Lignin Phosphate: A Biobased Substitute for Zinc Phosphate in Corrosion-Inhibiting Coatings

Lignin, due to its availability, molecular structure, reported barrier properties, and chemical modification prospects, is gaining increasing attention for its potential in biobased functional coatings. Herein, softwood kraft lignin (KL) was surface functionalized (phosphorylated), yielding lignin phosphate (KLP) to engineer a functional pigment for assessing its inhibitory properties in epoxy-based anticorrosive coatings. The aim was to emulate the conventional inhibitive mechanism of zinc phosphate by introducing partial solubility to KLP. This solubility facilitates the formation of a passivation layer (iron phosphate), which is a prerequisite for the inhibition mechanism at the interface between the metal and coating when it is exposed to corrosive conditions. Therefore, the utilization of KLP as a biobased inhibitive pigment signifies an innovative approach in the field of anticorrosive coatings. KLP was synthesized by reacting KL with phosphorus pentoxide (P₂O₅) and was characterized using Fourier Transform Infrared Spectroscopy (FTIR) and Nuclear Magnetic Resonance (NMR) spectroscopy. Subsequently, KLP was incorporated into an amine-cured Bisphenol-A (BPA) epoxy coating (KLP-EA) with a dry film thickness of 80 μm and evaluated as per industrial salt spray testing for coatings (ISO 9227:2017). Furthermore, the inhibitive corrosion resistance of KLP-EA was evaluated against a commercially available zinc phosphate-based epoxy coating (C-EA) and an unmodified kraft lignin-based epoxy coating (KL-EA), which is recognized solely for its barrier mechanism. The polarization test demonstrated that KLP effectively inhibited corrosion, resulting in lower I_corr values. The EIS results of the KLP-EA coating showed higher impedance modulus (|Z|_0.01 > 10⁸ Ω·cm²), signifying exception barrier properties. The results from salt spray testing after 1000 h of exposure demonstrated that the KLP-EA exhibited on par performance compared to C-EA and significantly superior performance to KL-EA. Based on the analysis of a rust creep test (ISO 12944–9:2018), KLP-EA showed a rust creep value of 1.7 ± 0.2 mm, compared to 2.3 ± 0.2 mm for the coatings solely based on barrier properties of KL-EA and 1.8 ± 0.2 mm for C-EA. Additionally, the underfilm corrosion products in KLP-EA were analyzed using X-ray Photoelectron Spectroscopy (XPS), which verified the existence of iron phosphate (passivating film), replicating the conventional inhibitive mechanism of zinc phosphate. The current research findings thus provide a zinc-free biobased alternative in the domain of inhibitive anticorrosive coatings.