Ab initio density functional theory is used to study an important chemical corrosion phenomenon: carbonyl formation on metal surfaces. Our initial focus is on the structural and energetic aspects of the iron pentacarbonyl formation on an Fe(100) surface. The surface is represented by a slab model, and the adsorption of CO molecules is described by an idealized model that allows the extraction of a metal atom from the surface while minimizing the lateral interactions. The carbonyl formation undergoes three major steps: the chemisorption of CO molecules to form surface carbonyl species, the desorption of the complex from the surface to form a transition state, and the evolution into the final product. Although the chemisorption of CO on Fe(100) is energetically favorable, the desorption of the surface carbonyl species is endothermic, but the required energy can be largely compensated by the adsorption energy. The present study demonstrated that the metal atoms can be depleted by the surrounding CO molecules to form iron pentacarbonyl.