I also do not have instant access to mils/year data, but can offer some insight into the galvanic potential for anodized material. At a microstructure level, anodizing creates an insulating ceramic layer on the material, so the new galvanic potential is a function of the breakdown voltage for the ceramic. The non-uniformity of the ceramic layer means you can't substitute the value for the bulk ceramic material, but it certainly gives you an indication of how much the couple can change. If this information is important for your application, you really should perform a test.