Control of Corrosion of Carbon Steel in Sea Water by Polyaniline/Epoxy-(graphene)-Based Double-Layered Smart Coatings
Authors: Diraki Ahmad , Omanovic Sasha 
Published: December 2024

Carbon steel (CS) is the preferred construction material in most industries due to its cost-effectiveness and unique mechanical properties. However, CS is susceptible to corrosion, and it thus requires the application of proper corrosion control measures to lengthen its service life. With regards to the oil and gas industry, the pipelines used to transport the oil and gas from the offshore production well are exposed to two main forms of the corrosive environment: the saltwater that surrounds the pipeline's outer wall, and the acidic corrosive environment that attacks the inner pipeline wall, especially during the oil and gas reservoir service such as the acidizing procedure for enhancing oil recovery (EOR). The most common method employed to protect the pipeline's outer wall is the application of protective coatings. However, many of the currently used coatings are not effective for extended periods of service. Therefore, there is a need to develop coatings with improved anti-corrosive properties, which could have an extended service life and, if damaged/compromised, could counterbalance the damage by acting as smart coatings.
To address the outer-wallcorrosion of carbon steel pipelines exposed to a simulated sea environment (3.5wt.% NaCl), two types of coatings were developed and characterized in our laboratory: (i) a composite double-layered anti-corrosive coating based on the inner electrically-conductive polyaniline (PANI) layer doped with graphene oxide (GO) synthesized electrochemically directly on the CS surface, and on the outer commercial epoxy coating, and (ii) a smart double-layered coating based on the inner PANI layer electrochemically formed on the CS surface and doped with sodium caprylate (SC) and sodium dodecyl sulphonate (SDS) as a corrosion inhibitor mixture, and the outer commercial epoxy coating.
The use of the double-layer PANI/GO/epoxy coating resulted in significantly better long-term anti-corrosive properties in the protection of CS surface in 3.5 wt.% NaCl. The coating maintained its protection efficiency over two months of constant exposure to the corrosive electrolyte (Figure 1). The excellent corrosion protection properties of the coating were prescribed solely to the presence of the underlying PANI/GO layer, which represents a high barrier for the transport of hydrated corrosive ions to the CS surface, through the combined action of charge (passive oxide film formation), surface energy (hydrophobicity), and blocking mechanisms.
The smart double-layered coating based on the inner PANI layer doped with SC+SDS and the outer epoxy layer yielded a hydrophobic surface and good adhesion to CS. When damaged to allow penetration of the corrosive electrolyte through it to reach the CS surface, it relatively quickly (within one day) recovered its anti-corrosive properties, as a consequence of the potential-driven release of SC+SDS from the PANI layer and its adsorption on the exposed CS surface, and it then continued to offer high corrosion protection during the remaining 29 days of exposure in aqueous 3.5 wt.% NaCl, in comparison with the undoped coating that failed rather quickly (within 4 hours), Figure 2.

Figure 1