Is the boiler actually corroded (Metal is partially dissolved due to chemical reactions), or do you have lime deposits from hard water? If the metal is corroded, then there is little you could or should do, short of replacing the boiler.
One way to judge what you are dealing with is where the discoloration is - if it's mostly where the water is, and expecially where the water gets hot (at the bottom of the boiler), then it is most likely lime deposits. If the deposit is grey and more or less everywhere, then you might have chemical corrosion going on.
If it's lime, then you might be able to dissolve it with a strong citric acid solution. You can find citric acid in the canning aisle of a reasonably well-stocked supermarket or hardware store. Use the stuff that people use in their preserves, Dissolve as much of it as possible in distilled water (again, supermarket), and run it through your espresso maker. After that, rinse with lots of water - a couple of runs, I'd guess. This should help, and should be food safe