Since corrosion commonly occurs heterogeneously over a surface, studies on a microscopic level are desired to obtain a complete picture of the process. Here, we demonstrate the capability of nano-FTIR microscopy to spectroscopically determine the nature of different corrosion products and their spatial distribution with a resolution of 20 nm, two-three orders of magnitude better than conventional IR microscopy. A copper surface was exposed to a humid atmosphere containing formic acid, and in addition to cuprite the corrosion product copper formate was observed to form inhomogeneously in particles of some tens to a few hundreds of nm.