April 2025, Vol. 252, No. 4

Features

Exploring Anode Performance in Presence of AC Source

By Dr. Ahmed Mahgoub, CP SME, and Ahmed Abdelbagi, CP SME, Saudi Aramco, Saudi Arabia.  

Editor’s Note: This is the first of two parts of an article that focuses on cathodic protection and anode performance. Part 2 will be published in the May edition of P&GJ. 

(P&GJ) — Cathodic protection (CP) is a technique to reduce the corrosion of a metal surface by making that surface the cathode of an electrochemical cell. The cathode in an electrochemical cell is the electrode where reduction (and no corrosion) occurs.  

Prior to apply cathodic protection, corroding structures will have both cathodic areas and anodic areas (those areas where corrosion is occurring). It follows, then, that if all anodic areas can be converted to cathodic areas, the entire structure will become a cathode and corrosion will be eliminated. This can be achieved by direct current is forced onto all surfaces of the structure to be protected.  

This CP current shifts the potential of the structure in the active (negative) direction, resulting in a reduction in the corrosion rate of the metal. When the amount of current flowing is adjusted properly, it will overpower the corrosion current discharging from the anodic areas on the pipeline, and there will be a net current flow onto the pipe surface at these points. The entire surface then will be a cathode and the corrosion rate will be reduced. 

Al-based alloys are attractive as anodes in the CP of steels in seawater due to their low cost and high current capacity. The main disadvantage of Al as a sacrificial anode is its natural ability to form a very stable and passive oxide layer, which hinders corrosion of the metal itself and shifts the metal potential to less-active values.  

Moreover, Mg anode is widely used to protect steel because it is more electronegative than it and hence it is capable to supply electrons to the more electropositive steel, providing CP of this metal surface. However, the Mg sacrificial anode performance is impacted in the presence of AC interference as the stray current accelerates the transmission and diffusion of oxygen, so the corrosion rate under AC interference is higher than that with no AC interference.  

Furthermore, Zn ribbon anodes provide a very simple, cost effective, maintenance-free method of AC mitigation, however, Zn faces high consumption/corrosion rates in the presence of AC. Based on that, it is accessing the effectiveness of Al, Mg and Zn anodes in the presence of AC induced and stray current. 

AC Stray Current 

Hydrate formation is a well-known risk in subsea pipelines and several solutions are available to solve this problem. Hydrates can be prevented by using Subsea pipeline electrical heating technology. Direct electrical heating (DEH) controls the temperature on the pipeline and is expected to be an efficient and convenient way to deal with wax plug in the subsea pipeline. 

At the far-end cable connection point of the DEH system, the cable current enters the steel pipe, while part of the current (around 20-40 %) leaves the pipe and is transferred to the sea through the anodes. The electrical current in the sea water enters the pipe again at the near-end connection point. Sacrificial anodes shall prevent AC corrosion in the current transfer zone (CTZ) in addition to provide adequate CP system for the subsea pipeline, the anodes act as effective grounding for the pipeline.  

The use of this system is associated with drawbacks such as higher consumption of the sacrificial anodes of the CP system and the possible risk of AC corrosion of surrounding sacrificial anodes and the pipeline itself at coating defects [1].  

Williams [2] reported a considerable increase in corrosion of Al conductors in water when an alternating current was applied, the extent of which was dependent on time and applied AC level. Corrosion occurred mainly during the initial test period and the corrosion rate increased with applied AC in the test range of 50 A/m² to 2000 A/m².  

In addition, Forthun [3] focused to study further the effect of AC on corrosion of Al-Zn-In anodes, both in the presence and absence of connection to steel. In this regard, different test configurations were employed, i.e., a pair of coupled anodes (A/A experiments) and steel coupled to anode, which in turn was coupled to another identical pair of parallel-coupled anode and steel sample (SA/AS experiments).  

In both cases, a constant AC density was applied and experiments are designated by the current density level that was applied (relative the area of one anode sample, and assuming the overall current was transferred only between anode samples). 

Figure 1 demonstrated that there is a noticeable rise in anode corrosion rates as the applied AC level increases in both A/A and SA/AS tests. This trend continues until the applied AC level reaches i_ac 80-90 A/m², after which the corrosion rates start decreasing with any additional increase in the applied AC level.  

Observations from high AC level testing showed that the test solution experienced heat generation and increased thickness due to the creation of extreme quantities of corrosion product. Replicates were conducted at a current density of 10 A/m². In the SA/AS experiments at this AC level, only one of the anodes exhibited corrosion. The extent of corrosion seen was similar to the combined corrosion rate of both anodes in the A/A experiments. 

Figure 2 explained that when comparing tests conducted at the same alternating current (AC) level, there were no discernible variations between the anodes from the A/A and SA/AS experiments. In other words, the anodes’ appearance was not significantly impacted by their connection to steel. The corroded surfaces exhibited an irregularly dispersed form of deterioration. The formation of pits expanded in both extent and size, and further merged together as the level of AC increased correspondingly. 

