New 'Stainless Steel for Hydrogen' Alloy Could Replace Costly Titanium in Electrolyzers

Researchers led by Professor Mingxin Huang at the University of Hong Kong's Department of Mechanical Engineering have developed a new alloy, dubbed stainless steel for hydrogen (SS-H2), designed to survive the highly corrosive, high-voltage conditions inside water electrolysis systems that produce hydrogen. The material could allow cheap steel to replace some of the expensive titanium components currently used in hydrogen production.

In water electrolysis, electricity splits water into hydrogen at the cathode and oxygen at the anode; when the electricity comes from renewable sources, the resulting hydrogen is considered "green." Seawater is an attractive water source because it is abundant and spares freshwater supplies, but its salt content is highly corrosive to metal parts, especially on the oxygen-producing anode side. Conventional stainless steel resists corrosion because chromium in the alloy forms a thin, self-healing protective oxide layer — but at around 1,000 millivolts, that layer can break down in a process called transpassive corrosion, well before the roughly 1,600-millivolt potential at which water oxidation occurs in the researchers' test conditions.

In tests using a 3.5% sodium chloride solution, roughly the salt concentration of seawater, the new SS-H2 alloy resisted corrosion up to about 1,700 millivolts, beyond where even highly corrosion-resistant conventional stainless steels, such as 254SMO, begin to fail. The alloy's composition — iron with unusually large amounts of chromium, cobalt and manganese — relies on ordinary chromium-based protection at first, but at around 720 millivolts, the manganese begins forming a second protective layer over the chromium film, a mechanism the team calls "sequential dual-passivation."

The finding was counterintuitive because manganese is traditionally considered harmful to stainless steel's corrosion resistance. "Initially, we did not believe it because the prevailing view is that Mn impairs the corrosion resistance of stainless steel," said the study's first author, Kaiping Yu. Atomic-scale analysis eventually confirmed the effect. The team also tested the alloy in a salt-water electrolyzer and reported performance comparable to titanium structural materials, which are typically used for their corrosion resistance but cost far more than steel.