Understanding the efficacy of concentrated interstitial carbon in enhancing the pitting corrosion resistance of stainless steel

Abstract By introducing a high fraction of interstitial carbon through low temperature carburization, the pitting corrosion resistance of austenitic stainless steel can be significantly improved. Previous work attributed this enhancement to the improvement of passive film properties. However, we show here that interstitial carbon actually weakens the passive film on stainless steel. In fact, the enhancement in pitting resistance is a result of carbon reducing the metal dissolution rate in a local pit environment by many orders of magnitude, which extremely decreases the growth stability of a pit and prevents it from transitioning into stable growth. Electronic structure calculations show that carbon bonds to the metal atoms and that the metal–carbon bonds are 1.6 to 2.0 times stronger than the metal–metal bonds. Different from prior theories, we show that the significant increase of pitting resistance originates from the formation of covalent bonds between interstitial carbon and its neighboring metal atoms, resulting in a significantly reduced dissolution rate. This study indicates a new strategy for the design of corrosion resistant alloys, namely alloying with concentrated interstitials that form strong bonds with the matrix atoms.