Electrochemical test of the 45# carbon steel in inhibitors

Experiment setup

Specimen: 45# carbon steel

Media: 0.05 mol/L NaCl +1.2% of the inhibitor

Experiments：EIS, Potentiodynamic

Instrument: CorrTest single-channel potentiostat model CS350M

Electrodes: 3-electrode system: Pt electrode as the counter electrode, Saturated calomel electrode as the reference electrode, and 45# carbon steel as the working electrode. The diameter of the specimen is 1.27cm, with exposure area 4 cm²

Treatment of the specimens

Polish the metal specimen with 1000-grit, 2000-grit sandpaper in order. Then clean it with alcohol. Use 703 silicone rubber to seal the specimens, leaving 1cm in the middle for testing (the surface area is 4 cm²). You can test it after the silicone rubber is dry.

Fig. 1. Treatment of the specimens

Fig. 2. Inhibitor solution

Electrochemical experiment

• EIS

Green (WE) and white (Sense) connect together to the working electrode/specimen

Yellow (RE) alligator connects to the reference electrode

Red (CE) alligator connects to the counter electrode

Fig. 3. the setup of the test

Choose the technique “Potentiostatic EIS”. After OCP is stable, run it with the parameters set as below:

Fig. 4 EIS test parameter setting

Fig 5. EIS result with inhibitor #1

Fig 6. EIS result with inhibitor #2

Fig 7. comparison of the EIS results

The diameter of the capacitance arc of system with inhibitor #1 and modulus are larger, and the Rp value after fitting will also be larger. According to icorr=B/Rp, (where B is the constant Stern coefficient), we know that the corrosion current density of the specimen in inhibitor #1 is lower, indicating that inhibitor #1 has better inhibiting effect than inhibitor #2 for the specimen.

• Polarization curve

Choose the technique “Potentiodynamic”. Potential sweeps from -0.6 to 0.5V vs. OCP, and the scan rate is 0.5 mV/s.

Fig. 8. Parameters setting for potentiodynamic

Fig 9. Tafel fitting result for 45# steel in inhibitor #1

From the Tafel fitting result, we can see that the self-corrosion potential E0= - 0.276V, self-corrosion current density i0=0.122 μA/cm2, corrosion rate =0.00143 mm/a, pitting corrosion potential is 218 mV.

Fig 10. Tafel fitting result for 45# steel in inhibitor #2

For the Tafel fitting result of 45# steel in inhibitor #2, the self-corrosion potential E0= - 0.298 V, self-corrosion current density i0=0.15494 μA/cm2, corrosion rate =0.00182 mm/a, pitting corrosion potential is 202 mV. 

We can conclude that the inhibitor #1 has better inhibiting effect, which is in line with what we conclude from EIS result.