Anti-corrosive properties of water-borne acrylic composite coatings using electrochemical methods

Abstract

This study evaluates the anti-corrosive performance of waterborne acrylic (WBA) nanocomposite coatings on mild steel (MS), incorporating zinc phosphate (Zn₃(PO₄)₂), graphene oxide (GO), and polyvinylpyrrolidone (PVP). The WBA composites were synthesized and characterized using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and field emission scanning electron microscopy (FESEM). FTIR analysis confirmed the successful formation of polymer nanocomposites, revealing the presence of functional groups associated with the incorporated nanomaterials. XRD, distinguished between crystalline and amorphous structures within the coatings. FESEM, coupled with energy-dispersive spectroscopy (EDS), offered detailed morphological and elemental analyses, confirming the uniform distribution of nanomaterials within the polymer matrix. The corrosion resistance of the coated MS surfaces was evaluated in a 5% sodium chloride (NaCl) aqueous solution using electrochemical techniques such as potentiodynamic polarization (PDP), linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS). Additionally, environmental durability was assessed through salt spray tests and QUV® accelerated weathering, simulating harsh conditions to evaluate the longevity and performance of the coatings. Among the formulations, WBA-Zn₃(PO₄)₂ demonstrated the most effective corrosion resistance, surpassing other formulations. EIS measurements demonstrated a charge transfer resistance (Rct) of 63.9 kΩ·cm² and an inhibition efficiency (ηEIS) of 93.76%. The open circuit potential (OCP) was measured at 0.78 V This performance was attributed to the strong passivation effect of the phosphate ions, which minimized the anodic and cathodic reactions. PDP analysis showed the lowest corrosion current density (icorr) of 2 µA/cm² and a corrosion potential (Ecorr) of -668 mV, confirming strong passivation by phosphate ions. WBA-GO showed moderate protection due to its barrier properties, but its effectiveness was slightly lower due to defects in the graphene oxide (GO) layer. EIS measurements demonstrated a charge transfer resistance of Rct of 14.5 kΩ·cm² and ηEIS of 69.93%. The icorr was recorded at 8 µA/cm² with an Ecorr of -504 mV. Coatings containing PVP, such as WBA-PVP, WBA-GO/PVP, and WBAZn₃(PO₄)₂/PVP, exhibited significantly lower Rct values ranging from 1.42 to 8.3 kΩ·cm², with negative inhibition efficiencies between -116.92% and -207.04%. These negative values suggest that PVP-containing coatings disrupted passivation, leading to increased corrosion rates. The hydrophilic nature of PVP may have contributed to these adverse effects, compromising the coatings' barrier properties. Environmental exposure tests further confirmed WBA-Zn₃(PO₄)₂ superior durability, with minimal discolouration (ΔE = 9) and strong adhesion (4B rating) after 48 hours of salt spray exposure. WBA-GO (ΔE = 15) and WBA-PVP (ΔE = 22) showed greater degradation. Phosphate ions in WBA-Zn₃(PO₄)₂ minimized anodic and cathodic reactions, enhancing corrosion resistance. Optical profilometry provided 2D and 3D topographies of the bare metal surface, displaying the presence of iron peaks and confirming the corrosion susceptibility and the elemental composition of mild steel Results indicate that the WBA-Zn₃(PO₄)₂, emerged as the most effective anticorrosive coating, offering superior corrosion resistance, minimal blistering, and excellent adhesion due to its strong passivation and barrier properties. While, WBAGO displayed reasonable protective properties but were slightly less effective than WBA-Zn₃(PO₄)₂. The weakest performers were WBA-PVP and WBA-GO/PVP, which struggled with limited resistance and adhesion issues. These findings suggest that optimally formulated water-borne acrylic nanocomposites are eco-friendly alternatives with low VOC content and can offer a sustainable solution for corrosion protection in harsh environments.