Novel Approach Addresses Critical Weakness inVital Industrial Material
Researchers at the University of Wisconsin-Madison havedeveloped a promising new technique to address a significant vulnerability inone of industry's most widely used materials. Their breakthrough offers apotentially faster and less expensive solution to a problem that costsindustries millions in downtime and replacement costs annually.
Professor Kumar Sridharan and Assistant Scientist KasturiNarasimha Sasidhar have demonstrated that ultrasonic nanocrystal surfacemodification can successfully restore corrosion resistance in stainless steelthat has undergone sensitization, a process that severely compromises thematerial's resistance to rusting when exposed to certain high temperatures,such as during welding operations.
The findings, published in the March 5, 2025 issue ofMetallurgical and Materials Transactions, could have significant implicationsfor industries that rely heavily on stainless steel, including manufacturing,transportation, oil and gas, nuclear power, and chemical processing.
Understanding Sensitization and Its Impact
Stainless steel's remarkable corrosion resistance stemsprimarily from its chromium content, typically around 18%. This chromium formsa passive oxide layer that protects the underlying metal from corrosiveenvironments. However, when stainless steel is heated to temperatures betweenapproximately 425°C and 870°C (800°F-1600°F), a range commonly reached duringwelding and other industrial processes, it can undergo sensitization.
During sensitization, chromium combines with carbon in thesteel to form chromium carbide precipitates at grain boundaries. This depletesthe surrounding areas of chromium, creating microscopic regions withinsufficient chromium to maintain corrosion resistance. These depleted zonesbecome vulnerable to corrosive attack, which can lead to intergranularcorrosion and eventually to structural failure.
"This is a major problem for stainless steels,"explains Sridharan. "When stainless steel gets corroded, components needto be replaced or remediated. This is an expensive process and causes extendeddowntime in industry."
Ultrasonic Treatment Offers New Solution
The researchers' approach employs ultrasonic nanocrystalsurface modification, a process in which a hard pin taps the steel's surface atextremely high frequencies. This mechanical treatment creates significantplastic deformation in the material's surface and subsurface regions.
"We showed that ultrasonic nanocrystal surfacemodification can restore the corrosion-resistant state of the stainless steel,without needing any heat treatment, which is a really big deal," Sridharannotes.
Conventional remediation methods for sensitized stainlesssteel typically involve solution annealing, heating the entire component totemperatures above 1000°C (1832°F) followed by rapid cooling. This process isenergy-intensive, time-consuming, and can cause distortion in complexcomponents. Additionally, it may not be feasible for large structures or thosealready installed in service.
The UNSM approach offers a potential alternative that couldbe applied more selectively and without the need for high-temperatureprocessing, potentially saving significant time and resources.
Atomic-Level Investigation Reveals Mechanism
To understand why their approach was so effective, theresearchers collaborated with Madison-based CAMECA Instruments Inc. to employatom probe tomography, an advanced characterization technique that providesthree-dimensional mapping of elements at the nanoscale.
"CAMECA's atom probe tomography technology allowed theresearchers to look at the steel at the nanometer scale, in three dimensions,and to precisely measure the location of the elements in the material,"explains Sasidhar, who now works as a senior applications scientist at CAMECA.
The analysis revealed that the ultrasonic treatmenteffectively redistributed chromium within the material, equalizing itsconcentration in the previously depleted regions near grain boundaries. Thisredistribution restored the critical chromium levels needed for corrosionresistance.
The researchers also employed transmission electronmicroscopy and electron backscatter diffraction to characterize themicrostructural changes induced by the UNSM treatment. Their analysis showedthat the process creates deformation bands and a high density of nano-twins atdepths up to 200 micrometers below the surface, indicating significant plasticdeformation that facilitates the redistribution of chromium.
Industry Implications and Future Directions
While ultrasonic nanocrystal surface modification is notyet readily scalable for large industrial applications, Sridharan believes thisresearch could open doors to similar, more scalable surface modificationmethods to optimize the performance of stainless steels.