The study, reported in [3] clarified that the application of AC did not significantly affect the resulting anode efficiencies (standalone Al alloy) when low AC voltage was applied, however it was observed that at 2 V AC and higher, the anode capacity (and resulting current efficiency) was lower than the acceptance criteria set by DNV-B-401 [4] for testing the anode capacity of Al-based anodes (i.e., 2500 Ah/kg).  

In addition, Thorsteinsen [5] investigated the behavior of Al-Zn-In sacrificial anode material in the presence of AC. The anode capacity can be determined from his findings. The anode capacities are significantly lower than the theoretical capacity, as illustrated in Figure-3 [6]. The anode current capacity is consistently less than 1000 Ah/kg, regardless of the varying AC current densities. These tests suggest that the introduction of AC results in a decrease in anode capacity.  

The author [1] conducts a thorough case study to assess the impact of DEH on the CP system of an 11.5km subsea pipeline.  

The study highlights the necessary number of Al-Zn-In bracelet anodes needed to achieve the optimal level of protection in both the CTZ and the total length of the pipeline. This determination is based on the actual anode current capacity and corrosion rate, as demonstrated earlier. The conventional CP system only needed 40 anodes.  

However, in the case of both CP and DEH systems under worst-case conditions, up to 324 anodes were required (eight times the number of anodes needed in the conventional case). The results indicate that the DEH system significantly impacts both the CP design and the performance of the Al sacrificial anodes. 

Mg Anode Effectiveness  

Inductive AC interference on new and existing pipeline systems from crossing or parallelisms with overhead power lines is a serious concern [7]. Meanwhile, Mg anode is widely used to protect steel because it is more electronegative than it and hence it is capable to supply electrons to the more electropositive steel, providing CP of this metal surface. The effect of AC on the performance of Mg sacrificial anode, i.e., the DC potential, the output CP current as well as the corrosion rate by AC interference experiment shall be evaluated.  

Currently, it is unequivocal that AC significantly influences the electrochemical performance of Mg sacrificial anodes. Pookote [8] indicated that AC may lead to the potential shift of Mg sacrificial anode and expedite its decomposition. Freiman [9] examined the impact of AC current on the efficacy of Mg anode associated with onshore pipelines.  

Experimental results indicated that both the DC potentials of Mg and the pipeline exhibited a positive shift under AC. At the same time, the current efficiency of Mg sacrificial anode was decreased significantly. Furthermore, Bruchner [10] discovered that the polarity of the pipeline-Mg sacrificial anode CP system may be altered under AC. When the applied AC current density reached 39 A/m², the Mg anode became the cathode, while the shielded pipeline became the anode. 

Ding [11] performed an extensive study on the impact of AC voltages of 0V, 1V, 3V, and 5V on Mg sacrificial anodes. The electrochemical properties of the examined anode were analyzed by open circuit potential (OCP) analysis, electrochemical impedance spectroscopy (EIS), and polarization curve measurements.  

The sacrificial anode specimens utilized in this study were Mg alloy rods that were welded with wires and encased in electrical tubes. The test solution was extracted from the soil along the apron, with the following chemical concentrations: Na₂CO₃ 0.1599 g/L, NaCl 0.5124 g/L, Na₂SO₄ 0.1712 g/L, and NaHCO₃ 0.0864 g/L. 

The OCP of the sacrificial anode under different AC voltages was examined over a period of eight days, as depicted in Figure-4. The OCP of Mg anode experiences a sudden negative shift during the initial experimental phase in case of AC voltage, with the most significant potential offset occurring under an AC interference value of 5V, while NO potential offset is observed in the absence of AC interference.  

Subsequently, the corrosion potential begins to exhibit a positive change across all experimental circumstances. The activity on the sacrificial anode surface diminished over time, regardless of AC interference. The OCP exhibited greatest positive migration under 5V AC interference and minimum migration under 0V voltage. 

Furthermore, to examine the sacrificial anodic corrosion process and assess the variation in corrosion product film resistance (R_C) under varying AC interference voltages. Figure 5 illustrated the variation of R_C under various AC interference voltages based on EIS analysis. During the experiment, corrosion products build on the surface of the sacrificial anode, increasing the resistance of the corrosion reaction. In the absence of AC interference and with 1V AC interference, the rate of increase of R_C diminishes throughout the experiment.  

The rate of R_C initially slows and subsequently accelerates during the experiment when the AC interference voltages are 3V and 5V. The increased AC stray current alters the corrosion condition of the Mg sacrificial anode surface. The value of R_C progressively grew over the duration of the trial. The greater the AC voltage, the more significant the change in resistance. 

