Alginate-Occupied Carbonate Pigments for Dielectric Preservation and Scratch-Localized Corrosion Interruption in Polyolefin Coatings
Abstract
The corrosive process in the presence of chloride is intensified due to the coupling of moisture-wicking, porous pigments, polymeric defects, and corroded steel surfaces through electrolyte. Water-insensitive properties of the nonpolar matrix of polyolefin coatings are highly valuable since they provide the resistance to water uptake and chemical degradation, although scratches and interconnected porosities can contribute to the formation of a corrosion cell. In this work, a water-based polyolefin coating reinforced with 1 wt.\% sodium alginate-filled calcium carbonate pigment on carbon steel is studied. The used pigment consists of a thermal calcium-based carrier combined with a biopolymer having a high number of functional groups (carboxylates) able to precipitate a local protective film on a corroded steel. The performance of the developed coating is analyzed by measuring carrier porosity, thermogravimetric mass loss, electrochemical impedance spectroscopy, contact angle response, and scratch depth. After thermal treatment, the specific surface area and porosity of calcium carbonate increase from 4.73 to 16.24 m$^2$ g$^{-1}$ and from 0.0156 to 0.0332 cm$^3$ g$^{-1}$, respectively. Sodium alginate loading reduces the surface area and porosity to 3.145 m$^2$ g$^{-1}$ and 0.0098 cm$^3$ g$^{-1}$, resulting in 80.6\% of the surface decrease and in 70.5\% of pore filling in comparison with the carrier. After exposure to 3.5 wt.\% NaCl for sixteen days, low-frequency impedance values of modified coating remain about 100 G$\Omega$ cm$^2$, while unmodified polyolefin coating reduces its impedance by approximately two orders of magnitude. In addition, contact angle increases up to about 100$^\circ$ and scratch depth decreases down to about 40 $\mu$m. According to the findings, the occurrence of corrosion disruption is a result of the synergy of mineral reservoir filling, hydrophobic dielectric preservation, decreased wetting, and semi-sealing of the corroded surface by alginates.