[confusedstud]

I did an experiment to obtain the icorr of aluminium and used the voltage of -4 to 4V. Tap water has a pH of 8 while distilled water 7 so according to the pourbaix diagram of aluminium that i found here http://complex.gmu.edu/people/peixoto/subpages/ece590s08/Pourbaix.pdf shows that aluminium would either be in the passivity region or immunity region in the 2 pH of the solution.

So if this were to be the case, aluminium shouldn't corrode right? So the icorr in either case should be 0? But in the experiment with distilled water that was not the case the icorr was about 9micro-ampere (roughly) and so we calculated the corrosion rate using kaicorr/nD and got about 4.06mpy. Which is pretty low so can we explain that it could be the presence of ions within the distilled water that causes the icorr value to be non-zero?

But both tap and distilled water in a perfect setting with no ion (even if that's impossible) the icorr should be zero right?

[Borek]

Typical pH of distilled water is somewhere around 5.6 due to dissolved CO₂.

Not that it changes much, not that I can help further.

[Enthalpy]

Is Pourbaix really a valid tool to investigate the corrosion of a passivating alloy?

If I grasp it properly, it evaluates static equilibria - but an oxide layer is a dynamic protection instead.

Then it implicitly supposes that an alloy is a uniform phase, which is very wrong. By measuring a potential, it makes nearly no difference between the alloys - but some aluminium alloys resist eternally in seawater while others lose 1mm a week.

And what about salt for instance? Seawater is horrible to most aluminium alloys but sweet water hugely less so, despite the pH hasn't changed that little but through NaCl - it's the chloride ion that weakens the oxide layer.

Or take the 304 stainless steel. It guarantees <0.06% C, but if must be welded, users specify the 304L with guaranteed <0.02% C, as the weld seams in 304 would corrode. Said to be a matter of chromium carbide formation during welding that reduces the Cr contents at the grains joints in the seam. How could an electric potential tell that?

If Pourbaix works, then for alloys with a homogeneous matrix that forms no passivation layer. Any other case must be too complicated to caracterize with a single number.