Stainless steel is widely known for its impressive corrosion resistance, but it is not invincible. When exposed to environments containing chloride ions, such as seawater, the risk of corrosion increases. To achieve the sleek, clean appearance the material is famous for, manufacturers typically grind the surface to smooth it. However, this finishing process reduces corrosion resistance, and the underlying mechanism has remained poorly understood.
Researchers at Tohoku University have now shed light on this phenomenon. They found that grinding leaves fine scratches on the surface, which can cut into tiny manganese sulfide particles embedded within the steel. Where these scratches intersect with manganese sulfide particles, the surface becomes more vulnerable to damage.
Using a common grade of stainless steel (Type 304), the team examined corrosion behavior in saltwater conditions. Grinding alone did not trigger corrosion; instead, pitting occurred only in regions containing manganese sulfide (MnS) particles. This indicates that surface scratches by themselves are insufficient to weaken corrosion resistance and that MnS inclusions play a key role in the process.
The researchers then analyzed how grinding alters both the protective surface layer and the MnS inclusions. While the chemical composition of the protective layer remained largely unchanged, its thickness became uneven after grinding. In contrast, the MnS inclusions suffered significant damage--they were deformed, cracked, and in some cases forced deeper into the steel.
"These changes to the MnS inclusions caused by grinding were the most important factor behind the reduced resistance to pitting corrosion," explained Masashi Nishimoto, an author of the study and professor in Tohoku University's Department of Engineering. "Ultimately, we showed that grinding lowers resistance not only by disrupting the protective layer, but mainly by damaging MnS inclusions at the same time."
Nishimoto and his colleagues hope that clarifying the effects of surface finishing on corrosion will lead to improved guidance for grinding and surface-treatment methods that minimize risk. Future strategies will need to address the harmful impact of MnS inclusions after grinding. This could enhance the durability and reliability of stainless-steel components used in chemical plants, industrial machinery, and medical devices, where surface finishing is essential.
This research was published online in npj Materials Degradation on February 4, 2026.
- Publication Details:
Title: Grinding-induced degradation in the pitting corrosion resistance of stainless steel: insights into passive film and MnS
Authors: Siqi Wang, Masashi Nishimoto, Izumi Muto
Journal: npj Materials Degradation
