Steel center pin. I tried very hard to get ALL aluminum by reading all the specs and reviews of many rivets. I'm building a custom product with a very strong magnetic field and I shouldn't use a trivalent (ferrous) material. ALSO there is an electromechanical/chemical fact called dissimilar metals (aka galvanic corrosion). Two different types of metals coming into contact in a humid environment (e.g. in boats) will cause one of those metals to corrode. When steel and aluminum are together, aluminum corrodes much faster than aluminum alone.
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