An experimental study on the effect of salt spray testing on the optical properties of solar selective absorber coatings produced with different manufacturing technologies

Solar selective absorber coating (SSAC) is one of the key components of a solar collector, with its optical properties having a significant impact on the collector’s thermal performance. The key parameters characterizing the optical properties of an SSAC are the solar absorptance (absorptance of solar radiation) and the thermal emittance (emittance for long-wave radiation). However, some of high-performing SSACs suffer from some drawbacks, such as lower durability, lower resistance to corrosion and abrasion, which is particularly harmful for SSACs, as, for example, chlorides in the atmosphere have become a main contributor to corrosion in coastal areas with the increasing trend of global warming. In this paper, salt spray tests have been conducted on the SSACs manufactured by three common manufacturing technologies, i.e., the anode oxidation (AO) technology, the vacuum magnetron sputtering (VMS) technology, and the black chromium plating (BCP) technology, over the testing durations of 12 h, 24 h, 36 h, and 48 h, respectively, to examine the effect of the salt spray testing on the optical properties of SSACs manufactured by different manufacturing technologies. The salt spray testing is an accelerated aging testing method for evaluating the SSAC’s resistance to corrosion when it is under an extended exposure to a saline, or salted, spray (fog). The experimental results show that, in general, the SSACs manufactured by the BCP technology have excellent resistance to salt spray (i.e., to corrosion) and those manufactured by the AO technology have only reasonable resistance to corrosion, whereas the SSACs manufactured by the VMS technology have very poor resistance to corrosion. The results also demonstrate that there are noticeable differences in the optical properties of the SSAC samples even manufactured by the same technology but by different manufacturers, with some having significant differences. The causes for the differences have been further examined through the inspection of the physical appearance of the selected SSAC samples and the experimentally measured distributions of the monochromatic reflectance of solar radiation of the samples over the solar spectrum before and after the salt spray testing over different durations.