Experimental steel resists corrosion at 1,700 mV in salt water

A 3.5% sodium chloride solution—roughly the salt concentration of seawater—gave researchers at the University of Hong Kong a hostile test for a new alloy. SS-H₂, short for stainless steel for hydrogen, resisted corrosion at electrochemical potentials up to about 1,700 mV, above the approximately 1,600 mV water-oxidation potential under the team’s test conditions. Led by Prof. Mingxin Huang, the team is targeting a material that could make some hydrogen-production hardware less dependent on titanium.

An electrolyzer uses electricity to split water: hydrogen forms at one electrode, the cathode, and oxygen at the other, the anode. When renewable electricity powers the process, the hydrogen can have very low operational carbon emissions. Seawater is an attractive potential feedstock because it avoids using increasingly valuable freshwater, but its chloride ions readily attack alloys, while the anode faces strongly oxidizing conditions.

Conventional stainless steel has a built-in defense. Chromium reacts at the surface to form an extremely thin chromium-oxide film that separates the metal from its surroundings and can reform after damage. The protection has a limit, though: at around 1,000 mV, the film can begin producing soluble Cr(VI) species, a process known as transpassive corrosion. Even 254SMO, a stainless steel engineered for seawater resistance, meets that limitation at sufficiently high potentials. Titanium components, sometimes protected with gold or platinum coatings, tolerate the conditions better but cost more than steel.

Huang’s team gave stainless steel a second protective mechanism. SS-H₂ contains iron, 20.73% chromium, 20.2% cobalt, 17.7% manganese and 1.7% silicon by weight. At around 720 mV, manganese begins forming a second protective layer over the chromium-rich film. As the chromium defense approaches the point where it normally breaks down, the manganese-based layer takes over. Dr. Kaiping Yu, the study’s first author, said the result initially challenged the prevailing view that manganese harms stainless steel’s corrosion resistance.

So what changes, concretely? If the behavior holds beyond the researchers’ tests, SS-H₂ could allow electrolyzer designers to replace some expensive titanium structural components with steel, potentially lowering the cost of green-hydrogen systems and making salt-water operation more practical. The team has demonstrated the material in a salt-water electrolyzer, but the result remains experimental.