I always see the electromotive series referenced to hydrogen, not gold. I've also always seen aluminum more anodic than zinc. That is, an aluminum block will protect a zinc block. That's why aluminum boats don't use zinc anodes. It may not hurt the aluminum hull much but it won't protect anything either. Aluminum should offer more protection than zinc except that it quickly oxidizes and aluminum oxide creates a protective barrier stopping further reaction. That is probably why, as Charlie says, the aluminum anodes are not just a chunk of aluminum; they have been alloyed to be "self cleaning".