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 signi cantly improved. Previous work attributed this enhancement to the improvement of passive lm properties. However, we show here that interstitial carbon actually weakens the passive lm 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 signi cant increase of pitting resistance originates from the formation of covalent bonds between interstitial carbon and its neighboring metal atoms, resulting in a signi cantly 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. Main TextTo meet the fast-growing demand of alloys serving in harsh environments, great efforts have been made to design alloys combining enhanced mechanical properties and improved corrosion resistance. 1, 2 One strategy to achieve this goal is to modify the alloy surface using surface engineering techniques, such as carburization. The deleterious precipitation of intergranular Cr carbides during carburization of stainless steels by standard methods can be prevented by low temperature carburization (LTC), 3, 4, 5 which allows the diffusion of carbon atoms into the alloy matrix resulting in a high concentration, without precipitation of carbides. Generally, carbon atoms are incorporated in the octahedral sites of face-centered cubic (FCC) structures, forming a subsurface zone of concentrated interstitial carbon, with carbon fraction up to 0.15 and the mean depth on the order of 10 mm. 6 Even low concentrations of interstitial carbon can improve the corrosion properties of iron, 7, 8 but the bene ts are small relative to those imparted by higher concentrations. Concentrated interstitial carbon was widely reported to improve the corrosion resistance of stainless

Effect of quenching in aqueous polyvinylpyrrolidone solutions on the microstructure and pitting corrosion resistance of AISI 1045 carbon steel

“…[ 34 ] Li et al reported that the bonds between interstitial carbon and metal atoms in stainless steels were 1.4–2.0 times stronger than the metal–metal bonds. [ 35 ] Note that in this study, it was impossible to measure the interstitial‐carbon concentration within each phase. However, it seemed reasonable to assume that the increase in the pitting corrosion resistance could be attributed to the reduction in the metal dissolution rate resulting from the interstitial carbon in the local environment of the pits and that interstitial carbon inhibited the transition from metastable pitting to stable pit growth.…”

Free‐Energy Parameterization and Thermodynamics in Si–Ge–Sn Alloys

“…In our previous work, we have shown that by calculating DFT energies for a series of supercells with changing compositions and counting the bonds between the different species in them, the different cohesive energies can be well fitted. [ 41,51 ] We have also shown that in more polar or ionic structures where charge transfer significantly affects the bond strength and the bond energy between two elements changes when other elements are present, the introduction of a bond synergy parameter can appropriately modify the bond strength as a function of composition, [ 52 ] which we, however, found to not be necessary here.…”

Review—Understanding and Controlling the Electrochemical Properties of Sulfide Inclusions for Improving the Pitting Corrosion Resistance of Stainless Steels

“…For example, carbon is known to decrease the corrosion resistance of stainless steels; however, recent studies reveal that interstitial carbon contributes to the improvement of dissolution resistance of stainless steels and inhibits pitting corrosion. [63][64][65][66] The pitting corrosion resistance of stainless steels varies with inclusion composition, and alloy design for modifying the inclusions has been found to be effective in the prevention of pitting. To conclude, a systematic investigation of the relationship between the electrochemical properties of microstructural features and the pitting corrosion resistance of stainless steels is essential to transform the experience-based knowledge into a well-established technology.…”