Galvanic corrosion based on wire beam electrode technique: progress and prospects

2.1 Corrosion mechanisms

Galvanic corrosion is the accelerated corrosion of a metal when it is connected to other metals with a higher potential, so it is also called bimetallic contact corrosion (Hack 2016). When galvanic corrosion occurs, low-potential active metals are usually dissolved as anodes, while high-potential metals are protected as cathodes. In addition, galvanic corrosion can also induce crevice corrosion, stress corrosion, pitting corrosion, and other types of local corrosion, which seriously affects the safety of metal equipment. Therefore, galvanic corrosion has received sufficient attention from relevant scientific researchers as a major safety hazard (Du 2013; Shi et al. 2017; Yang et al. 2000).

Figure 1 shows that three fundamental conditions are required for galvanic corrosion to occur. First, there are metal materials with different self-corrosion potentials. The potential difference determines the tendency of galvanic corrosion; second, there is an ion conduction channel. The ion conduction channel requires the coupling metal to be submerged in the electrolyte solution; third, there is an electronic conduction channel. The electronic conduction channel refers to the direct contact of the two metals or the connection via a conductor to form an electronic circuit (Randle 1994).

Figure 1:

Schematic diagram of galvanic corrosion process.

2.2 Influencing factors

Galvanic corrosion is influenced by a variety of factors that are both diverse and complex. Many specialists are dedicated to researching the mechanism of various influencing factors on galvanic corrosion. Related scientists represented by Chen et al. (2010) separated the key factors impacting galvanic corrosion into three categories: the material characteristics and geometric factors of the metal pair, and the environmental factors.

2.2.3 Environmental factors

2.2.3.1 Temperature

Temperature is a critical environmental factor in the corrosion process. From a thermodynamic standpoint, increasing the temperature lowers the activation energy of the anode and cathode reactions and speeds up the pace of galvanic corrosion. In the case of galvanic corrosion between active metals, raising the temperature accelerates self-corrosion reaction rate of the anode and cathode, increasing the galvanic corrosion rate (Hu 2020). The Arrhenius equation (Khaled et al. 2004) describes the link between temperature and corrosion current density as follows:

icorr=A⋅exp(−EaRT)

log(icorr)=log(A)−Ea2.303RT

where E_a represents the reaction activation energy in j⋅mol−1, R represents ideal gas constant, T represents reaction temperature in K.

The logarithm of the corrosion current density has been shown to be linearly related to the inverse of the temperature under different experimental conditions (Hu et al. 2021; Tamarit et al. 2008; Tovar et al. 2009), i.e., in accordance with the Arrhenius equation. The current emphasis of relevant research on the mechanism of temperature-influenced galvanic corrosion is on the middle and high temperature range (20–100 °C). However, because real engineering may entail low-temperature environments, such as deep sea galvanic corrosion, further study is required.

2.2.3.2 Oxygen content

The oxygen content in different environments varies greatly. The oxygen concentration of open systems like soil is substantially higher than that of static and closed systems like the deep sea. The quantity of oxygen in the corrosive environment has a significant impact on galvanic corrosion under normal conditions. When oxygen absorption corrosion occurs on the metal surface, the corrosion process is often under the control of the cathode. As a result, raising the oxygen concentration accelerates the transport of oxygen to the electrode surface, accelerating the rate of galvanic corrosion. However, for readily passivated metals such as aluminum (aluminum alloy) and stainless steel, raising the oxygen content aids in the creation and repair of the passivation film, inhibiting or even preventing the occurrence of galvanic corrosion.

In the 1970s, Shalaby (1971) discovered that adding inert gas to a solution might suppress galvanic corrosion, but the galvanic effect was significantly enhanced by the introduction of oxygen. Wang (2013) also discovered that the oxygen concentration of saltwater has a substantial impact on galvanic corrosion rate, with a strong linear connection between galvanic current density and oxygen content.

