Adding dispersants to circulating water?

We have a Vaillant Combi, whic is persistently (at 2 year intervals) having to have the hot water heat exchanger changed as it has clogged with iron oxide. This is despite the system having been flushed and filled with Fernox-MB1 AND a Magnaclean magnetic filter being incorporated into the return.

The boiler itself just seems to attract black iron, where it aggregates into lumps which block the works. Is there a suitable dispersant available that could be added to the circulating water to keep the iron dispersed thereby preventing the clogging and allowing the Magnaclean to work effectively?