1. Introduction

A planar SOFC stack is a sandwich repeated dozens of times: cathode, electrolyte, anode — and between every two cells, the interconnect. The interconnect does four jobs at once: it conducts electrons laterally from one cell to the next, it separates the oxidising air of one channel from the reducing fuel of the next, it distributes the gases, and it keeps the ceramic membrane pressed flat. For thousands of hours at 600–800 °C, in air on one face and wet hydrogen on the other, against the thermal expansion of zirconia-based electrolytes. The material that meets those demands is a ferritic stainless steel containing roughly 20–24 wt% chromium, and the physics of why it works is the physics of chromium oxide.

The original interconnects were ceramic lanthanum chromite, LaCrO3, which is oxidation-proof and electrically conducting but brittle and expensive to shape. The shift to metals, led by the development of steels such as Crofer 22 APU and ZMG232, reduced interconnect cost and enabled thin stamped designs — and made the interconnect a chromium metallurgy problem. The authoritative reviews are those of Fergus (2005), Quadakkers et al. (2003), and the alloy selection study of Yang et al. (2003).

2. Why chromia formers work

The interconnect needs a surface oxide that grows slowly, sticks, conducts electricity, and does not poison the rest of the stack. Chromia, Cr2O3, is the only practical choice. It is an intrinsic p-type semiconductor with reasonable conductivity at service temperature, unlike alumina, which is an excellent protector but an electrical insulator. It grows by parabolic kinetics at a manageable rate, unlike iron oxide, which grows fast and spalls. And it is formed by chromium levels that ferritic stainless metallurgy can carry without losing workability. Aluminium and silicon, which would form insulating oxides, must therefore be kept low — a deliberate deviation from ordinary stainless practice, where aluminium-killed deoxidation is routine.

Two further requirements complete the specification. The thermal expansion coefficient of the steel must match the cell (roughly 11–12 × 10–6 K–1), which ferritic structure provides; and the scale must stay thin for the entire service life, because electrical resistance across the stack is dominated by the scale: the accepted engineering metric is the area-specific resistance, ASR, of the interconnect surface, which oxide growth pushes upward continuously with time.

Why ferritic rather than austenitic stainless? The answer is thermal expansion. Austenitic grades expand roughly 50% more than the zirconia electrolyte and would crack the stack's seals on every thermal cycle; the body-centred-cubic ferritic structure sits naturally at the cell's expansion coefficient, and chromium, as the ferrite-stabilising element par excellence, is what makes that structure possible at the 20–24 wt% level. The microstructure is therefore not incidental: the interconnect is ferritic because the stack is ceramic, and it is ferritic because it contains chromium. Every other property — the thin stamped plates, the machined gas channels, the brazed seals — follows from that choice.

3. The design compromises

The chromium content of the steel is a three-way compromise. Chromium must be high enough to guarantee a continuous external chromia scale — roughly 20 wt% and above at SOFC temperatures for reliable behaviour. It should not be so high that sigma-phase embrittlement appears during long exposure in the 600–800 °C range, which bounds practical alloys near 22–25 wt%. And it interacts with the minor-element package: manganese is added so that the outer scale becomes a (Mn,Cr)3O4 spinel with lower chromium vapour pressure than bare chromia, and small additions of titanium or lanthanum are made to bind internal oxidation and improve scale adhesion. Crofer 22 APU, the reference material, is essentially Fe–22Cr with Mn, Ti and La in carefully controlled quantities — a composition that reads like a compromise between a stainless steel and a superalloy.

The scale itself is a graded structure: chromia at the steel interface, spinel on top, both slowly thickening. Because the ASR is proportional to scale thickness divided by its conductivity, the interconnect specification is effectively a scale-management specification: the chromium supplier's role is to deliver metal whose composition allows the steelmaker to hit every minor-element window, because an excursion in silicon or aluminium creates insulating internal oxides that ruin the ASR of entire production lots.

4. Chromium volatilisation and cathode poisoning

The spinel outer layer deserves a closer look, because it is the interconnect's quiet triumph. Manganese diffuses outward through the growing chromia and reacts at the scale surface to form a continuous (Mn,Cr)3O4 spinel; the spinel's chromium activity is lower than chromia's, so the vapour pressure of chromium species above the surface falls by an order of magnitude or more, while the electrical conductivity of the scale improves at the same time. The steelmaker therefore manages three oxide layers simultaneously — chromia for protection, spinel for vapour suppression and conduction, and the underlying alloy reservoir for healing — and all three are chromium-built. A supplier whose chromium brings uncontrolled silicon or aluminium disrupts exactly this layered design, which is why interconnect steel melts are among the most exacting chromium customers in the ferrous industry.

The chromium that makes the interconnect possible also creates its most difficult problem. In the oxidising, humid air of the cathode channel, chromium at the scale surface oxidises to volatile species — chromic acid vapour, CrO2(OH)2, is the dominant carrier at SOFC temperatures. The vapour is transported to the cathode, where it deposits and reacts with the perovskite: on strontium-containing cathodes such as LSM and LSCF, chromium reacts to form SrCrO4 at the electrochemically active sites. The cathode then loses its oxygen-reduction activity — the phenomenon known as chromium poisoning — and stack voltage decays even though the interconnect itself remains healthy. The laboratory and stack evidence is summarised by Konysheva et al. (2006) and by Badwal (2001).

