High temperature corrosion beneath carbonate melts of aluminide coatings for CSP application

• Published in: Solar energy materials and solar cells 2020-06, Vol.210, p.110514, Article 110514
• Main Authors: Audigié, P., Encinas-Sánchez, V., Rodríguez, S., Pérez, F.J., Agüero, A.
• Format: Article
• Language: English
• Subjects: Aluminide coatings; Aluminides; Aluminum; Chromium; Coatings; Corrosion; Grain boundaries; Gravimetry; High temperature; Inspection; Interdiffusion; Intermetallic compounds; Internal oxidation; Iron; Lithium ferrite; Molten carbonates; Molten salts; Oxidation; Scale (corrosion); Slurries; Spallation; Spraying; Steels; Substrates; Thermo-solar plants; Uniform attack (corrosion)
• ISSN: 0927-0248; 1879-3398
• DOI: 10.1016/j.solmat.2020.110514

Slurry iron-aluminide coatings deposited by spraying on 9 wt% Cr P91 alloy as well as uncoated P91 were exposed isothermally at 650 °C to a ternary molten salt mixture based on a Na, K and Li carbonate eutectic, under static and dynamic conditions. Uncoated P91 evidenced considerable mass gains and extensive spallation in both conditions. Indeed, P91 developed a very thick fast growing multilayered oxide scale which included LiFeO2, LiFe5O8 and (Fe,Cr)3O4. Under dynamic conditions, the metal loss was higher that when the test was carried out statically but this was not reflected in the gravimetric measurements likely due to spallation of the scales in both cases. The coated systems performed better than the uncoated material up to at least 1000 h according to metallographic inspection. However, the aluminide coating showed non-uniform attack and on the corresponding zones, a thick layer likely consisting of LiFeO2 developed over an internal oxidation zone corresponding to all of the coating initial thickness. K was also detected within the internal oxidation zone suggesting that the coating was internally attacked at least by K containing species (Li cannot be detected by EDS). K and perhaps Li seem to diffuse along the grain boundaries of the coating, leading to internal oxidation responsible for the degradation. On the non-degraded zones, the coating maintained the initial microstructure as very low coating/substrate interdiffusion occurred. A 20 wt% average Al content at the surface does not seem to be high enough to sustain a protective oxide. •Corrosion of P91 by molten carbonates at 650 °C takes place at very fast rates.•Dynamic conditions lead to higher corrosion degradation of coated and uncoated samples.•Non-uniform corrosion takes place on FeAl coated systems.•Mn from the substrate reaches the corrosion scale surface on coated and uncoated P91.