A Double-Layer Epoxy and Polyaniline/Graphene Oxide Composite Coating for Corrosion Protection of Carbon Steel Immersed in Seawater

Abstract

Offshore pipelines transport the produced hydrocarbon from the upstream production well to the downstream hydrocarbon separation facilities. Offshore oil and gas pipelines are still the safest way of transporting hydrocarbon, as evidenced by significant low failure rates in contrast with highway transportation, railroad, and ships, yet. The failures do occur and often with catastrophic results, especially to the marine eco-system. There are several origins of pipeline failures, but corrosion is the main one. Coatings are always used to control external corrosion of pipelines, in combination with impressed cathodic protection. However, the development of new coatings has always been an ongoing process. Recently, non-conducting epoxy coatings combined with conducting polymers (CPs) have been used to protect metals from corrosion [1]. Epoxy coatings are often used for corrosion prevention because of their exceptional mechanical, barrier and chemical properties [2-4]. CPs coatings have exhibited promising corrosion inhibition performance of metal substrates because they are environmentally stable, eco-friendly, non-toxic, and adhere firmly to the metal surface [5].In the present work, we developed and applied a double-layer coating on carbon steel (CS) to overcome the limitation of polyaniline/graphene oxide (PANI/GO) composites during an extended emersion time in the corrosive environment. We first synthesized polyaniline (PANI) coating incorporating a single-layer-GO directly on the carbon steel substrate by an electrochemical deposition method (inner layer), which was then followed by the application of a second (outer/top) layer of commercial pure neat epoxy resin. The corrosion resistance studies revealed that the two-layer PANI/GO/epoxy coating offered a significantly better long-term protection towards corrosion of the CS surface than the two-layer PANI/epoxy and, especially, than the single-layer epoxy coating.The coatings’ surface morphology was studied by field-emission scanning electron microscopy. The thickness mapping of the coatings was evaluated by a profilometer. The coatings’ surface wettability was also determined. The electrochemical/corrosion measurements were carried out in an aqueous 3.5 wt.% NaCl. Electrochemical impedance spectroscopy (EIS) and Tafel polarization were used to investigate the corrosion resistance of the coated and uncoated samples. CS samples coated with only PANI, or PANI/GO could not be instigated using Tafel and EIS techniques due to the samples’ relatively good electrical conductivity. Before each Tafel and EIS run, the samples were conditioned at open-circuit potential (OCP) for 1 hour. EIS measurements were performed at OCP in the frequency range between 10 mHz and 50 kHz applying a ±10 mV alternating voltage amplitude. Tafel polarization curves were recorded by anodically polarizing the samples from -200 mV to +200 mV vs. OCP at a scan rate of 1 mV s-1. All potentials in this paper are referred to SCE. All the measurements were performed at 22±1oC.Profilometry measurements revealed that the thickness of the PANI and PANI/GO coatings was 4.84±0.12 mm, while the epoxy coating was substantially thicker, 20.6±0.2 mm, and the double layers coating thickness was 24.9±0.14 mm. Our corrosion resistance results indicate the application of the epoxy coating resulted in a corrosion current decrease by almost two orders of magnitude, while the addition of the thin PANI sub-layer contributed to the further reduction by ca. two orders of magnitude. A surprising finding was that incorporating just 0.01 wt.% of GO into the PANI matrix resulted in a significant additional decrease in the corrosion current of almost two orders of magnitude. Long-term measurements (60 days, Figure 1) revealed that the corrosion resistance of the PANI/GO-epoxy coating remained stable. The PANI-epoxy coating started failing after ca. 37 days, while the resistance of the single-layer epoxy coating was substantially lower and decreased first after ca. 4 days of immersion, and then again started decreasing after ca. 19 days of immersion. The sustained high impedance value for the entire immersion time demonstrates excellent barrier performance of the double-layer PANI/GO-epoxy coating.[1] D.W. DeBerry, Journal of the Electrochemical society, 132 (1985) 1022.[2] V. Mišković-Stanković, J. Zotović, Z. Kačarević-Popović, M.D. Maksimović, Electrochimica acta, 44 (1999) 4269-4277.[3] Y. Wei, L. Zhang, W. Ke, Corrosion science, 49 (2007) 287-302.[4] Y.-X. Lan, C.W. Weng, M.M. Ahmed, K.-H. Luo, W.-F. Ji, W.-R. Liu, J.-M. Yeh, Materials Chemistry and Physics, 264 (2021) 124446.[5] S. Kim, T.-H. Le, C.S. Park, G. Park, K.H. Kim, S. Kim, O.S. Kwon, G.T. Lim, H. Yoon, Scientific reports, 7 (2017) 1-9. Figure 1