November 25, 2023 | Researchers at the University of Hong Kong have made a breakthrough in developing a stainless steel for hydrogen applications.
Under the leadership of Professor Mingxin Huang at the Department of Mechanical Engineering at the University of Hong Kong (HKU), the new steel “SS-H2” (stainless steel for hydrogen) with high corrosion resistance was developed. This characteristics makes it ideal for use in plants which are designed to produce hydrogen from seawater.
The discovery was published in the journal Materials Today under the title “A sequential dual-passivation strategy for designing stainless steel used above water oxidation”. The research accomplishments are currently undergoing patent applications in numerous countries, with authorization already granted for two of them.
New steel with great potential for industrial application
Currently, expensive gold- or platinum-coated titanium components are required for water electrolyzers in desalinated seawater or acidic solutions. The new developed steel SS-H2 allows expensive structural components to be replaced with cheaper steel. It is estimated that the use of the new steel can reduce structural material costs by a factor of 40, showing great potential for industrial applications.
“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,” explains Professor Huang.
The new steel’s performance in saltwater electrolyzers is comparable to today’s industrial applications that use titanium as a structural component to produce hydrogen from desalinated seawater or acid. However, the cost of the new steel is significantly lower.
Counter-intuitive discovery
Stainless steel is an important material widely used in corrosive environments. In this context, chromium is an essential element for corrosion resistance, as the oxidation of chromium forms a passive film that protects stainless steel in natural environments. However, as Professor Mingxin Huang’s team has shown, relying on this passivation mechanism is a mistake.
The stainless super steel 254SMO, as an example, has excellent resistance to pitting in seawater. But its application is limited due to transpassive corrosion. Conventional stainless steels experience transpassive corrosion at 1000 mV, which is below the required potential of 1600 mV for water oxidation.
Double passivation leads to higher corrosion resistance
The Chinese researchers have succeeded in preventing corrosion of their steel in media containing chloride up to a potential of 1700 mV using “sequential double passivation”. During double passivation, in addition to the passive layer based on chromium oxide, a secondary layer based on manganese is formed at 720 mV.
SS-H2 represents a fundamental breakthrough over traditional stainless steel due to this superior corrosion resistance.
“Initially, we did not believe it because the prevailing view is that manganese impairs the corrosion resistance of stainless steel. Manganese-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 Ph.D. is supervised by Professor Huang.
From the discovery of stainless steel to the preparation for official publication and hopefully industrial application, the team invested almost six years of work.
Further information: Kaiping Yu et al., A sequential dual-passivation strategy for designing stainless steel used above water oxidation, Materials Today (2023). DOI: 10.1016/j.mattod.2023.07.022