Running the entire length of each piece of foam, along the hinge is a 2½" wide, 20 gauge galvanized steel C-channel support beam. This is the heaviest gauge (thickest) reinforcement beam used in the industry, others tend to be 22 to 26 gauge. Galvanized steel is used for it's increases strength over aluminum, and also because of it's greater resistance to chemicals (chlorine and bromine). C-channel is much stronger than lesser expensive T-bar others may use.
If the possibility of this c-channel rusting has been made a concern to you, it really shouldn't be. The only way for this c-channel to rust is if the vapor barrier surrounding the foam is broken. If this happens the foam core of a cover will absorb water (getting heavier and heavier) long before the c-channel loses any integrity.