Alternating Current Corrosion of Aluminium Sacrificial Anodes
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Direct Electric Heating (DEH) is applied to subsea oil production and transmission pipelines to prevent freezing of hydrates as wax during productions shut downs. To prevent clogging, the pipes are heated by application of alternating current (AC) voltage. As a result, a risk for AC corrosion is introduced, which is the motivation and subject of this thesis. The steel pipes are coated and applied conventional cathodic protection (CP) by use of AlZnIn sacrificial anodes. The present work focused on the risk of increased rates of AC influenced corrosion of the AlZnIn anodes. Anode samples coupled to steel samples were investigated under applied AC by use of laboratory scale test cells in synthetic seawater at room temperature. In these experiments, which lasted for one week, the applied AC was varied in the range 0.5 to 150 A m-2, and the anode-steel area ratio (AR) was set to either 10:1 or 100:1. Corrosion rates were assessed by weight loss measurements and properties of surface deposits and corroded surfaces were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. For better assessment of anode-steel coupling in practice and the significance of anode-steel area, similar tests were performed by using a connection of two identical pairs of parallel-coupled anode-steel, with area ratio of 100:1. Equipment and methodology for investigation of AC corrosion were developed and evaluated. Results show that the anode corrosion rate increased with increasing applied AC level, while steel is sufficiently protected under the experimental conditions specified above. Anode corrosion rate was influenced both by current provided for the protection of the steel and self-corrosion and the attack was characterized by pit formation and coalescence of these at higher AC levels. High self-corrosion rates were attributed to successive alkalization, explained by hydrogen evolution, and acidification of the anode surface at each AC cycle, which destabilized the protective oxide layer. Corrosion was limited at high AC levels, explained by hydrogen blanketing of the anode surface and by hydrogen trapped within pores of the hydroxide surface film. AC corrosion of the anodes depended strongly on the anode-steel area ratio. In experiments with electrode pair configuration as described above, the significance of the steel samples vanished by increasing the anode-steel area ratio to 100:1. The system functioned as an anode?anode galvanic couple, which caused a significant decrease in the potential of the anodes, giving rise to runaway self-corrosion rates. In experiments with one anode-steel couple, the couple potential also decreased once AC was applied, increasingly in extent with applied AC potential. This further increased the current requirement by increasingly cathodic steel, thereby resulting in extreme anode corrosion at high AC levels in experiments with AR of 10:1. A subsequent positive shift in the couple potential to a stable level lower than the DC operation potential (-1.05 VSCE) of the AlZnIn anode was observed within 20 hours, caused by hydrogen evolution on steel. The time until the positive shift increased with applied AC level and decreased AR. Formation of calcareous deposits on the steel surface under DC conditions is an important aspect of CP in seawater because the deposits reduce the current requirement significantly. Such deposits did not appear to have a similar significance in the presence of applied AC. Increased water reduction by AC, causing pH increase on the steel surface higher than the DC case, reduced the protectiveness of the deposits by inhibition of electrically insulating CaCO3 formation. Preconditioning of the steel surface by CP under usual DC conditions to form the desired deposits did not have a clear influence on the AC corrosion of anodes. The decrease of both the AC and DC components of the cell current as a function of time under moderate applied AC levels, however, indicated the formation of calcareous deposits on steel. No calcareous scales were found to deposit on the anode surface. The decrease of cell current with time can also be attributed to the development of corrosion products on the anode surface. Decrease in the cell current was not appreciable for high AC levels (> 2 V RMS) with an AR of 10:1, explained by the destruction and instability of the calcareous deposits due to vigorous gas evolution. In conclusion, these results suggest that the lifetime of the CP system at high levels of applied AC (V(AC) > 2 V AC or iAC > 30 A m-2) may become significantly reduced in relation to the expected lifetime under DC conditions.