The purpose of this paper is to develop technically efficient and economically effective sacrificial anodes that can be used for cathodic protection (CP) of pipelines in marine environment and fill the knowledge gap in the use of carbon anodes for CP.
A sacrificial anode was produced via sand casting by adding varying weight-percent of coal and ferrosilicon to a constant weight-percent of grey cast iron. The hardness of the produced anodes was evaluated using a Rockwell hardness tester. The microstructure of the anodes was observed with scanning electron microscope/energy-dispersive spectroscopy (SEM/EDS). X-ray diffraction (XRD) was used to study the phases present. A potentiostat was used to assess the corrosion behaviour of the produced anodes and mild steel in 3.5 Wt.% NaCl solution.
The SEM results showed that some anodes had interdendritic graphite formation, while others had pronounced graphite flakes. The EDS analysis showed carbon and iron to be the prominent elements in the anode. Anodes Bc, B2 and B5 with a corrosion rate of two order of magnitudes were observed to have similar dendritic structures. Anode B4 is the most electronegative with an Ecorr of −670.274 mV Ag/AgCl and a corrosion rate of 0.052475 mmpy. The produced anodes can be used to protect mild steel in the same environment owing to their lower Ecorr values compared to that of mild steel −540.907 mV Ag/AgCl.
Alloying has been majorly used to improve the efficiency of sacrificial anodes and to alleviate its setbacks. However, development of more technically efficient and economically effective sacrificial anodes via production of composite has not been exhaustively considered. Hence, this research focuses on the development of a carbon based anode by adding natural occurring coal and ferrosilicon to grey cast iron. The corrosion behaviour of the produced anode was evaluated and compared to that of mild steel in marine environment.
Osundare, A., Oloruntoba, D. and Popoola, P. (2018), "Development of carbon anode for cathodic protection of mild steel in chloride environment", Anti-Corrosion Methods and Materials, Vol. 65 No. 2, pp. 158-165. https://doi.org/10.1108/ACMM-07-2017-1817Download as .RIS
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Petroleum pipelines are the backbone of the oil and gas transportation system (Hopkins et al., 2011). Major pipelines across the world transport large quantities of crude oil, natural gas and petroleum products. In modern cities, pipelines are essential in providing fuels for vital functions such as power generation, heating supply and transportation. Failure in the transportation of hazardous products by these pipelines is a potential cause of serious environmental damage, explosions, loss of income, lives and properties. Sources of failure include corrosion problems, amongst others (Hamid et al., 2015). Low alloy steel is an important metal used in marine structures and it has been in service for many years in marine applications. However, good corrosion protection is essential to guarantee a long-term safe operation of low alloy steel facilities such as pipelines and offshore platforms (Sun et al., 2013).
Cathodic protection (CP) is a technique to reduce the corrosion susceptibility of a metal surface by making it the cathode of an electrochemical cell (Parthiban et al., 2008; Szabó and Bakos 2006). This is accomplished by shifting the potential of the metal in the negative direction using an external power source (referred to as impressed current CP) or a sacrificial anode. For the impressed current system, a current is applied to the structure using a power supply, referred to as a rectifier, and an anode (high silicon cast iron [HSCI], metal scrap, graphite, magnetite and platinized materials) buried in the medium. In the case of a sacrificial anode system, the galvanic relationship between a sacrificial anode material, such as zinc or magnesium, and the structure is used to supply the required CP current (Abootalebi et al., 2010).
