Corrosion of 316H Stainless Steel in Flowing FLiNaK Salt

Abstract

Type 316H stainless steel samples were exposed to flowing FLiNaK salt for 1000h in a thermal convection loop with a maximum temperature of 650°C and a minimum of 540°C. Samples in the hottest part of the loop lost mass, with a maximum mass loss of 1.8 mg/cm2, while samples in the coldest parts of the loop gained mass. Analysis of the samples that gained mass showed an iron-rich layer on the sample surfaces, suggesting that iron, not chromium, accounted for the majority of the mass transfer in the loop. Analysis of the salt showed major increases in the Cr, Fe, and Mn content of the salt during exposure. The loop was modeled using the TRANSFORM code. Modeled values matched the experimental temperature measurements showing that TRANSFORM is capable of accurately simulating the conditions in the loop.

Corrosion of 316H Stainless Steel in Flowing FLiBe (LiF-BeF₂)

“…Typical procedures used to purify salt using H2-HF have been presented elsewhere [8][9][10]. Flowing salt experiments were conducted using a harp-shaped (~0.75 m tall by 0.5 m wide) TCL, which is shown in Figure 1 and has been described in detail elsewhere [18,19]. All of the components in contact with salt were made from type 316H stainless steel including the 26.7 mm OD x 2.9 mm wall thickness tubing and the specimens in the hot leg (HL) and cold leg (CL) which consisted of small dogbone tensile specimens (25mm long with a 1.5 x 0.76 mm gage, designated SS-3) and coupons (1.8 x 19 x 25 mm).…”

“…The temperature difference created a density gradient resulting in flow of ~2 cm/s determined by monitoring a "hot spot" created by a torch applied for 60 s to the bottom of the CL. After 1000 h, the salt was dumped into a similar SS pot by opening a SS valve below the CL [19]. The specimens were cleaned prior to exposure using acetone and methanol and after exposure by using deionized water sonication at 40°C for 1 h followed by cleaning in acetone.…”

“…Moving beyond static, isothermal compatibility experiments [5][6][7][11][12][13], flowing salt experiments in a thermal gradient are needed to study mass transfer, which is the major concern with potentially corrosive working fluids [13][14][15]. Historically, thermal convection loops (TCLs) [13][14][15][16][17][18][19][20] have been used as an initial assessment of compatibility before investing in a forced convection loop. Thus, this task conducted a monometallic TCL experiment with type 316H stainless steel tubing and 316H specimens with a peak temperature of 650°C and FLiBe salt provided by industry.…”

“…Thus, this task conducted a monometallic TCL experiment with type 316H stainless steel tubing and 316H specimens with a peak temperature of 650°C and FLiBe salt provided by industry. Similar procedures were used so that these results could be compared to previous results in flowing FLiNaK salt [19].…”

“…Images of the 316H thermal convection loop (a) before furnace installation and insulation showing the fill tank at top and dump tank at bottom, (b) during operation and (c) water collection system. The loop flow path is approximately 1m tall and 0.5m wide[19].…”