Molten Salt Electrochemistry Turns CO2 Into Carbon That Steels Itself Away

Steel is the backbone of modern civilization, but making it remains one of the largest industrial sources of carbon dioxide on the planet. As hydrogen-based direct reduction, electrolytic reduction, hydrogen plasma smelting, and molten oxide electrolysis edge toward commercial deployment, a curious problem has emerged: these carbon-free technologies produce iron with essentially zero carbon content, while the blast furnaces they are meant to replace yield pig iron loaded with 3 to 5 weight percent carbon. That gap matters enormously downstream, because electric arc furnaces, which account for roughly 70 percent of United States steel production, rely on carbon dissolved in their feedstock for chemical energy, for reducing iron oxide, and for foaming slag to protect their graphite electrodes. A team of researchers at the National Laboratory of the Rockies, working with a colleague at Carnegie Mellon University, now reports a strikingly elegant fix: an electrochemical process that pulls carbon straight out of carbon dioxide and drives it into mild steel, all inside a bath of molten salt.

The study, published in Discover Electrochemistry, demonstrates what the authors call electro-carburization through a two-step reaction. In the first step, carbonate ions in a molten equimolar lithium-potassium carbonate electrolyte are reduced at the steel cathode, depositing solid carbon and releasing oxygen anions. Gaseous carbon dioxide bubbled through the salt at 150 standard cubic centimeters per minute then dissolves and reacts with those oxygen anions to regenerate carbonate, closing the chemical loop. At the tin oxide anode, oxygen or carbon dioxide gas evolves, so the net reaction across the cell is simply the decomposition of carbon dioxide into solid carbon and oxygen. In the second step, the freshly deposited carbon atoms diffuse into the steel through vacancies, interstitial sites, and grain boundaries, enriching the iron matrix until it saturates and precipitates the hard carbide phase known as cementite, or iron carbide.

The choice of electrolyte was no accident. The lithium-potassium carbonate mixture melts at just 503 degrees Celsius, allowing experiments at 600 to 800 degrees Celsius, a range where previous work has shown that carbon formation is favored over the production of carbon monoxide. Lithium itself plays a decisive role: unlike sodium or potassium, whose metallization competes with carbonate reduction in mixed salts, lithium deposition is thermodynamically less favorable, so carbon emerges as the dominant cathodic product. That chemistry matters for anyone hoping to run this process industrially, because stray metal deposition would contaminate the product and waste electrical energy. The measured Faradaic efficiency for carbon deposition reached 75 percent in the team’s tests, a respectable figure for a high-temperature molten salt system.

Temperature proved to be the master variable governing what happens inside the steel. The researchers treated rods of AISI 1018 mild steel, nominally containing 0.18 weight percent carbon, at three bath temperatures: 615, 715, and 805 degrees Celsius. These temperatures bracket the eutectoid point of the iron-carbon system, the critical threshold near 727 degrees Celsius where ferrite transforms into austenite. Ferrite, the body-centered cubic phase of iron that dominates mild steel at room temperature, dissolves almost no carbon, at most 0.022 weight percent. Austenite, its face-centered cubic counterpart stable at higher temperatures, can hold up to 2.14 weight percent. At 615 degrees Celsius, below the eutectoid, the treated steel showed only sparse cementite precipitates, because the ferritic matrix simply could not absorb the incoming carbon. At 715 and 805 degrees Celsius, however, carbon dissolved readily into austenite, and upon slow cooling that carbon-saturated region transformed into pearlite, the distinctive lamellar mixture of ferrite and cementite that gives many steels their strength.

The microstructural evidence was captured in remarkable detail. The team sectioned the treated rods radially, polished them to a mirror finish with ion milling, and etched them in sodium metabisulfite to reveal carbides and grain boundaries under the scanning electron microscope. Energy dispersive X-ray mapping confirmed that the bright, etch-resistant features were carbon-rich. Near the surface of electrodes treated above the eutectoid temperature, pearlite zones dominated, while deeper inside the material the pearlite retreated to the grain boundaries, tracing the paths along which carbon had migrated. This grain boundary decoration is a signature of short-circuit diffusion, in which carbon races along the interconnected network of boundaries far faster than it can creep through the crystal lattice itself, saturating the boundaries and precipitating cementite there upon cooling.

