I have heard that some have used Evaporust to clean the springs but wonder how much the cast iron of the spring material degrades the nature of the Evaporust.
Firstly, leaf springs are spring steel, not cast iron. Per Ford's AMA specs for 1968, they used 5160, 5147 and 5155 steels in their leaf springs. Regardless, the chelating agent in Evaporust will not react with the iron in spring steel or cast iron, it will only react with iron oxide (I'm simplifying it a bit). It may also react with other materials present on the surface of the part, such as grease, oil, paint, etc.
Chelators are great for this application because you don't have to worry about overexposure affecting the remaining good metal. Same with alkaline solutions - they won't touch the base steel. Acids, on the other hand, may remove rust, but they will also attack the steel itself. Acids can still be useful, it just requires more care and attention to control the process to remove the stuff you don't want, without removing stuff you want to keep.
It's worth mentioning the safety aspect. Acids and alkalines may be faster and more effective than chelators, but they tend to be quite hazardous. Chelators are generally much safer, some are even non-toxic.
