air separators cannot prevent corrosion. they remove air but cannot ensure a lack of oxygen. they should be used anyway since these systems are prone to air problems, but it doesn't stop your corrosion issue. you can stop the corrosion issue by not using any ferrous components on the HDPE side of the system, or by using anti-corrosion additives.
the air that is in the system during the initial install and even afterwards will cause corrosion. but once that corrosion is finished, you're all done. I don't know what kind of volume you are talking about or how much iron and even if I did I don't have the math to say what is safe and what isn't, but I can say the initial fill of oxygen is not typically a problem for hydronic space heat in residential homes that have only a few pumps and maybe some fittings in a boiler for "ferrosity" to take the corrosion. grrrrr
Perhaps if you have a huge volume of initial fill water and a small amount of "target" for that oxygen it could be a bigger problem though.
of course, all hydronic systems worth their salt have air separators as well.