Pitting Corrosion of Very Clean Type 304 Stainless Steel

The localized corrosion susceptibility of a clean grade of Type 304 (UNS S30400) stainless steel (SS304), which was processed with the intent of having fewer and smaller inclusions, was compared with regular grade SS304. Transmmission electron microscopy (TEM) analysis revealed that inclusions were composed of multiple phases that were either multi-element oxide or manganese sulfide (MnS). Oxide inclusions, which had only multi-oxide phases, were the dominant inclusion type in clean SS304, whereas MnS-containing inclusions, having both oxide and MnS phases together, were more frequent in regular SS304. The average size of inclusions was larger in regular SS304. Cyclic polarization tests showed that pitting potentials of clean SS304 were higher than regular grade SS304. Under exposure to strong oxidizing acid with Cl⁻, MnS-containing inclusions initiated pitting, whereas oxide inclusions did not, indicating that pit initiation is more frequent in regular SS304 because it has more MnS inclusions. The fewer pit initiation sites in clean SS304 was the reason for its higher pitting corrosion resistance.