Whats The Effect of Imidazole on Carbon Steel Weldment in District Heating Water? (Material Science)

Sang-Jin Ko and colleagues investigated the effect of imidazole as a corrosion inhibitor on carbon steel weldment in alkaline district heating water. They showed that, depending on the concentration of imidazole, the ratio of interaction between carbon steel and imidazole affected inhibition efficiency. Their study recently appeared in the Journal Materials.

Corrosion of metal in aqueous systems has led to structural degradation and accidents. In huge fluid transport systems such as those used in the petrochemical industry and district heating systems, the effect of corrosion is more extensive because it is hard to use expensive high corrosion-resistant metals in such large systems. The corrosion in a district heating system directly affects the lifespan and function of pipes by causing metal ion solvation and corrosion byproducts. Weldments of carbon steel are especially susceptible due to properties such as having different microstructures of base metal.

To reduce this problem, low-cost water treatment methods have been applied including pH control, deaeration, and addition of inhibitors. The organic inhibitor is one of the major methods used to reduce corrosion rate by its adsorbing on metal surfaces. Many research studies have been conducted on the corrosion inhibition performance of imidazole in acidic environments such as in the piping of a petrochemical plant. However, there has been no study on the effect of imidazole in alkaline conditions such as a local district water heating environment.

Thus, Sang-Jin Ko and colleagues investigated the effect of imidazole as a corrosion inhibitor on carbon steel weldment in alkaline district heating water. Along with, the inhibition efficiency and electrochemical properties by potentiodynamic polarization test and electrochemical impedance spectroscopy. They showed that, as the concentration of imidazole increased up to 500 ppm, inhibition efficiency increased up to 91.7%. At 1000 ppm, inhibition efficiency decreased.

Figure 1: Inhibition efficiency (%) from potentiodynamic polarization test and value of ndl from EIS. © Ko et al.

Additionally, to clarify the effect of imidazole as a corrosion inhibitor and to observe surface morphology after corrosion, they obtained surface images using Optical Microscopy (OM) after 40 h immersion in the solutions of 0, 100, 300, 500, and 1000 ppm of imidazole. It has been shown from OM measurements that, 500 ppm imidazole solution offered the highest inhibition efficiency and corrosion resistance. In 300 and 1000 ppm solution samples, micro-scale pittings were observed implying the lower inhibition efficiency and corrosion resistance.

Figure 2. Optical microscopy images (15×) of carbon steel weldment after 40 h immersion in the solution with addition of (a) 0 ppm, (c) 100 ppm, (e) 300 ppm, (g) 500 ppm, and (i) 1000 ppm of imidazole and magnified images (45×) of (b) 0 ppm, (d) 100 ppm, (f) 300 ppm, (h) 500 ppm, and (j) 1000 ppm of imidazole; white dotted lines show weldment area and red circles show micro-scale pitting © Ko et al.

Moreover, they conducted atomic force microscopy (AFM) to investigate surface properties of carbon steel weldment after 6h immersion in 500 and 1000 ppm of imidazole. They showed that the surface coverage of imidazole at 1000 ppm is lower than that of imidazole at 500 ppm.

Figure 3. Topography and surface potential of carbon steel weldment after 6 h immersion in 500 and 1000 ppm of imidazole solution as measured by AFM with SKPFM mode. © Ko et al.

Finally, X-ray photoelectron spectroscopy (XPS) was conducted in order to investigate how imidazole adsorbed on to the carbon steel surface. They showed that, with 500 ppm of imidazole, the amount of pyrrole type interaction is 4.8 times larger than pyridine type interaction. At 1000 ppm of imidazole, the amount of pyridine type interaction is 3.49 times larger than pyrrole type interaction.

Figure 4. XPS results (N1s) of carbon steel weldment after 6 h immersion in (a) 500 ppm and (b) 1000 ppm of imidazole. © Ko et al.

Their results demonstrated that, depending on the concentration of imidazole, the ratio of interaction between carbon steel and imidazole affected inhibition efficiency.

“Our study shows that samples from 1000 ppm of imidazole solution show a lower inhibition efficiency than samples from 500 ppm of imidazole.”

— they concluded.

Featured image: Molecular structure of imidazole. © Sang-Jin Ko et al.


Reference: Ko, S.-J.; Choi, S.-R.; Hong, M.-S.; Kim, W.-C.; Kim, J.-G. Effect of Imidazole as Corrosion Inhibitor on Carbon Steel Weldment in District Heating Water. Materials 2021, 14, https://doi.org/10.3390/ma14164416


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