2.2.3.3 Media flow state

The flow of the medium can accelerate the mass transfer process in solution, reduce or even eliminate concentration polarization on the surface of coupling metal, preserving the initial potential difference between the anode and cathode metals involved in galvanic corrosion, resulting in a faster rate of corrosion.

Dai et al. (1992) and Zhu et al. (1992) demonstrated that raising the flow rate can considerably increase the galvanic corrosion rate within a specific flow rate range. Dong et al. (2010) investigated the galvanic corrosion behavior of carbon steel and stainless steel under high flow velocity (13.7, 20, 27 m/s). Figure 2 depicts the evolution of galvanic current over time. Compared with the static solution, the flow of media can significantly improve the galvanic effect, which is manifested in the rapid increase of galvanic current in the early stages of corrosion, and the galvanic current is positively correlated with the flow rate. Sun et al. (2011) observed that when the flow rate of seawater increases, so does its influence on galvanic corrosion.

Figure 2:

Galvanic current of 1020 carbon steel and 304L stainless steel couple at different flow rates.

In addition to the related factors mentioned, the pH value and ion concentration (conductivity) of the electrolyte solution also have an impact on the galvanic corrosion process.

3.1 Technique principle

The wire beam electrode (WBE) is a unique type of surface testing technique, differing from traditional macroscopic electrochemical methods. This technique uses the principle of calculus to divide the sample surface into multiple insulated tiny electrodes. Each wire is insulated from its neighbors and constitutes a minisensor. These microelectrodes are capable of measuring more than just the average electrochemical parameters of the electrode surface as a whole. It can also be used as individual electrodes to collect and process corrosion information from small areas using its matching measurement and control system to characterize the electrochemical difference on the surface of metal materials (Bu et al. 2019; Weng and Zhao 2003). It also provides an efficient method for investigating the related mechanism and process of galvanic corrosion (Dong et al. 2011; Shi et al. 2013).

Another important characteristic of the WBE is that the surface area of each wire in the WBE is much smaller than the total working electrode area, so corrosion and electrochemical processes on each wire surface can be assumed to be uniform even if the whole electrode surface is electrochemically nonuniform. This assumption allows electrochemical techniques and theories describing uniform electrochemical processes to be applied to each wire in a WBE.

The WBE maps contain both spatial and temporal information on localized corrosion, and thus are useful for understanding the mechanisms of localized corrosion and for quantifying the effects of localized corrosion mitigation. The corrosion potential and current on the surface of the WBE can be measured using the electrochemical workstation. The measuring principle is depicted in Figure 3 by employing a high-resolution potential follower and a zero-resistance current meter to detect the open circuit potential of each metal wire relative to the reference electrode and the short-circuit current between each metal wire. As a result, the electrochemical parameters of each electrode on the surface of the WBE can be determined by measuring through the automatic cycle potential/current scanning method.

Figure 3:

Schematic of WBE: (a) Potential scan, (b) current scan.

3.2 Development process

Because of the WBE technique and the continuous development and improvement of relevant testing systems, accurate electrochemical parameters of metal surfaces can be obtained. It has been widely used for measuring surface potential and current fluctuations during the induction, development, and inhibition of metal local corrosion.

The WBE was initially designed to detect localized defects in organic coatings (Tan 1991a; Tan and Yu 1991b; Wu et al. 1995) and anti-rust oil films (Tan 1998). In the 1990s, Tan (1993) connected the microelectrode to the digital voltmeter, and the other end of the voltmeter was connected to the auxiliary electrode. A closed loop with the electrolyte solution was formed to obtain the electrochemical parameters of each wire surface. This test method had obvious shortcomings: a long test time, cumbersome operation steps, and low accuracy. After that, (Lin et al. 1997) introduced the computer to the system and realized the automation of the WBE test process. On this basis, Zhong et al. (1997) applied a CA3140 current amplifier and impedance transformation to the data test system and improved the accuracy of experiments. Wang et al. (2009) developed a PXI WBE instrument composed of modules controlled by LabVIEW software, which greatly improved the test efficiency and promoted the popularization and application of the test system. Li et al. (2013) added a high-precision digital multimeter module to the data test system, aiming for the real-time switching of corrosion current and potential.

