# What happens to the carbon in carbon steel when it rusts?

Carbon steel is iron containing some carbon in various forms of iron carbide.

When it rusts (oxidises), the iron becomes part of all kinds of crystalline and amorphous iron oxides and hydroxides, mostly as Fe3+.

Where does the carbon go in this process? Does it also oxidise to carbon dioxide or monoxide and leaves the system as a gas? Does it combine with the iron to form siderite (iron carbonate)? Does it remain as graphite or some other form of elemental carbon? Something else?

• @DrMoishePippik not sure. Black things in rust could be magnetite, specular hematite and a bunch of other things that are unrelated to carbon. – Gimelist Jun 25 at 4:49
• @DrMoishePippik but the carbon is part of the iron carbide in the beginning. Do we know that when iron oxidises the carbon is left behind as elemental carbon? This is what my question is about. – Gimelist Jun 25 at 5:17
• apologies - my previous (now deleted) comments referred to cast iron, in which most of the carbon is graphite inclusions in the iron matrix. See answer below. – DrMoishe Pippik Jun 25 at 18:23

Cast iron has considerable free carbon as graphite, but carbon steel has iron carbide in cementite, $$\ce{Fe3C}$$ in a ferrite ($$\ce{α-F}$$e) matrix, along with other phases and additives. In a corrosive environment, the ferrite and cementite form a galvanic couple, causing increased corrosion of the $$\ce{α-F}$$e while effectively protecting the $$\ce{Fe3C}$$. According to Acta Metallurgica Sina,, "So, the ferrite phase diminishes gradually, and the accumulated cementite multiplies on the surface of the specimens along with the development of corrosion."
That said, what now happens to the remaining $$\ce{Fe3C}$$? Wikipedia states, "While cementite is thermodynamically unstable, eventually being converted to austenite [$$\ce{γ-Fe}$$] and graphite at higher temperatures, it does not decompose on heating at temperatures below... 723 °C..."