A team led by Professor Huang Mingxin from the Department of Mechanical Engineering at the University of Hong Kong has made significant progress in the field of stainless steel. This latest innovation focuses on the development of a stainless steel specifically designed for hydrogen applications, known as SS-H2. This achievement is part of Professor Huang's ongoing "Super Steel" project, which has achieved significant phased results in creating COVID-19-resistant stainless steel in 2021 and developing super-strong, super-tough super steel in 2017 and 2020.
The new steel developed by the team is highly resistant to corrosion and therefore has potential applications in producing green hydrogen from seawater, a new sustainable solution that is still in the pipeline.
The performance of the new steel in salt water electrolysers is comparable to current industrial practice of using titanium as structural components to produce hydrogen from desalinated seawater or acid, yet the cost of the new steel is much cheaper. The findings have been published in the journal Materials Today. The research results are currently applying for patents in multiple countries, two of which have been authorized.
Revolutionary Advances in Corrosion Resistance
Since its discovery a century ago, stainless steel has been an important material widely used in corrosive environments. Chromium is an important element in ensuring the corrosion resistance of stainless steel. A passive film created by the oxidation of chromium (Cr) protects stainless steel in the natural environment. Unfortunately, this traditional single passivation mechanism based on chromium has hindered the further development of stainless steel. Due to the further oxidation of stable Cr2O3 into soluble hexavalent chromium, traditional stainless steel inevitably suffers from lateral passive corrosion at a voltage of about 1000mV (saturated calomel electrode, SCE), which is lower than the potential required for water oxidation of about 1600mV.
For example, 254SMO super stainless steel is the benchmark among chromium-based anti-corrosion alloys and has excellent resistance to pitting corrosion in seawater; however, phase transition corrosion limits its application at higher potentials.
Professor Huang's research team adopted the "sequential double passivation" strategy to develop a new type of SS-H2 with excellent corrosion resistance. In addition to a single Cr2O3-based passive layer, a second Mn-based passive layer is formed on the previous Cr-based passive layer at ~720mV. This continuous dual passivation mechanism prevents SS-H2 from corroding in chloride media to an ultra-high potential of 1700mV. Compared with traditional stainless steel, SS-H2 has achieved a fundamental breakthrough.
Unexpected discoveries and potential applications
"At first, we were not convinced because manganese is generally believed to impair the corrosion resistance of stainless steel. Manganese-based passivation was a counter-intuitive finding that could not be explained by existing knowledge of corrosion science. However, when a large number of atomic-level results were presented to us, we were convinced. In addition to being surprised, we could not wait to exploit this mechanism," said Dr. Yu Kaiping, the first author of the article and a doctoral student supervised by Professor Huang.
The team invested nearly six years in this work, from the initial discovery of this innovative stainless steel, to achieving a breakthrough in scientific understanding, to finally preparing it for formal publication and potential industrial applications.
"Unlike the current corrosion community, which mainly focuses on corrosion resistance at natural potentials, we focus on developing high-potential corrosion-resistant alloys. Our strategy overcomes the fundamental limitations of traditional stainless steels and establishes a paradigm for alloy development suitable for high potentials. This breakthrough is exciting and leads to new applications." Professor Huang said.
Currently, electrolysers for desalinating seawater or water in acidic solutions require expensive gold or platinum-coated titanium as structural components. For example, the current total cost of a 10MW PEM electrolyzer system is approximately HK$17.8 million, of which structural components account for 53% of the total cost. The breakthrough achieved by Professor Huang's team makes it possible to replace these expensive structural components with more economical steel. It is estimated that the application of SS-H2 is expected to reduce the cost of structural materials by approximately 40 times, showing great industrial application prospects.
"From experimental materials to actual products, such as grids and foams for water electrolyzers, the task at hand is still full of challenges. Currently, we have taken a big step towards industrialization. We have cooperated with a factory in the mainland to produce several tons of SS-H2-based wire." Professor Huang added: "We are working on applying the more economical SS-H2 to hydrogen production from renewable resources."
Compiled source: ScitechDaily