I got confused because I assumed tinning iron and steel is a modern technology. The German wikipedia entry on brigandines states however that the metal plates in a brigandine were often tinned.
Is there any truth to brigandines being tinned? Any tinned armour?
The Royal Armoury has two brigandines with tinned plates:
https://collections.royalarmouries.org/object/rac-object-36987.html
https://collections.royalarmouries.org/object/rac-object-13483.html
I assume that while modern electroplating was out of the question, simply dipping the piece into molten tin was possible; after all, the melting point of tin is fairly low. Corrosion resistance is a useful feature, and a piece of armour was already a quite pricey purchase.