When hydrogen meets steel

On the night of September 11, 2024, an approximately 100-meter-long section of the Carola Bridge in Dresden collapsed into the Elbe River. The culprit: cracks in the bridge's steel tension structure caused by hydrogen. The Carola Bridge is by no means the first building to be affected by hydrogen. Other well-known examples include the London skyscraper 122 Leadenhall Street, popularly known as the Cheesegrater, and the partial replacement of the Bay Bridge in San Francisco. In both cases, failure of the steel bolts resulted in renovation costs running into millions.

The process is known as hydrogen embrittlement. Certain corrosion processes in the presence of water release atomic hydrogen – the smallest element in the periodic table – at the surface of steel components. Owing to its small size, hydrogen easily diffuses into the steel, promoting crack formation and propagation.

The fact that hydrogen attacks metals has been known since the 19th century. However, the complex mechanisms behind hydrogen embrittlement are still not fully understood – despite a plethora of studies. Empa researchers from the Joining Technology and Corrosion Laboratory are now investigating an aspect of hydrogen embrittlement that has received very little attention to date: the interaction of hydrogen with the so-called native oxide layer on steel.

The native oxide layer, also known as the passivation layer, is a thin layer that forms naturally on the surface of the majority of metals and alloys. It is what gives stainless steels their corrosion resistance. The type and composition of this layer, which is only a few nanometers thick, differs from steel to steel. Some oxides are significantly more stable and resistant to hydrogen than others, thus shielding the steel from embrittlement. This is what Empa researchers Chiara Menegus and Claudia Cancellieri set out to investigate. They are focusing specifically on the interface between the metal and its oxide layer. “Hydrogen accumulates in regions where the material is disordered,” explains doctoral student Menegus. “The metal-oxide interface is one such area.”

Chiara Menegus and Claudia Cancellieri at Empa's HAXPES system. Image: Empa

HAXPES, short for Hard X-ray Photoelectron Spectroscopy, is an analysis method based on the photoelectric effect, for the discovery of which Albert Einstein was awarded the Nobel Prize in Physics in 1921. Electrons are “kicked out” of the material using X-rays, allowing conclusions to be drawn about the chemical nature of the sample. While conventional X-ray photoelectron spectroscopy is limited to the surface of the material, the “hard” version – HAXPES – probes much deeper into the sample thanks to high-energy X-rays and allows precise characterization of multi-layered structures and buried interfaces. HAXPES has applications in the development of microelectronic devices, solid-state batteries and functional thin films as well as in catalysis and corrosion research. The only facility in Switzerland is located in Empa's Joining Technologies and Corrosion Laboratory in Dübendorf.