I'm not sure if this is important but I've noticed that aluminum has only been confirmed to actually stop end positive magic systems.
End Positive Magic Systems:
In allomancy aluminum destroys metals and cannot be pushed or pulled with iron or steel. It also can protect a person from emotional allomancy.
In Forgery (another end positive magic) Ralkalest (which has be theorized to be aluminum) cannot be forged with a soul-stamp.
End Neutral:
In Feruchemy aluminum can be used to store identity (although we've never seen it used).
End Negative:
In Hemalurgy aluminum can steal allomantic enhancement powers.
Does anyone have an explanation to this?