Tests reveal why titanium additions do not fully prevent pitting corrosion in stainless steel

Stainless steel is renowned for its resistance to corrosion, but it can succumb to pitting corrosion in environments with chloride ions. This type of corrosion often starts at microscopic particles called inclusions. Titanium (Ti) is added to stainless steel to enhance its resistance to pitting corrosion by transforming soluble manganese sulfide (MnS) into more stable Ti-containing sulfides.

However, even with Ti additions, pitting corrosion is not entirely eliminated. The specific electrochemical characteristics of the different inclusions present in Ti-containing stainless steel and which ones can initiate pitting have not been well understood. Researchers at Tohoku University have now shed light on this complex issue.

They examined a ferritic stainless steel, Fe-16%Cr alloy (Type 430), and created model specimens with controlled inclusion compositions. By employing microscale electrochemical measurements, they compared four types of inclusions: MnS, (Mn,Ti)S, TiS₂, and titanium nitride (TiN). Their findings revealed that while Ti-containing inclusions were more stable than MnS, (Mn,Ti)S was the most soluble.

Moreover, the potential at which pitting occurred corresponded to the range in which (Mn,Ti)S dissolved in the model specimens. Crucially, pits formed at the sites of (Mn,Ti)S inclusions, with manganese and sulfur accumulating around these initiation points. These results suggest that even minute quantities of (Mn,Ti)S inclusions can serve as triggers for pitting corrosion in Ti-containing stainless steel.

The study underscores the significance of controlling the composition of sulfide inclusions to boost the corrosion resistance of Ti-containing stainless steel. Moving forward, the researchers intend to investigate the impact of calcium (Ca), which is often co-added with Ti during stainless steel production, in pursuit of further enhancing the pitting corrosion resistance of these materials.