• Chaudhari, Tushar
• Rajagopalan, Narayanan
• Dam-Johansen, Kim

Abstract

Lignin, due to its availability, molecular structure, reported barrierproperties, and chemical modification prospects, is gaining increasingattention 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 forassessing its inhibitory properties in epoxy-based anticorrosivecoatings. The aim was to emulate the conventional inhibitive mechanismof zinc phosphate by introducing partial solubility to KLP. Thissolubility facilitates the formation of a passivation layer (ironphosphate), which is a prerequisite for the inhibition mechanism at theinterface between the metal and coating when it is exposed to corrosiveconditions. Therefore, the utilization of KLP as a biobased inhibitivepigment signifies an innovative approach in the field of anticorrosivecoatings. KLP was synthesized by reacting KL with phosphorus pentoxide(P₂O₅) and was characterized using FourierTransform Infrared Spectroscopy (FTIR) and Nuclear Magnetic Resonance(NMR) spectroscopy. Subsequently, KLP was incorporated into anamine-cured Bisphenol-A (BPA) epoxy coating (KLP-EA) with a dry filmthickness of 80 μm and evaluated as per industrial salt spray testingfor coatings (ISO 9227:2017). Furthermore, the inhibitive corrosionresistance of KLP-EA was evaluated against a commercially available zincphosphate-based epoxy coating (C-EA) and an unmodified kraftlignin-based epoxy coating (KL-EA), which is recognized solely for itsbarrier mechanism. The polarization test demonstrated that KLPeffectively inhibited corrosion, resulting in lower <i>I</i>_corr values. The EIS results of the KLP-EA coating showed higher impedance modulus (|<i>Z</i>|_0.01 &gt; 10⁸ Ω·cm²),signifying exception barrier properties. The results from salt spraytesting after 1000 h of exposure demonstrated that the KLP-EA exhibitedon par performance compared to C-EA and significantly superiorperformance to KL-EA. Based on the analysis of a rust creep test (ISO12944–9:2018), 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 barrierproperties of KL-EA and 1.8 ± 0.2 mm for C-EA. Additionally, theunderfilm corrosion products in KLP-EA were analyzed using X-rayPhotoelectron Spectroscopy (XPS), which verified the existence of ironphosphate (passivating film), replicating the conventional inhibitivemechanism of zinc phosphate. The current research findings thus provide azinc-free biobased alternative in the domain of inhibitiveanticorrosive coatings.

Topics

• surface
• corrosion
• x-ray photoelectron spectroscopy
• zinc
• lignin
• electrochemical-induced impedance spectroscopy
• Nuclear Magnetic Resonance spectroscopy
• creep
• creep test
• molecular structure
• Phosphorus
• infrared spectroscopy
• elemental analysis