At present, magnesium (Mg), zinc (Zn) and aluminium (Al) are the most commonly used sacrificial metals for CP (Pathak et al., 2012; Parthiban et al., 2008; Yu and Uan 2006) Aluminium has attained considerable merit as the basis for a galvanic anode in oil and gas environments mainly due to its low density, large electrochemical equivalent, availability, thermal and electrical conductivity, high current capacity, low specific weight and reasonable cost (Shibli and Gireesh, 2005; Smoljko et al., 2012; Singleton et al., 2011; Shibli et al., 2007; Liu et al., 2014). However, aluminium suffers from self-passivation by the formation of a thin, continuous, adherent and passive layer of γ-Al2O3 on its surface, which makes it unsuitable to be used as a sacrificial anode. This setback is corrected by alloying with certain metals whose primary role is to hinder the formation of a continuous, adherent and protective oxide film on the surface of the alloy, thus permitting continuous galvanic activity of the aluminium (Muoz et al., 2002; Xiao et al., 2011; Khireche et al., 2014; El Shayeb et al., 2001). Alloying elements such as zinc (Zn), titanium (Ti), mercury (Hg) and indium (In) have been used to produce aluminium alloys for sacrificial anodes (Bessone, 2006; Gurrappa, 2005; Bahadori, 2014; Rajani and Kleiner, 2007). Magnesium, another commonly used sacrificial anode, suffers from local cell action and formation of products on its surface, drastically reducing its efficiency to about 50 per cent. This implies that only one-half of its corrosion current flows to the protected structure, the rest is wasted on small local reactions on the magnesium surface. To cater for this setback, an alloy AZ63 (6 Al, 3Zn, 0.2 Mn) is mostly preferred over pure magnesium, as it prevents pitting of the anode in service. More studies are still being carried out to improve the efficiency of these anodes. Zinc anodes are usually 99.99 per cent Zn or an alloy with a few tenths of a percent depending on the environment to be used. It has an excellent efficiency of about 95 per cent, but it gets easily used up (Bradford, 2001).
Since the birth of lithium ion battery at the end of the 1980s and early 1990s, many kinds of anode materials have been studied. Nevertheless, graphitic carbon is still the only commercially available product. This could be attributed to an attractive combination of relatively low cost, abundance and moderate energy density (Nitta et al., 2015; Wu et al., 2003). The prominence and competence of carbon anodes in the extraction of aluminium via Hall–Heroult electrolysis further attest to the competence of carbon as an anode material (Capral Aluminium, 2012). As a result, modification of carbonaceous anode materials has been a research focus.
Alloying has been majorly used to improve the efficiency of sacrificial anodes and alleviate its setbacks. However, the development of more technically efficient and economically effective sacrificial anodes via production of composite has not been exhaustively considered. Hence, in this present study, a sacrificial anode will be produced by adding coal and ferrosilicon to a grey cast iron and its corrosion behaviour in 3.5 Wt.% NaCl solution will be evaluated and compared to that of mild steel in same environment.
2.1 Material preparation
Coal lumps were reduced to smaller particles using a pulveriser. The coal and ferrosilicon particles were then ground separately for hours in a ball mill to obtain very fine particles. Sieve analysis was then carried out on the ground particles to obtain 75-µm particle size.
2.2 Carbon anode production
The chemical composition of the grey cast iron is shown in Table I. A varied weight-percent of coal and ferrosilicon of ratio 30:0, 25:5, 20:10, 15:15, 10:20 and 5:25 were added to 70 Wt.% of grey cast iron as indicated in Table II. The grey iron was melted in an indirect electric arc furnace and held at a temperature of 1,500°C for 15 min for homogenization. Coal and ferrosilicon were added to the melt minutes before pouring into a pre-heated mould with dimensions Ø 16 mm × 200 mm.
2.3 Hardness test
The hardness values of the alloy were evaluated on a hardness testing machine using the digital Rockwell hardness tester adopting an HRA (60 kgf) scale. Sample preparation and testing procedure were performed in accordance with ASTM E18-16 (2016) standard. Five hardness indents were made on each specimen, and readings within the margin of ±2 per cent were taken for computing the average hardness values of the specimen.
2.4 Anode characterization
The microstructure of the anode was characterized using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). X-ray diffraction (XRD) was conducted to observe the phases present in the anodes.
