Self-Protecting Epoxy Coatings with Anticorrosion Microcapsules

Abstract

The corrosion of steel substrates causes damage that is costly to repair or replace. Current protective coatings predominately rely on environmentally harmful anticorrosive agents and toxic solvents to protect the underlying substrate. The use of lawsone (2-hydroxy-1,4-napthoquinone) together with a water-based epoxy coating provides an environmentally friendly alternative for common protective coatings. Microencapsulated lawsone embedded in an epoxy coating allows the anticorrosive agent to remain dormant until released by damage and delivered directly onto the steel substrate. UV-vis analysis confirms successful encapsulation of lawsone in a polyurethane shell wall and reveals up to 8 wt % lawsone in the capsule cores. Uniform dry film thickness and inflicted damaged are verified with ultrasound and optical microscopy. Visual and electrochemical analysis demonstrates that this self-protective scheme leads to a 70% corrosion inhibition efficiency in a neutral salt water solution.

Figure 1

Lawsone structure interacts with a metal ion (M) during the corrosion process and forms a 1:1 or 2:1 metal complex where Z = 2 or 3.

Figure 2

Schematic of a self-protecting coating. (a) Undamaged coating with embedded microcapsules containing an anticorrosive agent (e.g., lawsone). (b) Mechanical damage ruptures embedded capsules releasing their content into the damage area. (c) Evaporation and diffusion of the carrier solvent produces a solid protective barrier that passivates and protects the underlying steel substrate.

Figure 3

Characterization of microcapsule diameter and shell wall thickness. (a) Optical image of microcapsules as prepared. (b) SEM image of a capsule cross section embedded in epoxy. (c) Histogram of capsule diameter as measured from optical images. (d) Capsule shell wall thickness measured from SEM cross-sectional imaging. False color added to highlight the shell interior (green), the shell wall (yellow), and the surrounding epoxy (blue).

Figure 4

Lawsone content of microcapsules as measured by UV–vis analysis. The dashed blue line represents the amount of lawsone initially added into the encapsulation beaker and the amount of lawsone that would be present inside the microcapsules if all of the lawsone was successfully encapsulated.

Figure 5

TGA results of type 0, 2, 5, and 10 microcapsules along with pure core materials.

Figure 6

Thermal stability of microcapsule components. (a) Isothermal TGA near room temperature (29 °C) for 24 h of hexyl acetate-filled microcapsules, lawsone, and the shell wall material (PU). (b) Mass of microcapsules kept under various storage conditions over time at room temperature (22 °C) and at an elevated temperature (35 °C).

Figure 7

Optical confirmation of lawsone release following scribe damage in an opaque phenolic coating. Optical images of (a) self-protecting coating with lawsone-filled microcapsules. (b) Control coating with hexyl acetate-filled microcapsules.

Figure 8

Reduction in the corrosion product in coatings with lawsone capsules. Optical (a,b) with a false colored corrosion product and SEM (c,d) images of the corroded samples after submersion in the NaCl solution for 5 days. (a,c) Coatings containing 20 wt % type 0 capsules. (b,d) Coatings containing 20 wt % type 10 capsules.

Figure 9

Corrosion current as a function of lawsone loading for coatings containing 20 wt % microcapsules and 10 wt % microcapsules. Each data point represents one sample.

Figure 10

Corrosion inhibition efficiency as a function of lawsone loading for coatings containing 20 wt % microcapsules and 10 wt % microcapsules. Maximum and minimum IE % values were calculated for each data point shown in Figure 9.

Figure 11

Electrochemical test cell with three electrodes and a 5 wt % NaCl electrolyte (75 mL). The steel samples measured 30.5 by 10.0 by 0.5 cm. The silicone seal was 0.5 cm thick. The electrolyte chamber measured 7.6 by 10 cm at the base and had a 4 cm diameter opening at the top. The coated steel substrate acts as the working electrode, a platinum wire (7.5 cm long, 0.5 mm diameter) serves as the counter electrode, and a silver/silver chloride electrode (7.5 cm long, 6 mm diameter) serves as the reference electrode.