Nanobubbles as corrosion inhibitor in acidic geothermal fluid
Introduction
Nanobubbles with a typical diameter of 50–200 nm are currently a growing research area of broad disciplines, with a significant focus on understanding their nature in bulk aqueous solutions (Lohse and Zhang, 2015). Despite the growing understanding of the pivotal role of the nanobubbles, the comprehensive scientific understanding of the behavior of nanobubbles still remains poor and provides room for more investigation. In contrast to micrometer-sized bubbles, a noteworthy property of nanobubbles is their longevity in liquid solutions and stability at high temperatures. Nanobubbles can exist for many hours, several days, or even a couple of months (Michailidi et al., 2020) rather than milliseconds theoretically expected (Weijs and Lohse, 2013). Surface nanobubbles are stable with respect to a temperature increase up to the boiling point of bulk water (Zhang et al., 2014). Surface nanobubbles have also been recognized to make a strong impact on the solid-liquid interface as they change the two-phase contact to a three-phase contact. This includes the effect of surface nanobubbles that changes the wettability and slippage on the solid surface (Lauga and Stone, 2003; Niavarani and Priezjev, 2010; Wang and Bhushan, 2010; Yen, 2015; Li et al., 2016). Thus, the use of nanobubbles holds promise in enabling to optimize the surface condition by controlling the interface hydrodynamically, allowing a broad range of engineering applications. Moreover, the use of nanobubbles will benefit from preventing chemical pollution and reducing cost for maintaining infrastructure in gas, oil, and geothermal industries, compared with the chemical products commonly used.
In geothermal infrastructures, material degradation due to corrosion is a major concern that leads to efficiency loss and eventual failure of the system. For example, steel instruments in the acidic geothermal fluid at moderate-to-high temperatures are subject to intensive corrosion (Nogara and Zarrouk, 2018a), increasing the operating costs of geothermal facilities. Well-planed material selection and corrosion engineering, taking into account environmental and economic considerations, are essential to reduce corrosion damage. It has been well documented that carbon steels, which are commonly used in a wide range of applications, are susceptible to intensive corrosion damages in highly corrosive geothermal fluids. In contrast, high-cost titanium, nickel and high nickel-based alloys, and (super-)ferritic and (super-)austenitic stainless steels have higher resistance levels to most geothermal fluid environments (Nogara and Zarrouk, 2018b). However, selecting the right materials presents a complex problem because the composition of geothermal fluids varies in a single operating day and from field to field. In addition to the costly corrosion-resistant material, numerous studies have alternatively sought to chemical solutions on the material surface for inhibiting corrosion. As the main corrosion process comprises anodic and cathodic reactions, corrosion inhibitors can generally be classified as anodic inhibitors, cathodic inhibitors, and mixed inhibitors (Tang, 2019). A protective coating of the material surface has also been found to be effective for inhibiting corrosion, by using microarc oxidation technologies (Sun et al., 2019) and organic compounds such as several Schiff base compounds (Shokry et al., 1998), acetylenic alcohols, propargyl alcohol and its derivatives (Finšgar and Jackson, 2014), Polyvinylamide derivatives (Tiu and Advincula, 2015), Pyridazinium derivatives (El-Hajjaji et al., 2019), Imidazole and Imidazoline derivatives (Solomon et al., 2019; Sun et al., 2019). Despite such longstanding study efforts, steel corrosion remains to present daunting challenges due to its intrinsic complexity. As nanoscale corrosion pathways have been found to be a significant impact on the corrosion behavior of steel (Hayden et al., 2019), there is a growing demand for nanoscopic operations on effective mitigation of corrosion (Dwivedi et al., 2017). Therefore, nanobubbles are expected to be an effective additive that can inhibit steel corrosion nanoscopically in geothermal infrastructures.
Here, we investigate, for the first time to our knowledge, whether nanobubbles can inhibit steel corrosion in acidic geothermal fluids. We test corrosion of the mild-carbon steel specimens (coupons) immersed in acidic geothermal fluid on-site at a geothermal power plant, with injecting the nanobubbles continuously generated by air for 7 days. We examine the effectiveness of corrosion inhibition by air-nanobubbles, combined with weight loss measurements of the immersed coupons, and micro-morphological observations and chemical composition analyses on the surfaces of tested coupons. These data also allow to understanding mechanisms with respect to how nanobubbles inhibit nanoscopically the bulk steel corrosion in the acidic geothermal fluid. Nanobubbles hold the potential for being a novel, environment-friendly, inexpensive and easy-to-use approach for inhibiting steel corrosion in the intensively corrosive medium.
Section snippets
Experimental setup for on-site immersion corrosion tests
The immersion corrosion testing was conducted on-site at the Hatchobaru Geothermal Power Plant (Kyushu Electric Power Co., Inc.) in Oita Prefecture, Japan. We used acidic geothermal fluid exiting the separator of the power plant (Fig. 1), and its chemical composition and electrical conductivity are summarized in Table 1. The silica concentration was around 860 mg/L. The test coupons for corrosion experiments were immersed in the geothermal fluid overflowed continuously in two heat-resisting
Visual inspection and weight losses of immersed coupons
Immersed coupons were sampled from the reference fluid and the fluid mixed with air-nanobubbles after 24, 48, 72, 96, 120, 144, and 168 h of immersion testing. Even after 24 h of immersion testing, both the coupons from the reference fluid and air-nanobubble fluid already lost much of their metallic luster (Fig. 2). Corrosion damages were then more significant in the coupons that were immersed for a longer time. Corrosion damages with abrasion were apparent in the coupons after 72 and 120 h of
Discussion
We have examined, for the first time to our knowledge, the behavior of steel corrosion immersed in an acidic geothermal fluid with continuously injected nanobubbles. We found that nanobubbles generally prevented the coupons from corrosional weight loss. Results of our immersion corrosion experiments revealed that the weight loss of coupons immersed in the fluid with air-nanobubbles was reduced as much as 50 % of the weight of the coupons in the untreated reference fluid. In addition,
Conclusions
We studied the use of nanobubbles as a corrosion inhibitor, for the first time to our knowledge, by testing immersion corrosion of low-carbon steel coupons in acidic geothermal fluid on-site a geothermal power plant. We performed weight loss measurements of the coupons immersed in the untreated geothermal fluid and the geothermal fluid with continuous injection of the air-nanobubbles for 7 days, and analyzed microstructure and chemical composition of the coupon surfaces using SEM and EDX,
Data availability
All data used in this work required to reproduce the results presented can be provided upon request to the corresponding author.
CRediT authorship contribution statement
Asuki Aikawa: Data curation, Resources, Formal analysis, Investigation, Methodology, Validation, Visualization, Writing - original draft. Arata Kioka: Conceptualization, Data curation, Resources, Formal analysis, Investigation, Funding acquisition, Methodology, Validation, Visualization, Writing - original draft, Supervision. Masami Nakagawa: Conceptualization, Resources, Investigation, Funding acquisition, Project administration, Writing - review & editing, Supervision. Satoshi Anzai:
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgment
The authors are grateful to A. Ueda, T. Yokoyama, R. Itoi, K. Yonezu, Y. Kiyota, K. Tagomori, and T. Kodama for discussion and their help throughout the work presented in the manuscript. The manuscript benefited from constructive comments by three anonymous reviewers. This work was partly supported by the project “Research and development of geothermal power generation technology — Development of technology for advanced use of geothermal energy — Development of chemical processing systems to
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