A cost-effective intense blue colour inorganic pigment for multifunctional cool roof and anticorrosive coatings

An enormous attention has been paid to cobalt aluminate (CoAl2O4) blue pigments owing to their significant hue of blue colour. Nevertheless, several serious disadvantages, such as high cost due to the high cobalt consumption and poor solar reflectance, are associated with this pigment. Therefore, the development of cost effective and sustainable blue pigments with reduced Co content and improved solar reflectivity is a major focused area in pigment research. Herein, a new series of multifunctional blue coloured inorganic pigments with low cobalt content were prepared via the solid-state method. The structural and spectroscopic evaluations were indicated that the strong blue colour could be generated because of the distorted tetrahedrally organized cobalt chromophore. This structural distortion of the chromophore geometry was linked to the covalency and electron affinity factors. Colour (b* = −59.21) and solar reflectance (R* = 64%) measurements of the optimized composition (NaZn0.9Co0.1PO4) was observed to be superior than the commercially available CoAl2O4 blue pigment. Meanwhile, the optimized pigment loaded epoxy coated steel substrate in marine medium exhibited impressive corrosion resistance properties. The anticorrosive mechanism, further established through the XPS results, validated that the phosphatized inhibitive film served as the barrier for corrosion. The estimated Co content in NaZn0.9Co0.1PO4 was found to be around 10 times lower than the CoAl2O4 blue pigment indicating the cost-effective nature. The optimized pigment was able to retain its optical properties in the prepared acrylic coatings on Al sheet and concrete block. Further, the temperature shielding studies ensured a reduction of ~4 °C in comparison with CoAl2O4 coatings. Therefore, the developed multifunctional blue pigment could be employed for the economic production of blue exterior paint for anticorrosive cool roof coatings. [Display omitted] •Innovative blue inorganic pigment, cobalt doped NaZnPO4 with strong blue hue.•Cost-effective pigment composition due to 10 times reduced cobalt content compared to commercial CoAl2O4.•Tetrahedral chromophore structure renders intense blue colour superior to CoAl2O4.•High solar reflectance and impressive corrosion resistance provided multifunctionality thus avoided duplex exterior coating protocol.•Pigment coatings displayed ~4 °C reduction in interior and surface temperature.