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Found in everything from kitchen appliances to sustainable energy infrastructure, stainless steels are used extensively due to their excellent corrosion (rusting) resistance. They're an important material in many industries, including manufacturing, transportation, oil and gas, nuclear power and chemical processing.

However, stainless steels can undergo a process called sensitization when subjected to a certain range of high temperatures—like during welding—and this substantially deteriorates their corrosion resistance. Left unchecked, corrosion can lead to cracking and structural failure.

"This is a major problem for stainless steels," says Kumar Sridharan, a professor of nuclear engineering and engineering physics and materials science and engineering at the University of Wisconsin–Madison. "When stainless steel gets corroded, components need to be replaced or remediated. This is an expensive process and causes extended downtime in industry."

Sridharan and Kasturi Narasimha Sasidhar, an assistant scientist in Sridharan's group, have demonstrated a new approach for restoring stainless steel's corrosion resistance that could be much faster and potentially less expensive than conventional high-heat remediation methods.

To fundamentally understand why their approach was so successful at restoring corrosion resistance, the researchers harnessed an advanced technique called atom probe tomography in collaboration with Madison-based company CAMECA Instruments Inc. (AMETEK), which has ties to UW–Madison.

The team detailed its findings in a paper March 5, 2025, in the journal Metallurgical and Materials Transactions.

For their approach, the researchers used a technology called "ultrasonic nanocrystal surface modification" on a sample of sensitized stainless steel. In this process, a hard pin taps the steel's surface at extremely high frequencies.

"We showed that ultrasonic nanocrystal surface modification can restore the corrosion-resistant state of the stainless steel, without needing any heat treatment, which is a really big deal," Sridharan says.

While ultrasonic nanocrystal surface modification is not readily scalable, Sridharan says this research could open a door to similar, more scalable surface modification methods to optimize the performance of stainless steels.

Of course, the researchers wanted to understand why their approach was effective. Traditional microscopy techniques like scanning or transmission electron microscopy alone were insufficient to yield the data necessary to answer their questions.

"CAMECA's atom probe tomography technology allowed the researchers to look at the steel at the nanometer scale, in three dimensions, and to precisely measure the location of the elements in the material," says Sasidhar, now a senior applications scientist at CAMECA Instruments Inc.

Stainless steel contains about 18% chromium, which is what makes it corrosion resistant. As sensitization occurs, chromium gets depleted in tiny areas in the stainless steel. Notably, these tiny areas of depletion are responsible for the drastic loss in corrosion resistance.

The team discovered that the ultrasonic nanocrystal surface modification treatment equalized the chromium concentration in the tiny depleted regions, which restored the corrosion resistance.

Sridharan says UW–Madison's connection to CAMECA, a world-leading manufacturer of atom probe tomography equipment, played a key role in this breakthrough. The precursor to CAMECA's atom probe tomography business was Imago Scientific Instruments Corporation, a company founded in 1998 by Tom Kelly, a former professor of materials science and engineering at UW-Madison. In 2010, Imago was acquired by AMETEK and incorporated into the CAMECA business unit.

"The company has strong historical links to UW–Madison," says Robert Ulfig (BSNEEP '94, MSMS&E '97), senior applications and business developer at CAMECA and a co-author on the paper, who worked closely with Sridharan during his graduate studies. "It's exciting that we were able to collaborate with the university to make this impactful discovery."