A team led by Professor Mingxin Huang at the Department of Mechanical Engineering of the University of Hong Kong (HKU) has developed a strategy to enhance the corrosion resistance of stainless steel, enabling its potential application for green hydrogen production from seawater. A paper on the work is published in the journal Materials Today.

Since its discovery a century ago, stainless steel has always been the important material widely used in corrosive environments. Cr-based passive films play an essential role in establishing the anti-corrosion property of stainless steel. However, stainless steel developed from such a single-passivation strategy often suffers from either pitting or transpassive corrosion, resulting in corrosion-induced failure of many infrastructures and therefore large economic loss in various industrial sectors.

… Here, we show that, by engaging a new “sequential dual-passivation” strategy, it is feasible to overcome the fundamental limitation of the single-passivation mechanism, and thereby further enhance the corrosion resistance of stainless steel.

Specifically, the low-potential-passivated Cr and the high-potential-passivated Mn construct a continuous-growing passive film that prevents corrosion in chloride media. While Mn has great benefits in developing advanced alloys and functional oxides, it is often considered a harmful alloying element that deteriorates the anti-corrosion properties of stainless steel. Here, we also show that the detrimental Mn can be turned into profitable by passivating before the preceding Cr-based layer decomposes, hence preventing its transpassive dissolution and dramatically increasing the protective potential. Such a fascinating combination makes our stainless steel exceptionally corrosion-resistant.

—Yu et al.

Yu et al.

The performance of the new steel in salt water electrolyzer is comparable to the current industrial practice using titanium as structural parts to produce hydrogen from desalted seawater or acid, while the cost of the new steel is much less.

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 the corrosion resistance of stainless steel. Passive film is generated through the oxidation of chromium (Cr) and protects stainless steel in natural environments. Unfortunately, this conventional single-passivation mechanism based on Cr has halted further advancement of stainless steel. Owing to the further oxidation of stable Cr₂O₃ into soluble Cr(VI) species, tranpassive 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 instance, is a benchmark among Cr-based anti-corrosion alloys and has superior pitting resistance in seawater; however, transpassive corrosion limits its application at higher potentials.

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

From the initial 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 nearly six years to the work.

Currently, water electrolyzers in desalted seawater or acid solutions require expensive Au- or Pt-coated structural components. The total cost of a 10-megawatt PEM electrolysis tank system in its current stage is approximately HK$17.8 million (US$2.3 million), with the structural components contributing up to 53% of the overall expense, the researchers said.

The development made by Professor Huang’s team makes it possible to replace these expensive structural components with more economically steel. As estimated, the employment of SS-H₂ is expected to cut the cost of structural material by about 40 times.

Resources

• Kaiping Yu, Shihui Feng, Chao Ding, Meng Gu, Peng Yu, Mingxin Huang (2023) “A sequential dual-passivation strategy for designing stainless steel used above water oxidation,” Materials Today doi: 10.1016/j.mattod.2023.07.022