Those manifolds are corroding all the time, even after having them cleaned up, unless you have them Dow Coated to prevent further corrosion
I installed one last year on a early 1966 street Hemi motor in a 1963 Dodge Polara 440 hardtop for a friend and his Son, it was brand new and part of it was corroded black and part was dark gun metal grey from corrosion
I had a old 426 Hemi Cragar magnesium blower manifold cleaned and Dow coated by Mert Littlefield blower company a long time ago, it looks exactly the same now as it did when we had it done is what my customer is saying
I had a old set of Motor Wheel Magnesium rear rims, what a pain to keep up