HKU Develops Low-Cost Stainless Steel for Green Hydrogen from Seawater

A new stainless steel alloy uses a unique dual-protection layer to withstand the harsh conditions of seawater electrolysis, potentially slashing material costs by 40 times.

Researchers at the University of Hong Kong (HKU) have developed a new stainless steel alloy, named SS-H₂, specifically designed to resist corrosion in the harsh electrochemical environment of seawater electrolysis for green hydrogen production.

The alloy employs a sequential dual-passivation strategy, forming a protective manganese-based layer in addition to the conventional chromium oxide layer. This allows it to withstand potentials up to 1700 mV in chloride-containing environments—a critical threshold for water oxidation.

Key Details

• Material: SS-H₂, developed by Professor Mingxin Huang's team in HKU's Department of Mechanical Engineering.
• Mechanism: A Cr₂O₃-based layer forms first, followed by a Mn-based layer at around 720 mV, providing protection up to 1700 mV.
• Application: Direct seawater electrolysis for hydrogen production, performing comparably to titanium-based materials at a significantly lower cost.
• Cost Impact:
  • In a 10 MW PEM electrolyzer, structural components account for ~53% of total cost (~HK$17.8 million).
  • Replacing titanium with SS-H₂ could reduce structural material costs by approximately 40 times.
• Patent Status: Multiple patents filed; two granted. Tons of SS-H₂-based wire have already been produced in collaboration with a mainland Chinese factory.
• Publication: The study, titled "A sequential dual-passivation strategy for designing stainless steel used above water oxidation," was published in Materials Today (2023).

The Breakthrough

Conventional stainless steel fails at high potentials (~1000 mV) due to transpassive corrosion of its chromium oxide layer. Even advanced super stainless steel 254SMO cannot withstand the ~1600 mV required for water oxidation. SS-H₂ overcomes this fundamental limitation by forming a dual passivation layer that remains stable at much higher voltages.

Researcher Perspectives

"Initially, we did not believe it because the prevailing view is that Mn impairs the corrosion resistance of stainless steel."
— Dr. Kaiping Yu, first author

"Our strategy overcame the fundamental limitation of conventional stainless steel and established a paradigm for alloy development applicable at high potentials."
— Professor Mingxin Huang, HKU Department of Mechanical Engineering

Broader Context

Direct seawater electrolysis remains a major challenge due to corrosion, chlorine side reactions, catalyst degradation, and limited durability. SS-H₂ offers an alloy design approach to improve durability and reduce costs, though further engineering is needed to integrate it into commercial electrolyzer products.