New Stainless Steel Green Hydrogen Breakthrough: SS-H2

Stainless steel isn't rust-resistant because it is chemically inert; in fact, it resists rust since it reacts in a controlled and predictable manner which helps to protect it. Chromium, which is contained in stainless steel at a concentration usually over 10.5 per cent, reacts with oxygen to form chromium oxide (Cr2O3), a very thin film - only a few nanometres thick - that sticks firmly to the surface of the metal and prevents further oxygen and chloride ions from getting to the metal underneath. This process is known as passivation, and the oxide layer that is produced is called the passive film. It is the same fundamental chemistry that A-level students study when looking at reactivity and oxidation, but it has been designed so that it is strong enough to be used in constructing kitchens, bridges and chemical plants.

Passivation only functions within a particular range of electrochemical potential, that is to say within the voltage difference between the metal's surface and the surrounding solution; if that potential is raised high enough then the passive film itself may start to break down, a phenomenon known as transpassive corrosion in which the protective oxide either dissolves or becomes unstable instead of protecting the metal. This is the electrochemical analogue of a fire door that works perfectly at ordinary temperatures but fails when it gets too hot. The passive film of conventional stainless steel breaks down at about 1000 millivolts, which thus sets a strict upper limit on the conditions under which it can be used.

The process of water electrolysis - which is the method employed to produce green hydrogen by splitting water molecules into hydrogen and oxygen with the aid of an electric current - requires voltages of about 1600 millivolts if it is to operate efficiently on an industrial scale, a figure that results from the thermodynamics of the water-splitting reaction together with the extra overpotential that is necessary to make the reaction proceed at a useful rate. There thus exists a gap of several hundred millivolts between the voltage required by electrolysis and that which ordinary stainless steel can withstand, a gap which SS-H2 specifically addresses.

As the researchers currently understand the mechanism of SS-H2, it includes a second passivation event. When the applied potential rises above the level at which ordinary passive films would start to fail, manganese in the alloy produces its own protective compound at the surface, placing this compound either on top of or alongside the existing chromium oxide and thus extending the range over which the metal remains passivated. The fact that manganese, an element which has hitherto been associated with promoting rather than preventing corrosion in stainless steels, acts in this way is still not fully accounted for by existing corrosion theory, which is the very reason why Dr Yu referred to the result as counter-intuitive rather than merely an incremental improvement.

This is relevant to anyone considering a career in materials engineering since it serves as a reminder that even though corrosion science is a well-developed field possessing established tools such as Pourbaix diagrams - which show which reactions are dominant at a given pH and potential - there is still room for true surprises. The mechanism known as sequential dual-passivation will need to be further characterised, probably by using techniques such as X-ray photoelectron spectroscopy in order to determine exactly what compounds form at the surface and in what amounts, before textbooks can be rewritten. For chemical and materials engineers, SS-H2 also shows a more general lesson in that cost reduction in emerging industries such as green hydrogen does not always result from coming up with a completely new process; at times it comes about by getting an old and well-understood material to endure conditions for which it was never intended.