2.5 Electrochemical test
Corrosion testing was conducted using electrochemical methods in accordance with the ASTM G59-97 (2014) standard. The experiments were carried out using an AutoLAb potentiostat (VersaSTAT 400) with versa STUDIO electrochemical software. The corrosion behaviour of the anodes was investigated in 3.5 Wt.% NaCl solution at room temperature (25°C). The experiments were performed using a three-electrode corrosion cell set-up comprising the anode and mild steel as the working electrode, saturated silver/silver chloride as the reference electrode and platinum as the counter electrode. The working electrodes were prepared by attaching an insulated copper wire to one face of the sample using an aluminium conducting tape, and cold mounting it with epoxy resin. The working electrode was immersed in the 3.5 Wt.% NaCl for a suitable time to stabilise the open-circuit corrosion potential (OCP). Potentiodynamic polarisation was measured using a scan rate of 1 mV/s at a potential initiated at −250 mV to +250 mV on OCP. After each experiment, the electrolyte and the test samples were replaced. The linear Tafel segments of the anodic and cathodic curves were extrapolated to corrosion current densities (Icorr) and corrosion potential (Ecorr). Three repeat tests were carried out for all compositions of the anodes, and the reproducibility and repeatability were observed to be good, as there were no significant differences between results from triplicates.
3. Results and discussion
Figure 1 shows the hardness values of the anodes produced. The hardness of the anodes does not depend on the concentrations of the coal particles added but on the ferrosilicon additions. This could be attributed to the reduced tendency for carbides’ formation as ferrosilicon additions increased (Seidu et al., 2014).
The SEM images of the produced anodes are shown in Figures 2 to 7 and that of coal in Figure 8. It was observed that all the anodes had a combination of graphite and ferrite phases. Graphitization increased with decreasing coal content and peaked in Anode B3 (15 Wt.% coal and 15 Wt.% FeSi). Figures 2 and 3 show an interdendritic graphite formation representing Type D graphite flakes with random orientation (ASTM A247). This could be because of decreased graphitization and increased chilling tendency caused by the addition of 30 Wt.% coal for Bc and 5 Wt.% coal and 25 Wt.% ferrosilicon for B5.
On the basis of the EDS analysis of the anodes, carbon, oxygen, silicon and iron are the prominent compounds inherent in the anodes (as depicted in Figures 9-11). The oxides could be attributed to the additions of coal as shown in Figure 12. Anode Bc with 30 Wt.% additions of coal had increased silicon content owing to the presence of silicon in the coal added. From the XRD patterns, the major phases present are rich in iron, while the other phases are rich in carbon and silicon (Figures 13 to 15).
3.3 Electrochemical test of produced carbon anodes in 3.5 Wt.% NaCl solution
Figure 16 and Table III show the potentiodynamic curves and potetiodynamic data of the produced anodes in 3.5 Wt.% NaCl solution. The corrosion rate of the anodes improved with the additions of coal. The anodes having corrosion rates with two orders of magnitude were observed to have similar interdendritic microstructure as seen in Figures 2, 4 and 7. These anodes were also observed to have very high Icorr compared to Anodes B1, B3 and B4 with a pronounced graphite formation. The better corrosion resistance of anodes with interdendritic graphite structures could be attribute to the very little disparity in the anodic and cathodic site existing in its structure compared to anodes with pronounced graphite structures. The pronounced graphite flakes, formation of rare compounds and inhomogeneity in these anodes could be responsible for the increased corrosion rate (Charng and Lansing, 1982). Anode B4 has the lowest Ecorr of −670.274 mV and a corrosion rate of 0.052475 mmpy. Anodes required for CP should have a low dissolution rate and be more electronegative to avoid polarity reversal when in contact with steel (Guessoum et al., 2011). All produced anodes had lower Ecorr compared to mild steel (−540.907 mV) in the same environment, with the highest being Anode B3 with −623.832 mV. This implies that the produced anodes will protect the mild steel in same environment (Figure 17).
Addition of coal to grey cast iron improves its corrosion resistance. The corrosion behaviour of the anodes depends on the type of structure obtained after production. Anode with interdendritic graphite structures was observed to be more corrosion-resistant compared to anode with pronounced graphite flakes structure. All produced anodes are more electronegative with a lower corrosion rate compared to mild steel in 3.5 Wt.% NaCl solution; hence, making more electronegative anodes suitable to protect mild steel in the same environment.
Chemical composition of grey cast iron
Weight percent of coal used for anode production
Potentiodynamic polarization data obtained from Tafel plot for produced anodes immersed in 3.5 Wt.% NaCl solution
|Sample||Ecorr (mV) Ag/AgCl||Icorr (µA)||Corrosion rate (mmpy)|
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