The table is from a text on called "Handbook on Corrosion Engineering". But I saw it on Wikapedia: http://en.wikipedia.org/wiki/Galvanic_corrosion

There is a course outline on it which Charlie's and others might find interesting

http://www.npl.co.uk/upload/pdf/guide_to_conservation_of_metals_20071105124733.pdf. Look at paragraph 3.6 which discusses protective oxidation on metals and performance in salt solutions.

To me, the effectiveness of an anode seems to depend on the degree of noble difference (i.e., the greater the difference, the more protection provided.)