Everything rusts. Iron, exposed to oxygen and water, dissolves into iron oxide — rust — a flaking, porous substance that occupies more volume than the original metal but has none of its strength. The process is electrochemical: iron atoms lose electrons to oxygen, becoming ions that dissolve into water, then precipitate as hydrated oxides. The solid dissolves into powder.
Other materials decay differently. Concrete spalls as water penetrates its surface, freezes, expands, and fractures it from within. Polymers degrade under ultraviolet light, their long molecular chains breaking into shorter fragments. Wood rots as fungi digest its cellulose. Even stone — the archetype of permanence — is slowly dissolved by acid rain, by freeze-thaw cycles, by the patient work of lichens.
Material decay is dissolution without a solvent in the traditional sense. The dissolving agent is time itself, assisted by water, oxygen, light, and life. The process is irreversible — you cannot un-rust iron, un-rot wood, un-crumble concrete. The dissolution is absolute: the original substance is gone, replaced by something simpler, more stable, and less useful.
The structures we build are temporary arrangements of matter against entropy. They all dissolve eventually. The question is only how fast.
In Fiction
J.G. Ballard's The Crystal World (1966) imagines a forest in which matter is crystallizing — the opposite of decay, but equally fatal. Trees turn to jewel. People turn to statue. The dissolution here is not into chaos but into too much order. Ballard understood that decay and crystallization are two forms of the same process: the movement of matter toward a state from which it cannot return.