The findings are particularly significant for industrieswhere stainless steel components are subjected to welding or otherhigh-temperature processes during fabrication or repair. These include:
- Nuclear power generation, where components must maintainintegrity in corrosive environments for decades
- Chemical processing facilities, where corrosion can leadto containment failures and hazardous leaks
- Oil and gas infrastructure, particularly in offshoreenvironments
- Food processing equipment, where corrosion can lead tocontamination issues
- Biomedical implants, where material integrity is criticalfor patient safety
Robert Ulfig, senior applications and business developer atCAMECA and a co-author on the paper, highlighted the significance of theuniversity-industry collaboration: "The company has strong historicallinks to UW–Madison. It's exciting that we were able to collaborate with theuniversity to make this impactful discovery."
Backdrop & Context
Stainless steels represent a family of iron-based alloyscontaining a minimum of 10.5% chromium, which provides corrosion resistancethrough the formation of a chromium-rich passive oxide layer on the surface.Additional elements like nickel, molybdenum, and nitrogen are often added toenhance specific properties.
The most common types of stainless steel include austenitic(300 series), ferritic (400 series), martensitic, and duplex varieties, eachwith distinct microstructures and properties suited for different applications.Austenitic stainless steels, particularly Type 304 (18% Cr, 8% Ni) and Type 316(with added molybdenum for improved corrosion resistance), are the most widelyused grades in industrial applications.
Sensitization has been a recognized problem in stainlesssteels since the early 20th century, particularly in welded components.Traditional solutions have focused on either:
1. Preventing sensitization through the use of low-carbongrades (304L, 316L) or stabilized grades containing elements like titanium orniobium that preferentially form carbides instead of chromium
2. Remediating sensitized components through solutionannealing heat treatments
The ultrasonic nanocrystal surface modification techniqueemployed in this research represents a mechanical approach to materialsprocessing that has gained attention in recent years for its ability to modifysurface properties without thermal processing. The technique involves applyingultrasonic vibrations at frequencies of 20 kHz or higher through a tool thatimpacts the material surface thousands of times per second.
CAMECA Instruments Inc., the company that collaborated onthe atom probe tomography analysis, has historical ties to UW-Madison. Its atomprobe tomography business evolved from Imago Scientific InstrumentsCorporation, founded in 1998 by Tom Kelly, a former professor of materialsscience and engineering at UW-Madison. Imago was acquired by AMETEK in 2010 andincorporated into the CAMECA business unit.
Key Takeaways:
• Researchers at UW-Madison have demonstrated thatultrasonic nanocrystal surface modification can restore corrosion resistance insensitized stainless steel without heat treatment.
• Sensitization occurs when stainless steel is exposed totemperatures between approximately 425°C and 870°C, causing chromium depletionnear grain boundaries and making the material vulnerable to corrosion.
• The ultrasonic treatment works by redistributing chromiumto equalize its concentration in previously depleted regions, as confirmedthrough advanced atom probe tomography analysis.
• Conventional remediation methods require high-temperatureheat treatments that are energy-intensive, time-consuming, and can causecomponent distortion.
• The new approach could potentially offer faster and lessexpensive remediation for stainless steel components across various industries,including nuclear power, chemical processing, and manufacturing.
• The research represents a successful collaborationbetween UW-Madison and CAMECA Instruments Inc., a company with historical tiesto the university.
• While not yet readily scalable, the findings could leadto the development of similar surface modification techniques that could beapplied in industrial settings.
• The study employed multiple advanced characterizationtechniques, including atom probe tomography, transmission electron microscopy,and electron backscatter diffraction, to understand the mechanism behind thetreatment's effectiveness.
• The treatment creates deformation bands and nano-twins todepths of approximately 200 micrometers below the surface, indicatingsignificant plastic deformation that facilitates chromium redistribution.
• This research addresses a long-standing challenge inmaterials science and could have significant economic implications by reducingdowntime and replacement costs in industries that rely heavily on stainlesssteel components.
