Effect of AC interference on the corrosion behavior of cathodically protected mild steel in an artificial soil solution. Part I: Investigation on formed corrosion product layer

Metallic structures like pipelines are highly susceptible to corrosion effects. Different techniques have been used to protect these structures from corrosion. Among them, cathodic protection (CP) is one of the most widely used methods to mitigate corrosion in metallic structures. Nevertheless, despite the perceived effectiveness of this technique under different conditions, it has proved unreliable in some conditions. In the presence of AC interference, for instance, cathodically protected metallic structures like pipelines may still be affected by localized corrosion. This can be attributed to the electrolytic dissolution of metals induced by the AC polarization interference.

The AC-induced corrosion phenomena have been extensively investigated in the past decades using different models. Some of the identified critical parameters that influence the AC corrosion include AC frequency, AC current and the magnitude of the CP currents. Nevertheless, despite the remarkable works, there is still a lack of clear understanding of the AC-induced corrosion process and underlying mechanisms. Besides the corrosion mechanism, the protection mechanism during cathodic polarization is not fully understood.

The main consequence of the current lack of understanding of AC corrosion is the associated difficulty and challenges in conducting corrosion risk assessment in the presence of AC interference. This has been mainly attributed to the impact of the AC-induced currents and the incubation period characterized by AC-induced corrosion, often assumed to consist of steel surface alkalization. Thus, this requires urgent research attention considering the threat of AC corrosion for different metallic structures, including cathodically protected pipelines.

Herein, Mr. Mario Markić, Mr. Serkan Arat and Professor Wolfram Fürbeth from DECHEMA Research Institute investigated the formed corrosion product layers as well as the surface layer conversion during AC polarization of cathodically protected mild steel. In their approach, high-speed potential measurements were performed to determine the effects of the polarization parameters on the alternating voltage. Additionally, the resulting corrosion layers were characterized using three different techniques: optical microscopy, Raman spectroscopy and electron probe microanalysis. Their main objective was to provide a better understanding of AC corrosion processes and mechanisms. Their research work is currently published in the journal, Materials and Corrosion.

The researchers showed that the properties of the resulting AC-induced corrosion product were not only dependent on AC polarization parameters as thought before but also on the CP potential. These properties included the amount of oxygen and the thickness of the product layers. The corrosion slowed down to the minimum level when moderate CP current densities were applied. Although the corrosion mechanism did not appear to change with the variation of the AC/CP parameters, regular corrosion was observed when the CP was insufficient. Furthermore, as reported in the previous studies, the authors confirmed that a more severe corrosion attack was due to lower frequencies.

In a nutshell, the research team reported the combined spectroscopic, microscopic and electrochemical investigations of AC-induced corrosion of mild steel under different AC parameters and CP conditions. The results demonstrated the need to consider AC interference during corrosion risk assessments owing to its predominant influence on the steel potential in the presence of AC sources. In a statement to Advances in Engineering, Professor Wolfram Fürbeth, the lead author said the study provided valuable insights that provide a better view and understanding of AC-induced corrosion mechanisms and would pave the way for more research into this area.