Corrosion Resistances of Steel Pipes Internally Coated with Enamel

The corrosion resistances of enamel-coated steel pipe in 3.5 wt% NaCl solution was evaluated and compared with those of epoxy-coated pipe using open-circuit potential, linear polarization resistance, and electrochemical impedance spectroscopy tests. T-001c enamel slurry and GP2118 enamel powder were sprayed to steel pipe in wet and electrostatic processes, respectively. The phase composition and microstructures of the two enamels were characterized with x-ray diffraction and scanning electron microscopy (SEM). The surface roughness of enamels and their bond strength with steel substrates were quantified to understand coating quality. SEM images revealed that both types of enamel coatings have a solid structure with isolated bubbles. Electrochemical tests showed a high corrosion resistance of the enamel coatings as verified in visual inspection on the tested samples. In particular, the GP2118 enamel-coated samples consistently outperformed the epoxy-coated samples.