New “Ultra Stainless Steel” Could Improve Green Hydrogen Production

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A research project led by Professor Mingxin Huang at the Department of Mechanical Engineering at the University of Hong Kong (HKU) has made a completely new breakthrough in relation to conventional stainless steel and the development of stainless steel for hydrogen (SS-H)2).

This marks another major achievement by Professor Huang’s team in their ‘Super Steel’ project, following the development of anti-COVID-19 stainless steel in 2021 and ultra-strong and ultra-hard Super Steel in 2017 and 2020 respectively.

The new steel developed by the team exhibits high corrosion resistance, enabling its potential use for green hydrogen production from seawater, where a new sustainable solution is still in the pipeline.

The performance of the new steel in salt water electrolyzer is comparable to the current industrial practice where titanium is used as structural parts to produce hydrogen from desalinated seawater or acid, while the price of the new steel is much cheaper.

The discovery has been published in Materials today in the paper titled “A sequential double passivation strategy for the design of stainless steels applied over water oxidation.” The research results are currently applying for patents in several countries, and two of them have already received authorization.

Since its discovery a century ago, stainless steel has always been an important material widely used in corrosive environments. Chromium is an essential element in establishing stainless steel’s corrosion resistance. Passive film is formed through oxidation of chromium (Cr) and protects stainless steel in natural environments. Unfortunately, this conventional single passivation mechanism based on Cr has stopped further development of stainless steel. Due to the further oxidation of stable Cr2O3 to soluble Cr(VI) species, transpassive corrosion inevitably occurs in conventional stainless steel at ~1000 mV (saturated calomel electrode, SCE), which is below the potential required for water oxidation at ~1600 mV .

254SMO super stainless steel, for example, is a benchmark among Cr-based anti-corrosion alloys and has superior pitting resistance in seawater; however, transpassive corrosion limits its use at higher potentials.

Using a “sequential dual-passivation” strategy, Professor Huang’s research team developed the new SS-H2 with superior corrosion resistance. In addition to the single Cr2O3-based passive layer, a secondary Mn-based layer is formed on the previous Cr-based layer at ~720 mV. The sequential double passivation mechanism prevents SS-H2 from corroding in chloride media to an ultra-high potential of 1700 mV. SS-H2 demonstrates a fundamental breakthrough compared to conventional stainless steel.

“At first we didn’t believe it, because the prevailing opinion is that Mn impairs the corrosion resistance of stainless steel. Mn-based passivation is a counterintuitive finding that cannot be explained by current knowledge in corrosion science. But when numerous atomic-level results were presented, we were convinced. Beyond being surprised, we can’t wait to exploit the mechanism,” said Dr. Kaiping Yu, the article’s first author, whose Ph.D. is supervised by Professor Huang.

From the first discovery of the innovative stainless steel to achieving a breakthrough in scientific understanding and ultimately preparing for the official publication and hopefully its industrial application, the team devoted almost six years to the work.

“Unlike the current corrosion environment, which mainly focuses on resistance at natural potentials, we specialize 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.” said Professor Huang.

At present, expensive Au or Pt-coated Ti is required for structural components of water electrolyzers in desalted seawater or acid solutions. For example, the total cost of a 10 megawatt PEM electrolysis tank system in its current phase is approximately HK$17.8 million, with the structural components contributing up to 53% of the total cost. The breakthrough by Professor Huang’s team makes it possible to replace these expensive structural components with more economical steel. As estimated, the use of SS-H2 is expected to reduce the cost of construction material by about 40 times, showing a great foreground for industrial applications.

“From experimental materials to real products, such as masks and foams, to water electrolyzers, there are still challenging tasks at hand. Currently, we have taken a big step towards industrialization. Tons of SS-H2-based wire have been produced in collaboration with a factory from the mainland, we are moving forward to use the more economical SS-H2 in hydrogen production from renewable sources,” Professor Huang added.

Reference: Yu K, Feng S, Ding C, Gu M, Yu P, Huang M. A sequential double passivation strategy for the design of stainless steels applied over water oxidation. Materials today. 2023:S1369702123002390. doi: 10.1016/j.mattod.2023.07.022

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