Water is the source of all life, so much so that the earth would have continued to be a barren rock hurtling through space if water had not started raining down on the surface through some miracle of chemistry and physics. The very first single celled organisms that came about on our planet millions of years ago existed entirely under the oceans of the world, and it took several eons before these critters evolved enough that they could start walking on dry land thereby giving rise to modern societies.
However, for all of its benefits water can also be a rather destructive force in certain situations. Water is comprised of a hydrogen atom followed by two oxygen atoms, and this abundance of oxygen means that it can make Metal Kards rust if they come into contact. It’s not just metal cards that are at risk if exposed to water either. Suffice it to say that paper performs even more poorly when it is in close proximity to any kind of moisture. The reason behind this is that water can loosen up its molecular structure and force it into odd and unpleasing shapes.
That doesn’t mean that you should worry about water affecting your business cards, though, because you can easily make them waterproof by adding a plastic laminate coating. Alternatively, you could make metallic business cards out of metals that are less prone to rust like aluminum or stainless steel. Iron cards can look quite rustic, but iron is among the most rust prone metals out there so it might be a good idea to avoid using it here. Just make sure that you give waterproofing the importance it deserves.