In concentrated solar power (CSP) tower plants, receiver systems must withstand harsh desert conditions and high temperatures while maintaining high efficiencies in converting solar radiation to thermal energy within a heat transfer fluid. To enhance economic efficiency and reduce maintenance downtime, the high-absorbing receiver coatings must perform for hundreds of hours without needing repair. In particular, thermocycling influences their durability and can lead to crack formation and delamination of the overlay coatings currently used in CSP tower plants. Therefore, self-healing diffusion coatings based on Cr and Mn forming black, highly absorbing Cr–Mn-oxides in situ are alternative coatings for increased durability.
In this work, Cr-Mn-diffusion coatings were deposited on VM12 ferritic-martensitic steel. Oxidation tests were conducted in air at temperatures between 527 and 710°C for up to 100 h to demonstrate the initial oxide formation and investigate black spinel formation. Solar absorptances of up to 93 % and optical coatings efficiencies of up to 80 % are obtained with the best coating performance at around 650 °C. The solar absorptance remains stable at 92 – 92.5 % after a slight drop in solar absorptance after around 50 h of exposure, which was proven during long-term thermocyclic exposures for up to 640 h at 650 °C. A mechanism of oxide formation and coating consumption is proposed, showing the importance of Mn outward diffusion and, thereby, destabilization of highly absorptive Mn-stabilized cubic spinel. At the beginning of the exposure, Mn$_2$O$_3$ and cubic Mn$_x$Cr$_{3-x}$O$_4$ (β-spinel) form, which transform to tetragonal Mn$_x$Cr$_{3-x}$O$_4$ (α-spinel) and, with continuing exposure, also to Cr$_2$O$_3$.
With respect to the substrate material, it is found that Cr-Mn-coated VM12 performs up to 2 % better concerning solar absorptance in comparison to P91 steel. For analysis, metallographic investigations are combined with methods such as Raman spectroscopy, XRD, spectrophotometry, and qualitative and quantitative EPMA measurements.