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Abstract
This study presents a process for preparation of cellulose–lignin barrier coatings for hot-dip galvanized (HDG) steel by aqueous electrophoretic deposition. Initially, a solution of softwood kraft lignin and diethylene glycol monobutyl ether was used to prepare an aqueous dispersion of colloidal lignin particles (CLPs) via solvent exchange. Analysis of the dispersion showed that it comprised submicron particles (D = 146 nm) with spherical morphologies and colloidal stability (ζ-potential = −40 mV). Following successful formation, the CLP dispersion was mixed with a suspension of TEMPO-oxidized cellulose nanofibers (TOCN, 1 and 2 g·L–1) at a fixed volumetric ratio (1:1, TOCN–CLPs), and biopolymers were deposited onto HDG steel surfaces at different potentials (0.5 and 3 V). The effects of these variables on coating formation, dry adhesion, and electrochemical properties (3.5% NaCl) were investigated. The scanning electron microscopy results showed that coalescence of CLPs occurs during the drying of composite coatings, resulting in formation of a barrier layer on HDG steel. The scanning vibrating electrode technique results demonstrated that the TOCN–CLP layers reduced the penetration of the electrolyte (3.5% NaCl) to the metal–coating interface for at least 48 h of immersion, with a more prolonged barrier performance for 3 V-deposited coatings. Additional electrochemical impedance spectroscopy studies showed that all four coatings provided increased levels of charge transfer resistance (Rct)—compared to bare HDG steel—although coatings deposited at a higher potential (3 V) and a higher TOCN concentration provided the maximum charge transfer resistance after 15 days of immersion (13.7 cf. 0.2 kΩ·cm2 for HDG steel). Overall, these results highlight the potential of TOCN–CLP biopolymeric composites as a basis for sustainable corrosion protection coatings.
Highlights
For more than a century, organic coatings and paints have been utilized effectively to protect metallic surfaces against corrosion.[1]
The graph obtained from the as-prepared colloidal lignin particles (CLPs) demonstrates a bimodal distribution of fine and large particles with a volumeweighted mean of D3,4 = 11.1 μm
Driven by the need for more sustainable coatings, this study highlights a process for preparation of water-dispersible CLPs and their codeposition with different concentrations of TOCNs
Summary
For more than a century, organic coatings and paints have been utilized effectively to protect metallic surfaces against corrosion.[1]. Lignocellulosic feedstocks are the most abundant biomass source obtained from plant matter that are primarily constituted of cellulose, hemicellulose, and lignin,[5] altogether representing a potent renewable resource in the development of biobased economy and products.[6] In the hierarchical structure of plants’ cell walls (i.e., a natural composite), cellulose provides the structural scaffold,[7] while lignin rigidifies the structure and provides a water-resistant barrier for the polysaccharide components.[8] The inherent cofunctionality of cellulose and lignin in manmade composites has demonstrated promising mechanical properties and barrier performance against water, water vapor, and oxygen permeation[9−13] characteristics that have the potential for application in corrosion protection These studies were mainly focused on preparation of freestanding films for applications such as ultraviolet (UV) shielding and packaging, while the anticorrosion performance of a cellulose−lignin coating has not, to the best of the authors’ knowledge, been investigated previously. The findings of this study highlight the significant potential of water-borne biopolymeric coatings for the protection of HDG steel surfaces
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