The most dramatic finding, however, came from tuning the applied cell voltage. Running the cell at minus 2.4 volts, the researchers deposited carbon far faster than it could diffuse into the steel. The result was a porous carbon-rich layer more than a millimeter thick, roughly 1.1 millimeters, encrusted with entrapped salt that X-ray diffraction identified as lithium-potassium carbonate, lithium carbonate, lithium oxide, and some graphite. The deposit was roughly 25 percent porous, built of amorphous carbon sheets with salt particles scattered throughout. At minus 1.8 volts, the opposite regime prevailed: carbon deposition was slow enough that thermochemical diffusion consumed essentially every atom as it arrived, leaving only a thin residual layer about 130 micrometers thick, mostly salt, with minimal carbon left on the surface.

Those two regimes, diffusion-limited and deposition-limited, translate directly into different steel microstructures. Electrodes held at minus 2.4 volts for 28 minutes at 715 degrees Celsius developed a heavily carburized zone extending roughly 30 micrometers inward, filled with fine pearlite whose lamellae were spaced less than a micrometer apart. Beyond about 40 micrometers, the structure transitioned back to a ferritic matrix, though grain boundary cementite persisted much deeper. Electrodes held at minus 1.8 volts showed a thinner pearlite-rich skin of about 20 micrometers, with pearlite colonies decorating grain boundaries down to 150 micrometers and beyond. Quantitative image analysis of backscatter electron micrographs confirmed that both electrochemical conditions produced significantly more cementite than control samples that were either untreated or merely heat-treated in carbon dioxide without salt or applied voltage.

The numbers align well with theory. The expected diffusion distance for carbon in low-carbon steel at 715 degrees Celsius over 28 minutes is about 28 micrometers, matching the observed depth of the heavily carburized layer at the higher voltage. The steadily rising current at minus 2.4 volts, climbing from 730 to 780 milliamperes over the test, reflects the growing effective surface area as carbon accumulated on the cathode, while the current at minus 1.8 volts stabilized near 65 milliamperes after the initial double-layer charge. Cyclic voltammetry revealed three reduction peaks, including one attributed to transition metals in the steel that vanished once the electrode became encapsulated in its own carbon coating, cutting the electrolyte off from the metal surface.

Why does this matter beyond the laboratory? Pre-carburizing green iron or scrap steel before it enters an electric arc furnace would lower the melting point of the charge, since carbon-free iron melts nearly 300 degrees Celsius higher than carbon-rich pig iron, slowing furnace startup and inflating energy consumption. Dissolved carbon also delivers chemical energy through oxidation to carbon monoxide, a contribution that accounts for roughly 30 percent of a typical furnace’s energy input, and it eliminates the enthalpic penalty of dissolving lump coal or coke added at the melt. Unlike conventional carburizing agents, the electrochemical route introduces no sulfur or ash impurities, and the excess carbon deposited at high voltages could even be harvested as a solid carbon product. The researchers note that temperatures near the eutectoid, around 715 degrees Celsius, achieved carburization comparable to 805 degrees Celsius while sparing the tin oxide anode from the thermal shock and degradation seen at the higher temperature.

Challenges remain before molten salt electro-carburization can feed gigaton-scale steelmaking. The team worked with small two-millimeter rods, and electrode geometry, scale-up, salt handling, and long-term anode stability all await study. Yet the demonstration that a single dial, the applied cell voltage, can switch the process between coating deposition and deep carburization gives engineers a level of control that conventional thermochemical treatments lack. As the steel industry races to reconcile its carbon-free future with the carbon-hungry furnaces of the present, this two-step dance of electrochemistry and diffusion in a vat of molten salt offers a compelling bridge, one that turns the industry’s own waste gas into the very ingredient its next-generation feedstocks are missing.

Subject of Research: Electrochemical carburization of mild steel using molten carbonate salt electrolysis with carbon dioxide as the carbon source

Article Title: Electro-carburization of mild steel through two-step reaction in molten-salt electrolyte

Article References: Pennell, S., Wu, I., Hoover, H., Olushina, O., Vidal, J., Webler, B., Bell, R., & Rippy, K. (2026). Electro-carburization of mild steel through two-step reaction in molten-salt electrolyte. Discover Electrochemistry, 3(1), Article 30. https://doi.org/10.1007/s44373-026-00121-6

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00121-6

Keywords: electro-carburization, molten carbonate salt, mild steel, carbon dioxide conversion, cementite, pearlite, electric arc furnace, green steel, carbon diffusion, electrochemistry, iron-carbon phase diagram, carburization