Investigating the Accelerated Electrochemical Corrosion Protection Performance of Coal-Tar and Bitumen Enamel Coatings for Pipelines

Corrosion of pipelines is an extreme challenge for oil and gas industry. The harsh environment of pipelines application has great effect on selecting the appropriate coatings for them. Different types of coatings are used for corrosion protection of pipelines in industry and meanwhile, coal-tar and bitumen enamel coatings are widely applied for enhancing their service-life [1-3]. Therefore, in this research, the effects of industrially used coal-tar and bitumen enamel coatings on mild steel substrates are considered. The degradation process of coatings is examined in the synthetic groundwater by electrochemical methods using Potentiostat/Galvanostat (PGSTAT 30). The electrochemical cell consists of the sample, Pt wire, and saturated calomel electrode (SCE) as working, counter, and reference electrode, respectively. Accordingly, the polarization tests are repeated 7 times for accelerating the degradation of coatings. For potentiodynamic polarization tests, open circuit potential (OCP) of system at equilibrium state after immersion is recorded as corrosion potential (Ecorr). Then, the polarization curve was plotted by sweeping the applied potential from -0.4 to +0.4 V with respect to OCP at scan rate of 1 mV/sec on 1 cm2 anode. Corrosion current density (icorr) was determined from Tafel plot by extrapolating the linear portion of the curve to Ecorr. Tafel constants including anodic (βa) and cathodic (βc) slopes were calculated for anodic and cathodic parts of Tafel plot, respectively. Then, the corrosion protection performance of coatings is determined by electrochemical impedance spectroscopy (EIS). The EIS tests were carried out in frequency range of 10-2 to 105 Hz with AC amplitude of 10 mV at OCP. All electrochemical tests are repeated three times. The variation of OCP during immersion in synthetic groundwater indicates that the coated substrates have more positive OCP values compared to bare one. Meanwhile, the OCP value of the coal-tar enamel coated sample is more noble than bitumen enamel coated sample. The results derived from potentiodynamic polarization tests show that coal-tar coated sample has more positive Ecorr, lower icorr, lower porosity, higher polarization resistance (Rp) and higher corrosion protection efficiency (PE); revealing enhanced corrosion protection capability of coal-tar enamel coating compared to bitumen enamel coated sample. Indeed, the corrosion rate for bare substrate, bitumen enamel coating, and coal-tar enamel coating is 0.07502, 13th Annual Electrochemistry Seminar of Iran Materials and Energy Research Center (MERC), 22- 23 Nov, 2017 292 0.00904, and 0.00159 mm/year, respectively. Moreover, the EIS results including Bode, phase and Nyquist plots present that the coal–tar enamel coating is more appropriate for achieving enhanced corrosion protection for pipelines due to their higher corrosion resistance. The results of EIS are fitted by electrical equivalent circuit by using NOVA software which show that the coating resistance and charge transfer resistance for coal-tar enamel coating is significantly more than bitumen enamel coating. The enhanced corrosion protection capability of this sample can be attributed to the pore structure and chemical composition of coal-tar enamel coating. Figure 1 shows the overview of the obtained electrochemical data. Figure 1. (a) The variation of OCP; (b) The Bode plots; (c, d) The Nyquist plots for bare substrate, than bitumen enamel coating, and coal-tar enamel coating.