Everything is relative when dealing with galvanic corrosion of metal (which, technically, is not electrolysis, which is the breakdown of the fluid conductor).
The less "noble" the metal the more reactive it is (resulting in greater galvanic corrosion of that metal).
Less Noble to More Noble order:
Less Nobel (more reactive)Mild Steel
Cast Iron
Brass
Stainless Steel (certain types in certin electrolytes)
More Nobel (less reactive(So, it looks like cast iron would corrode more relative to brass; however, steel would corrode more rapidly relative to cast iron).
http://www.answers.com/topic/galvanic-corrosion-1
