbaix diagram only shows the potential-pH equilibrium diagram of 1 substance with water. When dealing with 2 substances (e.g. Al and ethanol), we can overlap both diagrams for each substance to look for "sharing" domain(s). The problem is, I don't think pourbaix diagram for ethanol has ever been established. As far as I know, only diagrams for methane, methanol (methyl alcohol) and methanal (formaldehyde) are available. Nevertheless, I think  you should go ahead with the actual measurement. As long as you have a proper setup, I believe you should be fine.

The classical Pourbaix diagram only applies to a metal in deionized water at 25C.  However, there are plenty of software packages that do the calculations to draw diagrams at temperatures other than 25C and in aqueous electrolytes that contain ions other than M+, H+ and OH-.  The issue with ethanol is that it is not an ionizing electrolyte, so it will not participate in many of the electrochemical corrosion reactions in the same way that water does.  This does not mean that you cannot always estimate the reactions at conditions for low concentrations of ethanol in water.  However, for moderate to strong EtOH concentrations, an assumption that the activity of water equals 1 and therefore the dissociation constant is 10^-14 is no longer valid.

There is at least one commercial thermodynamics package that performs the calculations required to draw Pourbaix diagrams in the mixed non-aqueous/aqueous solutions where the activity of water is not essentially equal to 1.  However, this program can be quite expensive, simply because it has technology built into it that is not available from other sources.

thanks for your answers.they helped me.