You want to look at the galvanic compatability of the mixed metals. For differences of 0.25v or less, these metals are generally safe to use together without any form corrosion inhibitor. Up to 0.50v difference, which is categorised a mildly reactive, a 10% concentration of corrosion inhibitor changed yearly should suffice. Above 0.5v which is categorised as highly reactive, and around a 25% mix of corrosion inhibitor will be needed, which probably should be changed every 3-6 months. Copper->Aluminium is typically around a 0.6-0.8v difference, depending on the actual aluminium alloy being used.