The polarization resistance progressively increased as the corrosion current diminished throughout the experiment. The corrosion tendency and rate of the sacrificial anode decreased with the extension of the experimental duration. In comparison to the polarization resistance and corrosion current measured at the same experimental duration, it is evident that the polarization resistance was diminished under AC interference, while the corrosion current was elevated relative to conditions without AC interference.  

It proof that AC interference voltage will diminish the corrosion propensity of the sacrificial anode while accelerating the dissolution rate of Mg anodic. The greater the AC interference voltage, the more pronounced the promotional impact. Furthermore, the control of the sacrificial anode can be discerned from the slope trends of the cathode and anode polarization curves. Following a designated experimental duration, the slope of the anodic polarization curve increasingly surpassed that of the cathodic polarization curve due to the AC stray current enhancing the transmission and diffusion of oxygen.  

Consequently, the corrosion rate under AC interference exceeds that without AC interference; moreover, a higher AC interference voltage correlates with an accelerated corrosion rate. 

Tang [12] focused to study further the effect of AC on the performance of Mg sacrificial anode used to provide CP for buried pipeline, i.e., the DC potential, the output CP current as well as the corrosion rate combined with electrochemical measurements by AC interference experiment. Mg and steel were employed in this work with a simulated backfill solution of 4 g/L Na₂SO₄ was adopted as the aggressive solution, which was made from analytic grade reagents and deionized water.  

The experimental setup was adopted to conduct the AC interference experiment of steel – Mg sacrificial anode CP system under AC of 100 and 300 A/m² for various times. 

After 96 hours of exposure in the simulated backfill solution, the corrosion rates of Mg and carbon steel were measured by weight-loss measurements and plotted against AC current density (i_ac). The results are shown in Table-1. It was observed that as i_ac increased, the corrosion rates of both Mg and carbon steel also increased.  

When AC was not present, the corrosion rate of Mg was 6.88 mm/yr. Applying AC to the system, the corrosion rate was accelerated dramatically to 34.9 mm/yr at applied AC current density 300 A/m². Moreover, it was concluded that Mg sacrificial anode could not provide enough protection under AC interference of 50 A/m² (or larger). 

---

_Reference  

1. _{Ahmed Mahgoub. “Direct electrical heating impaction for subsea pipeline integrity,” Materials Performance Magazine. Vol. 60, No. 4, (2021), pp. 28-31.}  
2. _{J.F. Williams, Alternating Current and Aluminum. Materials Protection, 1967: p. 50-52.} 
3. _{Kari Forthun. “Alternating current corrosion of aluminum sacrificial anodes,” NTNU, Master Degree, 2013.}  
4. _{DNV-RP-B401. “Cathodic Protection design,” 2021.}  
5. _{Andreas Thorsteinsen. “Alternating current corrosion of cathodically protected steel in marine environment,” NTNU, Master Degree, 2012.}  
6. _{Hesjevik & Olsen. “Cathodic protection design of submarine pipelines with direct electric heating,” Corrosion 2013, NACE, Paper No. 2443.}  
7. _{Ahmed Mahgoub. “Mitigation of AC Induced Voltages on Buried Pipelines Influenced by E.H.V Power Lines. A Journal of Institute of Corrosion,” Corrosion Management, Issue 165, January/February 2022.}  
8. _{S.R. Pookote, D.T. Chin. “Effect of alternating current on the underground corrosion of steels,” Mater. Perform. 17 (1978) 9–15.} 
9. _{L.I. Freiman, M. Yunovich. “Special behavior of steel cathode in soil and protection assessment of underground pipe with a buried coupon,” Prot.Met. 27 (1991) 437–447.} 
10. _{W.H. Bruchner, Soil corrosion of steel by AC, Mater. Protec. 4 (1965).} 
11. _{Q. Ding, I. Chu, T. Shen. “Study on the Electrochemical Performance of Sacrificial Anode Interfered by Alternating Current Voltage,” International Journal of Corrosion Volume 2018.}  
12. _{D. Tang, Y. Du, X. Li, Y. Liang, and M. Lu, “Effect of alternating current on the performance of magnesium sacrificial anode,” Materials and Corrosion, vol. 93, pp. 133–145, 2016.}  
13. _{Lindemuth, H. Hernandez. “AC Mitigation and Cathodic Protection for a Long Regulated Pipeline,” CORROSION 2010, paper no. 105 (Houston, TX: NACE, 2010).}  
14. _{Y. Du, and  D. Tang. “Investigation on corrosion of zinc ribbon under alternating current,” Corrosion Engineering Science and Technology, vol. 50, 2015.}  
15. _{M. Lu, and Y. Du. “Electrochemical Studies on the Performance of Zinc Used for Alternating Current Mitigation,” Corrosion, Vol. 71, No. 6, 2015.}  

Related Articles