Corrosion testing of diffusion-coated steel in molten salt for concentrated solar power tower systems

Scale Formation and Degradation of Diffusion Coatings Deposited on 9% Cr Steel in Molten Solar Salt

“…The Al-coating formed an even thinner oxide scale compared to the chromized P91 specimen and did not show substantial microstructural alteration as shown in Figure 9. The enhanced stability of aluminized P91 is in good correlation with other studies in literature [12,25,26], which reported improved scaling behavior for slurry aluminized P91 in molten nitrate salts. Post-exposure XRD analysis revealed that the oxide scale grown on the aluminized specimen after 1000 h exposure in MSS at 600 • C consisted of corundum, hematite and FeAl 2 O 4 (see Figure 6).…”

“…Figure 8 shows the cross-sectional BSE-image, elemental line scans, and distribution maps of the chromized P91 specimen after 1000 h exposure in MSS at 600 • C. In contrast to the 200 µm thick and highly non-protective oxide scale grown on the uncoated P91 specimen, the oxide scale grown on the chromized specimen was only 10 µm thick and consisted of an outer hematite and an inner Cr-rich oxide layer. The oxide scale formed at 600 • C in this study exhibited a very similar duplex morphology as the one grown on chromized P91 after exposure at 560 • C investigated in an earlier study [26]. Alongside the presence of hematite, Na was enriched within the outer layer of the oxide scale up to nearly 5 at.% (Figure 8b), which evidently led to the local formation of sodium ferrite, as identified by the XRD analysis (see Figure 6).…”

“…Notwithstanding, underneath the inner layer of the scale, Cr-depletion was observed to some extent, since the Cr-concentration dropped to 11.6 at.% at the surface of the coating (see Figure 8b). Nevertheless, the Cr-reservoir was evidently still adequate to counter-act the depletion of Cr to the salt, as was forecasted in an earlier study [26]. However, it should be mentioned that the oxide scale grown on the chromized P91 could not hinder the transfer of Na + cations from the salt melt to the scale.…”

“…With increasing exposure temperature, the decomposition of nitrate salts is accelerated, as highlighted in [1,8,14]. This leads to a higher equilibrium content of oxidizing agents in the salt melt, according to Reactions 1 and 2 and thereby enhanced Cr-depletion and thus an earlier onset of breakaway for P91 compared to lower exposure temperatures [26]. This is in good agreement with the study by Bradshaw and Carling, who reported an increasing extent of dissolved Cr cations in the salt melt with increasing temperature [8].…”

“…Corrosion resistance of ferritic-martensitic steels in molten nitrate salt can be increased via diffusion coatings, thereby enabling their employment in the receiver panels. Recent studies showed that aluminide coatings can significantly increase the corrosion resistance of ferritic-martensitic steels in molten nitrate salts by forming Al-rich protective oxide scales [6,12,[24][25][26]. This study aims at the elucidation of corrosion and protection mechanisms by identifying the chemical composition of oxide scales grown on uncoated and diffusion coated ferritic-martensitic steels in molten solar salt (MSS) using complementary characterization methods.…”