A research project led by Professor Minxin Huang from the Department of Mechanical Engineering at the University of Hong Kong (HKU) has achieved a breakthrough in conventional stainless steel and the development of stainless steel for hydrogen (SS-H2). This marks a major achievement in Professor Huang’s team’s Supersteel project, following the development of stainless steel for protection against COVID-19 in 2021 and super-durable and super-strong super steel in 2017 and 2020, respectively.

The new steel developed by the team shows high corrosion resistance, allowing it to be potentially applied in the production of green hydrogen from seawater, where a new sustainable solution is still being developed. The new steel’s performance in a brine electrolyzer is comparable to current industrial practice, where titanium is used as structural parts to produce hydrogen from desalinated seawater or acid, while the new steel costs much less.

The discovery was published on: Today’s Ingredients In an article titled “Sequential Double Passivation Strategy for the Development of Stainless Steels Used on Water Oxidation.” The research is currently filed for patents in several countries, two of which have already been approved.

Stainless steel has always been an important material widely used in corrosive environments since its discovery a century ago. Chromium is an important element in determining the corrosion resistance of stainless steel. The passive film is formed by the oxidation of chromium (Cr) and protects stainless steel in the natural environment. Unfortunately, this common Cr-based one-time passivation mechanism has stalled the further development of stainless steel.

Due to further oxidation of stable Cr₂HE₃ Based on the soluble forms of Cr(VI), transpassive corrosion inevitably occurs in conventional stainless steel at ~1000 mV (saturated calomel electrode, SCE); this is below the potential required for water oxidation at ~1600 mV.

For example, super stainless steel 254SMO is the benchmark among Cr-based anti-corrosion alloys and has excellent resistance to spot correction in seawater; however, transpassive corrosion limits its application at higher potentials.

Professor Huang’s research group developed a new SS-H2 with excellent corrosion resistance using the “sequential double passivation” strategy. In addition to a Cr-based passive layer₂HE₃, the secondary Mn-based layer is formed on the previous Cr-based layer at ∼720 mV. The sequential double passivation mechanism prevents the corrosion of SS-H2 in chloride environments up to ultra-high potential of 1700 mV. SS-H2 shows a fundamental breakthrough compared to traditional stainless steel.

“At first we did not believe it because the prevailing view was that manganese worsened the corrosion resistance of stainless steel. Manganese-based passivation is a counterintuitive discovery that cannot be explained by current knowledge in corrosion science. However, when numerous high-level atomic results were presented, we were convinced. We are not only amazed but eager to use the mechanism,” he said. Dr. Kaiping Yu, first author of the paper and a Ph.D., under the guidance of Professor Huang.

From the initial discovery of the innovative stainless steel to achieving a breakthrough in scientific understanding and ultimately preparing for formal publication and hopefully industrial application, the team devoted nearly six years to this work.

“Unlike the current corrosion community, which generally focuses on resistance at inherent potentials, we specialize in the development of alloys that are resistant to high potentials. Our strategy has overcome the fundamental limitations of conventional stainless steels and created a paradigm for the development of alloys suitable for high potentials. This breakthrough brings exciting and new applications.” ” Professor Huang said.

Currently water electrolyzers in desalinated seawater or acidic solutions require expensive Au or Pt-coated Ti for structural components. For example, the total cost of a 10 MW PEM electrolysis tank system is currently approximately HK$17.8 million, with structural components accounting for 53% of the total cost.

The discovery made by Professor Huang’s team makes it possible to replace these expensive structural components with more economical steel. It is estimated that the use of SS-H2 is expected to reduce the cost of structural material by approximately 40 times, demonstrating excellent best practices in industrial applications.

“From experimental materials to real products such as meshes and foams for water electrolyzers, there are still challenges ahead. So far we have made a big step towards industrialization. Tons of SS-H2-based wires have been produced in cooperation with a factory on the mainland. More economical in producing hydrogen from renewable sources “We are making progress in implementing SS-H2,” added Professor Huang. Source