The advocates of the carbonic-acid theory hold that carbonic acid attacks iron, converting it into a ferrous carbonate. The hydrogen thereby released combines with oxygen of the air, or other sources, which in turn decomposes the ferrous carbonate to ferrous hydroxide, or rust, releasing in the latter reaction the same amount of carbonic acid as was originally present. This attacks a new portion of iron and forms a second particle of rust. A moderate supply of oxygen would render this process indefinitely continuous on the initial small supply of carbonic acid. The theory is logical, but too great importance seems to be attached to it by those who assume it to be the only method, or even principal method of rusting.

The claim of the hydrogen-peroxide theory is that iron, oxygen, and water react