Alternating Current Corrosion of Aluminum

Abstract

Formation of wax and hydrates due to production stops is a well-known problem in the oil industry. Conventionally the pipelines are heated beyond a critical temperature to avoid this problem. This is done by a method called direct electric heating (DEH), where an external alternating current (AC) voltage is applied to the system. The DEH system is a 1-phase system, which is not electrically insulated from the surrounding seawater. In consequence, a potential risk of AC corrosion is present. Recent work in this field has shown that the steel used is sufficiently protected by a coating system in combination with a cathodic protection (CP) system. Sacrificial anodes of aluminium alloyed with zinc and indium (AlZnIn) are conventionally used in the CP system. Earlier work has also shown that the corrosion rate of AlZnIn anodes increases significantly as a function of applied AC. However, the AC corrosion mechanisms are not fully understood. This project is part of on-going research where the main goal is a better understanding of the AC corrosion mechanisms that takes place. For this purpose aluminium with 99.99% purity was investigated under influence of AC. In additional the anodes curently used in DEH system was investigated under influence of applied AC. All experiments were exposed to AC for a period of 96 hours. The applied AC on AlZnIn was varied between 10 A/m2 and 100 A/m2 while the applied AC on aluminium with 99.99% purity was varied between 20 A/m2 and 200 A/m2. Evaluation of the experiments involved weight loss testing, DC potential measurements on anode, AC potential measurements on anode, DC potential measurements between anodes, phase angle measurements between applied alternating voltage and AC response and waveform analysis. In additional the surface and surface deposits was analysed using scanning electron microscopy and X-ray diffraction.

Results show that the corrosion rate increased when increasing the applied AC. This was found for both materials tested. The results also shows that AlZnIn was more susceptible for AC corrosion compared to aluminium with 99.99% purity. A maximum of the corrosion rate was found when the applied AC density was 60 A/m2. This was found for both of the tested materials. Macroscopic and microscopic characterization of the surface showed that the size of the pits formed was increasing in size with increasing applied AC up to when the applied AC density was 60 A/m2. The phase angle measurements showed a decreasing phase angle between the applied voltage and AC response when increasing the applied AC. The measured average phase angle varied between 8 degree and 25 degree depending on the material tested and the range of applied AC densities. The former was found when 200 A/m2 was applied to aluminium with 99.99% purity. At the same time the generated DC increased with applied AC. It is therefore found a decreasing ratio between the faradic AC contribution and generated DC. This implies that corrosion is mainly caused by the faradic AC at lower applied AC densities (iac<60 A/m2) while there is a combined mechanism between the faradic rectification effect and faradic AC contribution at higher AC densities (iac>60 A/m2).