To date, the WBE technology has been widely applied in various corrosion research fields, such as under-scaling corrosion, atmospheric corrosion, galvanic corrosion, corrosion inhibitor evaluation, and coating corrosion resistance evaluation. The statistical analysis of WBE technique applied to various types of local corrosion is shown in Table 1.

3.3 Advantages

Compared with the traditional electrochemical test method represented by the weightlessness method, the WBE technique has obvious advantages:

1. Traditional electrochemical methods can obtain the average corrosion parameter information on the electrode surface but cannot reflect the nonuniformity of corrosion. The WBE technique can remedy this and accurately measure the distribution information of the electrode potential and the galvanic current on the metal surface (Zhang et al. 2015).

2. The related test system of the WBE has the advantages of a high degree of automation, fast measurement speed, mature development, and continuous measurement.

3. The test process has no high requirements for the flatness of the metal surface. It can be carried out even under complicated conditions (coating, corrosion product coverage, etc.). Therefore, this technique can be applied to monitor the corrosion parameter information distribution characteristics of the alloy interface under the complex electrode surface state.

4.1 The influence of different factors

The WBE is widely used in the field of galvanic corrosion to study the corrosion behavior and mechanism of different metal coupling systems, as well as the influence of various corrosion factors on galvanic corrosion, because it has the characteristics of different metals arbitrarily arranged to form various pairs. Tan et al. (2012) developed a novel local corrosion intensity index (LCII) to characterize the nonuniform distribution of the current density on the surface of the microelectrode using the bottom-based measurement features of the WBE technique.

where the i_max represents the maximum anode current density, and the i_tot represents the total anode current density.

The inhomogeneity of current distribution on the metal surface can be quantitatively described using LCII. When the anode current is evenly distributed on the surface of 100 microelectrodes in an ideal and uniform corrosion test environment, the LCII is close to 0.01. but when the corrosion height tends to be localized, the LCII is close to 1. In the actual experiment process, the corrosion is generally considered uniform when the LCII is less than 0.1, and the corrosion process can be regarded as localized when the LCII is higher than 0.1.

Temperature has caught the attention of associated researchers as the key environmental component that influences galvanic corrosion, and a great number of experimental experiments have been carried out. Chen et al. (2018) investigated the galvanic corrosion of WBE in a 0.6 M sodium chloride solution at three different temperatures (35 °C, 55 °C, and 80 °C), and the WBE was made of three-metals: Cu, Cu–Sn alloy, and Cu–Zn alloy (Figure 4). It was found that the average galvanic current of the Cu and Cu–Sn alloy, which shifted from the cathode at 35 °C to the anode at 80 °C, increased as temperature increased during the immersion time. In contrast, the average galvanic current of the Cu–Zn alloy can be ranked as 35 °C > 55 °C > 80 °C. Cao (2016) investigated the galvanic corrosion process of stainless steel and brass by the WBE technique, as well as the effect of temperature on the nonuniformity of corrosion. Figure 5 depicts the local corrosion intensity index (LCII) at various temperatures. The results reveal that with the increase in temperature, the LCII reaches its maximum value at 50 °C, and then begins to decline at 70 °C.

Figure 4:

Distribution of copper and copper alloys in WBE.

Figure 5:

Nonuniformity index of galvanic corrosion at different temperatures.

