If you look at the thermodynamic properties of carbon dioxide and compared that with the known atmosphere of Venus, it turns out that Venus is covered with an ocean of supercritical carbon dioxide fluid that is about 3 km deep. Supercritical carbon dioxide at Venusian surface conditions maybe a more potent solvent than sulfuric acid (high temperature supercritical water can dissolve almost anything including gold). I have no clue what the long term effect this would have on a nickel coin. One thing is certain, no one is going down to the Venusian surface to check that coin anytime soon.

Gary