“Cannot be explained” – New ultra stainless steel stuns researchers

Led by Professor Mingxin Huang in HKU's Department of Mechanical Engineering, the team developed a special stainless steel for hydrogen production (SS-H₂). The material resists corrosion under conditions that normally push stainless steel past its limits, making it a promising candidate for producing hydrogen from seawater and other harsh electrolyzer environments.

The discovery, reported in Materials Today in the study "A sequential dual-passivation strategy for designing stainless steel used above water oxidation," builds on Huang's long running "Super Steel" Project. The same research program previously produced anti-COVID-19 stainless steel in 2021, along with ultra strong and ultra tough Super Steel in 2017 and 2020.

A Cheaper Path Toward Green Hydrogen

Green hydrogen is made by using electricity, ideally from renewable sources, to split water into hydrogen and oxygen. Seawater is an especially tempting feedstock because it is abundant, but it brings a serious materials problem: salt, chloride ions, side reactions, and corrosion can quickly damage electrolyzer components.

Recent reviews of direct seawater electrolysis continue to highlight the same core challenge. The technology could provide a more sustainable route to hydrogen, but corrosion, chlorine related side reactions, catalyst degradation, precipitates, and limited long term durability remain major obstacles to commercial use.

That is where SS-H₂ could matter. In a salt water electrolyzer, the HKU team found that the new steel can perform comparably to the titanium based structural materials used in current industrial practice for hydrogen production from desalted seawater or acid. The difference is cost. Titanium parts coated with precious metals such as gold or platinum are expensive, while stainless steel is far more economical.

For a 10 megawatt PEM electrolysis tank system, the total cost at the time of the HKU report was estimated at about HK$17.8 million, with structural components making up as much as 53% of that expense. According to the team's estimate, replacing those costly structural materials with SS-H₂ could reduce the cost of structural material by about 40 times.

Why Ordinary Stainless Steel Fails

Stainless steel has been used for more than a century in corrosive environments because it protects itself. The key ingredient is chromium. When chromium (Cr) oxidizes, it creates a thin passive film that shields the steel from damage.

But that familiar protection system has a built in ceiling. In conventional stainless steel, the chromium based protective layer can break down at high electrical potentials. Stable Cr₂O₃ can be further oxidized into soluble Cr(VI) species, causing transpassive corrosion at around ~1000 mV (saturated calomel electrode, SCE). That is well below the ~1600 mV needed for water oxidation.

Even 254SMO super stainless steel, a benchmark chromium based alloy known for strong pitting resistance in seawater, runs into this high voltage limit. It may perform well in ordinary marine settings, but the extreme electrochemical environment of hydrogen production is a different challenge.

The Steel That Builds a Second Shield

The HKU team's answer was a strategy called "sequential dual-passivation." Instead of relying only on the usual chromium oxide barrier, SS-H₂ forms a second protective layer.

The first layer is the familiar Cr₂O₃ based passive film. Then, at around ~720 mV, a manganese based layer forms on top of the chromium based layer. This second shield helps protect the steel in chloride containing environments up to an ultra high potential of 1700 mV.

That is what makes the finding so striking. Manganese is usually not viewed as a friend of stainless steel corrosion resistance. In fact, the prevailing view has been that manganese weakens it.

"Initially, we did not believe it because the prevailing view is that Mn impairs the corrosion resistance of stainless steel. Mn-based passivation is a counter-intuitive discovery, which cannot be explained by current knowledge in corrosion science. However, when numerous atomic-level results were presented, we were convinced. Beyond being surprised, we cannot wait to exploit the mechanism," said Dr. Kaiping Yu, the first author of the article, whose PhD is supervised by Professor Huang.

A Six Year Push From Surprise to Application

The path from the first observation to publication was not quick. The team spent nearly six years moving from the initial discovery of the unusual stainless steel to the deeper scientific explanation, then toward publication and potential industrial use.

"Different from the current corrosion community, which mainly focuses on the resistance at natural potentials, we specializes in developing high-potential-resistant alloys. Our strategy overcame the fundamental limitation of conventional stainless steel and established a paradigm for alloy development applicable at high potentials. This breakthrough is exciting and brings new applications," Professor Huang said.

The work has also moved beyond the laboratory. The research achievements have been submitted for patents in multiple countries, and two patents had already been granted authorization at the time of the HKU announcement. The team also reported that tons of SS-H₂ based wire had been produced with a factory in Mainland China.

"From experimental materials to real products, such as meshes and foams, for water electrolyzers, there are still challenging tasks at hand. Currently, we have made a big step toward industrialization. Tons of SS-H₂-based wire has been produced in collaboration with a factory from the Mainland. We are moving forward in applying the more economical SS-H₂ in hydrogen production from renewable sources," added Professor Huang.

Why the Timing Still Matters

Although the SS-H₂ study was published in 2023, its core problem has only become more relevant. Newer seawater electrolysis research continues to focus on the same bottlenecks: corrosion resistant materials, long lasting electrodes, chlorine suppression, and system designs that can survive real seawater rather than ideal laboratory solutions. A 2025 Nature Reviews Materials review described direct seawater electrolysis as promising but still held back by corrosion, side reactions, metal precipitates, and limited lifetime.

Other recent work has explored stainless steel based electrodes with protective catalytic layers, including NiFe based coatings and Pt atomic clusters, to improve durability in natural seawater. Researchers have also reported corrosion resistant anode strategies built on stainless steel substrates, showing that stainless steel remains a major focus in the effort to make seawater electrolysis more practical.

This newer research does not replace the SS-H₂ discovery. Instead, it reinforces why the HKU team's approach is important. The field is still searching for materials that can survive the punishing mix of saltwater chemistry, high voltage, and industrial operating demands. SS-H₂ stands out because it attacks the problem not only with a coating or catalyst, but with a new alloy design strategy that changes how stainless steel protects itself.

A Steel Breakthrough With Clean Energy Potential

SS-H₂ is not yet a plug and play solution for the hydrogen economy. The team has acknowledged that turning experimental materials into real electrolyzer products, including meshes and foams, still involves difficult engineering work.

Even so, the promise is clear. A stainless steel that can withstand high voltage seawater conditions while replacing expensive titanium based components could make hydrogen production cheaper, more scalable, and easier to pair with renewable energy.

For a field where cost and durability often decide whether a technology can leave the lab, a steel that builds its own second shield may be more than a materials science surprise. It could become a practical step toward cleaner hydrogen at industrial scale.