Zhang et al. (2009) scratched the surface of galvanized steel to investigate galvanic corrosion behavior in the damaged region and observed that the galvanized layer at the damaged area became the galvanic corrosion anode to speed up corrosion failure. After that, Zhang et al. (2011) carried out experiments using zinc/carbon steel bimetallic WBE with various area ratios and discovered that the smaller the area of the zinc electrode, the faster the anode corrosion rate. Simultaneously, the carbon steel receives enough cathodic protection, and the main anode gradually spreads from the zinc wire near the steel electrode to the distal zinc wire.

The use of WBE for studying the effects of different factors on galvanic corrosion provides a good characterization of the local electrochemical parameters over time, allowing for a better understanding of the corrosion process’s evolution.

4.2 Galvanic corrosion in special environments

4.2.1 Galvanic corrosion under sediment

The deposition of corrosion products will cause the scaling of equipment and further hinder the diffusion of corrosive media. Occluded galvanic cells are formed below the sediment due to the enrichment of corrosive media, which has obvious autocatalytic corrosion acceleration (Chen et al. 2017). Furthermore, based on previous study findings, galvanic corrosion under sediment possesses polarity reversal features, necessitating further investigation into the galvanic corrosion behavior of metals under sediment.

The galvanic corrosion under sediment in different environments can be simulated by coating the surface of the WBE with various sediments. Zhang et al. (2014) investigated the local galvanic corrosion behavior of carbon steel materials under mixed deposit cover (Figure 6), and found that when the surface temperature of carbon steel was 25 °C, the exposed area was always corroded as the anode, while the deposit-covered area was protected as the cathode; when the temperature was raised to 60 °C, the corrosion rate was accelerated, resulting in the continuous deposition of anodic corrosion products on the surface of the exposed electrode, raising its self-corrosion potential, and the phenomenon of polarity reversal occurred in the late stage of corrosion, i.e. the main anodic area was transferred from the initial exposed electrode to the deposit-covered area, thus continuously accelerating the corrosion of metal materials under the sediment. Yu (2014) used the same experimental method to investigate the galvanic effect of X65 carbon steel under no deposit and covered deposits, and discovered that the exposed electrode’s potential gradually increased with the corrosion process. The whole corrosion process also exhibits electrode polarity reversal features. Xiong et al. (2020) studied the corrosion of Q235 carbon steel in the flowback of shale gas containing CO₂/O₂, and plotted the current distribution of the WBE under varied immersion durations, as shown in Figure 7. Galvanic corrosion and polarity reversal also occurred between the plated and bare electrodes. During the first 3 h, the plated electrodes served as the cathode while the bare electrodes served as the anode. After 3 h of immersion, the bare electrodes were protected due to the change from the anode to the cathode.

Figure 6:

Photograph of WEB (a) and the WBE covered by mixed deposit (b).

Figure 7:

Galvanic current distribution maps of Q235 WBE (the bottom 5 × 10 electrodes were covered with deposit) in flowback water at 30 °C for different immersion times: (a) 3 h, (b) 4 h, (c) 24 h, (d) 48 h, (e) 72 h, and (f) optical photo of Q235 WBE after corrosion for 72 h.

The existence of the galvanic effect of the metal under the sediment can be explained by the following model (Figure 8): The deposit cover first obstructs O₂ diffusion, thus forming an oxygen concentration difference corrosion primary cell; the corrosion process of metal potential under sediment decreases continually and generates a galvanic effect with bare metal. This accelerates the corrosion process of the metal under the sediment. With the continuous accumulation of Fe³⁺ and Fe²⁺ under the sediment, prompting Cl⁻ from the solution migrates under the cover to maintain the charge balance, while the hydrolysis of metal chloride causes local acidity under the cover, speeding up corrosion under the sediment.

Figure 8:

Model of galvanic corrosion under the sediment.

4.2.2 Galvanic corrosion in crevices

Metal equipment will inevitably develop some tiny crevices in the metal surface throughout the manufacture and usage processes, and the connection of equipment (bolts, flanges) will also have many crevices. Because of the potential difference between the metal inside and outside the crevice, serious galvanic effect occurs frequently, and the deposition of corrosion products at the crevice causes corrosion to have a hidden nature, which can result in serious accidents and economic losses, it is necessary to investigate the galvanic coupling corrosion behavior that exists within the tiny crevice.

