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I am also trying to find out the same information. While I can not find any scientifically based research - all the "answers" Ive found indicate between 2:1 & 6:1, and most likely 4:1.
Alternate Answer:
Refer to the Pilling-Bedworth ratio (http://en.wikipedia.org/wiki/Pilling-Bedworth_ratio). It provides a theortetical volume ratio between the metal oxide and the elementary cell of the corresponding base metal. For iron, the ratio is roughly 2:1 (oxide to metal), but varies according to the chemical nature of the oxide, available oxygen, etc.
It is important to note that this is a theoretical ratio comparing relative volume and may not directly correspond with the ratio of measured thickness in actual practice because of the way the oxide (rust) product forms and layers resulting in varying density. Often, the physical thickness of the rust scale on carbon steel is reported to significantly exceed twice the loss in thickness of metal section. Many professionals in the corrosion industry cite an observed ratio of for carbon steel that is 4-10 times that of metal section thickness loss, particularly on heavy rust scaling in a chloride-laden marine environment. However, I am not aware of any published research or testing that supports this commonly accepted assertion.
This variance in ratios between the relative volume and thickness are possibly the result of voids within the formation of thick rust scale films. In my experience, we have observed rust scaling on carbon steel in a marine environment that is typically 3-4 times the thickness of the corresponding section loss in the base metal.
In summary, the answers could be different depending on whether the question concerns thickness of the corrosion product compared to metal section loss, or the relative volume between base metal lost and oxide formed. There is published data tha supports a 2:1 change in volume for iron. But, in terms of physical thickness of the measured rust scale compared to metal section loss, it appears the ratio could be greater.
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