Mitigation operates on three fronts. The alloy is formulated so its outer scale is the manganese–chromium spinel, whose equilibrium vapour pressure of chromium species is far lower than chromia's. Coatings — applied manganese–cobalt spinels, La2O3 layers and similar — suppress volatilisation further. And the cathode itself is chosen or treated to resist the deposit. None of these measures eliminates the source; they manage it, and the management chain begins with the chromium level and the manganese balance of the interconnect steel, which are fixed at the melt.

One further subtlety belongs to the steelmaking economics. Interconnect steel is produced in modest tonnages by stainless-mill standards, yet its composition windows are tighter than most aerospace alloys, and the chromium input must therefore be of a purity that lets the melt shop hit every window without arithmetic uncertainty. A chromium carrier that brings uncontrolled carbon, nitrogen or oxygen shifts the minor-element balance in ways the steelmaker must then trim — and every trimming operation costs time and metal. The low-gas, low-impurity chromium that the superalloy market has made standard is therefore the natural input for interconnect steel too, even though the end product is a humble stamped plate: the furnace does not know whether the chromium it receives is destined for a turbine blade or a fuel-cell stack, and neither does the certificate.

The parallel with the alloy's own history is worth closing. Ferritic stainless steels existed long before the SOFC, and the interconnect grades were developed by taking those steels and tightening everything: chromium to a precise window, aluminium and silicon to floors, manganese and rare-earth additions added, and every residual specified. The result is a material that would have seemed pointlessly expensive to a 1970s stainless mill and is now indispensable to a growing energy technology. Chromium is the constant across that evolution — the element whose oxide both protects the interconnect and threatens the cathode, and whose careful management is the quiet centre of the fuel-cell materials story.

5. What interconnect steelmakers need from chromium metal

The interconnect steel is a precision material, and its chromium budget is spent on purposes the steelmaker must certify. First, the chromium content window itself is tight — a few tenths of a percent — because the scale chemistry changes with chromium activity. Second, the residual elements that compete with chromium for the surface must be suppressed: aluminium below 0.1%, silicon below 0.05% in the strictest grades, because both form insulating oxides beneath the chromia and raise the ASR irreversibly. Third, the gas content of the chromium-bearing charge matters, because nitrogen and oxygen residuals contribute to precipitates that disturb the scale. The chromium metal supplied to interconnect steelmakers — usually as refined low-gas chromium or as chromium-bearing master input in the steel melt — is therefore specified against precisely the same sixteen-element discipline as the superalloy market, with the addition of unusually tight aluminium and silicon maxima.

There is a symmetry worth noting. The same chromium that gives the interconnect its protective, conductive scale is the chromium that later poisons the cathode. The industry has not found a way around this; it has learned to manage it with spinel chemistry and coatings. Every layer of that management — the alloy, the scale, the coating — is a chromium material, and every one of them inherits its quality from the chromium metal at the beginning of the chain.

The supply side of the story is equally concrete. Interconnect steel arrives at the stack builder as thin sheet — typically a fraction of a millimetre — with a surface quality that determines scale nucleation: the chromium must be distributed so uniformly that the scale forms everywhere at once, because a locally chromium-poor patch in the strip becomes a local fast-oxidation site and a hot spot in the stack. The steelmaker therefore imposes uniformity requirements that propagate backward to the chromium input, and the chromium supplier's lot-to-lot consistency becomes a stack-lifetime parameter. It is a long chain — chromium ore to oxide to metal to steel to stamping to stack — and every link is a chromium management decision.

The intermediate-temperature SOFC, operating near 500–600 °C, sharpens the same issues: lower temperature slows spinel formation and lowers chromia conductivity, so the scale budget tightens, while the chromium poisoning window remains open. The materials response is the same layered strategy, executed more precisely — thinner, better controlled scales, coatings as standard, and chromium-tolerant cathode chemistry. The interconnect story, in short, has a beginning and a middle but no end: as long as fuel cells oxidise fuel electrochemically, something made of chromium will be separating the gases.

Standing back from the details, the SOFC interconnect is one of the cleanest demonstrations of chromium's industrial value. The element does four jobs in one component — structural support, electrical conduction, gas separation and self-protection — and its oxide does the fifth, forming the conductive scale that makes the metal viable. No substitute element is on the horizon for any of these roles, and the fuel-cell industry's materials roadmap, from present stacks to the intermediate-temperature designs of the next decade, is written around chromium-bearing ferritic steels from beginning to end.

6. Outlook

The direction of travel in SOFC development is toward lower operating temperatures and longer lives, and both trends tighten the interconnect problem: lower temperature means lower chromia conductivity and slower spinel formation, and longer life means the ASR budget must be stretched further. The responses are already visible — coated interconnects as standard, chromium-tolerant cathodes, and stack designs that reduce the air-side chromium partial pressure. What will not change is the central position of chromium: the interconnect remains a chromia-forming steel because no alternative oxide offers its combination of protection, conductivity and cost, and the fuel cell industry will keep consuming refined chromium for as long as it builds stacks.