Pan et al. (2021) investigated the galvanic corrosion behavior of 6061 aluminum alloy at a crevice spacing of 206 μm in a 3.5 wt% sodium chloride solution. The WBE and the experimental device used are shown in Figure 9. And Figure 10 depicts the average corrosion potential and current of various electrode wires inside and outside the fissure as observed by the experiment. It is clear that the average corrosion potential inside and outside the crevice has decreased during the whole corrosion process, but the degree of decline is dramatically different. After 10 h of immersion, the average corrosion potential of the exterior of the crevice began to be higher than that of the interior, indicating that the galvanic corrosion of the anode region had moved from the outside to the inside of the crevice. This conclusion can also be obtained by analyzing the change in the average corrosion current.

Figure 9:

Schematic diagram of experimental device.

Figure 10:

The fluctuation of average corrosion potential and galvanic current immersed in crevice corrosion: (a) Average potential, (b) average current.

Welding is a common surface treatment process that fuses different metals at high temperatures and because of the electrochemical heterogeneity of the base material and the welding rod, the weld is typically a high-incidence location for galvanic corrosion. Some academics have begun to investigate the corrosion damage that occurs in the weld. Chen (2018) studied the galvanic corrosion behavior in stir friction welds based on the WBE technique and discovered that the metal at the bottom of the weld nucleus zone and the heat affected zone had a lower corrosion potential and was preferentially corroded as the anode.

4.2.3 Galvanic corrosion in the ocean

As the exploitation of terrestrial oil, gas, and mineral resources in the world reaches a bottleneck, the ocean has steadily evolved into an essential future resource reserve base. Countries throughout the world have intensified their exploration and exploitation of numerous marine resources in recent years. Contact of various metal materials in maritime equipment and installations causes galvanic corrosion, which leads to rapid corrosion and even failure of the materials, jeopardizing the safety and dependability of diverse marine metal equipment. Seawater is a highly conductive electrolyte due to its diverse composition, which includes numerous minerals, microorganisms, and a considerable amount of dissolved gas. As a result, galvanic corrosion in the water is frequently quick and difficult to find, and related research has become more valuable in recent years.

Yang (2017) investigated the galvanic corrosion behavior of stainless steel and carbon steel in the marine environment via the WBE technique and discovered that the galvanic effect between the two metals was more severe in the marine environment. It was also discovered that Cl⁻ had a larger influence on galvanic coupling corrosion than SO₄²- at the same concentration. The marine thermocline, which is a significant factor in galvanic corrosion of marine metallic equipment, is characterized by rapid fluctuations in temperature, pH, and dissolved oxygen content, and several researchers are committed to related studies. Deng et al. (2020) used a longitudinally arranged WBE to investigate the galvanic corrosion behavior of X70 pipeline steel in the seawater thermocline (Figure 11), and the corrosion current distribution maps (Figure 12) show that the lower part of X70 steel was corroded as the main anode in the early stages, the polarity reversal occurred after 10 days, and the galvanic current increased continuously during the corrosion process.

Figure 11:

Sketch of the WBEs.

Figure 12:

Variation of galvanic current of WBE in the simulated seawater thermocline: (a) 5 days, (b) 10 days, (c) 15 days, (d) 20 days).

Coatings in the ocean have the risk of being prone to failure, and metals under failed coatings are more likely to experience severe corrosion. Liu (2011) investigated the correlation between the coating failure process and the galvanic corrosion behavior under the coating, and the results revealed that there is a mutual synergistic effect between them. The coating breakage facilitates the formation of a galvanic corrosion cell, and the galvanic corrosion behavior also promotes the failure of the metal surface coating. The phenomenon of transfer of the primary anode region occurred throughout the corrosion process, the metal in the region of coating failure was corroded as the initial anode, but the cathodic reaction under the intact portion of the coating accelerated the upper part of the coating breakage, resulting in the formation of a new anode area. The anode transfer process continues, eventually breaking down and failing the whole coating.

