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.

Chloride salts are one candidate for a >700°C concentrating solar power (CSP) cycle, however, many reports from the literature suggest very high reaction rates between chloride salts and structural alloys. Historically, a specific methodology was established for evaluating halide salt compatibility based on solution kinetics. This study returned to that paradigm where the salts are purified and evaluated in sealed capsules before moving to a flowing experiment to determine a true corrosion rate in a temperature gradient for a commercial K–Mg–Na chloride salt. Isothermal testing focused on Ni‐based alloys 230 and 600 at 600°C–800°C. The results indicated there were promising combinations of salt chemistry, temperature, and alloy composition that reduce the extent of reaction. The results of the first monometallic thermal convection loop of alloy 600 run for 1,000 hr with a peak temperature of 700°C showed low attack with rates ≤9 µm/yr.