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Regarding this article, I have read that the corrosion rate and weight loss of a steel specimen after immersion in different corrosive media is negative. I'm surprised that this is really possible. Is there a chemical explanation for this?

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    $\begingroup$ You mean like electroplating? $\endgroup$
    – Jon Custer
    Jul 15, 2017 at 20:18
  • $\begingroup$ Immersion in saltwater and other corrosive media for corrosion protection. $\endgroup$
    – Acid
    Jul 15, 2017 at 23:42

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In the data cited in your reference, corrosion is measured by weight loss or gain. In the case of the steel coupon immersed in sodium carbonate, the weight gain is not due to "negative corrosion", but likely due to a tightly-adhering film of oxides that was not removed in cleaning. The oxides are made from the iron of the coupon plus oxygen from water, which is greater than the mass of the iron alone. (Normally, cleaning the coupon removes loosely adhering oxides, so there is a weight loss.)

According to Toyin, "the corrosion inhibition efficiency of $\ce{Na2CO3}$... indicates that a protective film is formed on the metal surface. The passivation of the mild steel may be caused by the formation of $\ce{Fe2O3}$ and $\ce{Fe3O4}$."

That said, it does appear that high carbon concentrations may be effective in greatly reducing the corrosion of mild steel.

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    $\begingroup$ If that's the case, does it mean that the higher weight gain, more corrosion take place? $\endgroup$
    – Acid
    Jul 16, 2017 at 9:10
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    $\begingroup$ Perhaps... it depends on how well the coupon is cleaned after the test: if the cleaning removes flocculent iron oxides (often reddish), then the weight would drop with corrosion, but if the (black) oxide adheres well and cannot be removed, then the weight might increase, depending on how much Fe dissolved before the passivating coat adhered. $\endgroup$ Jul 16, 2017 at 21:14

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