The cathodic reduction process generated a strong alkaline environment at the coating/metal interface, which expedited the peeling of the coating from the metal surface, corroding the substrate metal and converting the region under the coating from cathode to anode, according to Li et al. (2019). Liu et al. (2020) studied the galvanic corrosion behavior under the epoxy coating with point defects using the WBE depicted in Figure 13, and created an artificial coating defect at the 45# microelectrode. The current density distribution is shown in Figure 14. At the beginning of the corrosion process, the 45# microelectrode had a cathodic current peak, and then the anodic reaction accelerated, indicating that the anodic current peak continued to increase. Carbon steel and copper–nickel alloys also displayed electrochemical inhomogeneity under the epoxy coating, as well as polarity reversal of certain electrodes, throughout the corrosion process.

Figure 13:

(a) The uncoated WBE and (b) the epoxy-coated WBE with artificial defect.

Figure 14:

Current density distributions of the WBE at various immersion times: (a) 0.5 h, (b) 24 h, (c) 72 h, (d) 186 h, (e) 258 h, (f) 330 h.

4.3 Multi-metallic galvanic corrosion

Currently, most galvanic corrosion research is focused on bimetallic coupling systems, with little attention paid to the galvanic corrosion behavior of complicated coupling systems formed by multiple metal connections. Despite the fact that this complicated multimetal connection system is present in a wide range of marine vessels and chemical production equipment, it is frequently disregarded.

As a result, several academics have begun to undertake relevant experiments in recent years. For example, Wang (2013) combined WBE technique with electrochemical methods to investigate the corrosion behavior of TA2/B10/921A after coupling with one# steel/2# steel/921A in seawater, and discovered that both temperature and dissolved oxygen concentration have significant effects on the corrosion behavior of the complicated coupling system. The relationship between the coupling system’s galvanic current density and temperature satisfies the Arrhenius equation, and the galvanic current density increases linearly with increasing oxygen concentration in seawater. Ju et al. (2018) studied the local electrochemical characteristics of three coupled metal galvanic corrosion in a desalination unit using wire sensors made of three materials: aluminum–brass, titanium, and 316L stainless steel, and the open circuit potential (OCP) measurements at different Cl⁻ concentrations are shown in Figure 15. The findings indicate that there was nonuniformity in the potential and current density distribution of the three-metal coupled system, in which the aluminum-brass electrode was corroded as the anode, the titanium electrode was protected as the cathode, and the 316L stainless steel electrode was the secondary cathode. Furthermore, the corrosion rate of the electric couple depends on the concentration of chloride in the artificial seawater, with the corrosion process being most severe when the chloride concentration is 2.3 wt%.

Figure 15:

OCP (vs. SCE/V) distribution maps of WBEs after immersion in artificial seawater with different concentrations of chloride ions (wt.%): (a) 1.5%, (b) 1.7%, (c) 1.9%, (d) 2.1%, (e) 2.3%; and (f) 2.5%.

Li (2020) prepared the WBEs consisting of three single-component carbon steels (Q235, 45, and T9) and three carbon steel components simultaneously (Figure 16). The local corrosion behavior of the WBEs in 3.5 wt% NaCl solution was investigated, and it was discovered that the 45# and T9 carbon steel wires were affected by the accelerated effect of galvanic corrosion. The corrosion current density was much higher than that of the single-component electrode wires, and the Q235 electrode wires were protected as the cathode. It was also found that the corrosion potential of the three carbon steels has varied degrees of negative shift during the corrosion process.

Figure 16:

(a) Schematic diagram of the distribution of single carbon steel arranged in WBE and (b) the component gradient carbon steel arranged in WBE.