We examined the anti-scaling potential of polymeric nano-coated heat transfer surfaces against alkaline (CaCO3 and Mg(OH)2) and non-alkaline (CaSO4) scaling. The tested surfaces were coated with thin layers of poly-tryvinyltrimethyl-cyclotrisiloxane (pV3D3) and poly-divinylbenzene (pDVB) polymers using initiated chemical vapor deposition (iCVD) technique. Stainless steel and Cu/Ni (70:30) substrates were used to replicate the flash chambers’ walls and the brine heater tubes of the thermal desalination plants. Accelerated testing experiments were performed at room and boiling temperatures, two turbulences (200 and 500 rpm), and durations of 0.5, 2, 6, 24, 48 and 96 hours. ICP-OES was used to determine the surface densities of the deposited scales, and SEM was used to study the morphology of the surfaces before and after the tests. The results demonstrated the effectiveness of the iCVD coated surfaces in suppressing scale formation of the three types of scaling. The extent of scaling with respect to exposure durations provided good evidence on the heterogeneous nucleation scaling mechanism. The results proved that iCVD pDVB coating is, to some extent, more effective than the pV3D3 coating in reducing the CaCO3 and Mg(OH)2 scales. However, both coatings showed comparatively similar effectiveness in reducing the CaSO4 scale. The highest reduction in scale formation (91%) was obtained for iCVD pDVB coated Cu/Ni samples immersed in CaCO3 solution, at room temperature and low turbulence, for the duration of 6 hours. This is almost 11 folds less than the amount accumulated on the blank Cu/Ni sample at same conditions. The SEM images of the iCVD coated surfaces illustrated the clustering of the scale deposits, while the EDS spectra confirmed their ant-scaling potential. The SEM and optical microscopic images also showed the differences in the structure and shape of the deposited crystals of the three scales.

Initiated chemical vapor deposition (iCVD) coated surfaces for inhibiting scale formation in thermal desalination processes

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