Performance of zinc alloy sacrificial anode under cyclic immersion in seawater

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Date
2016-06
Authors
Tean Zi Yuan
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Zinc alloy sacrificial anodes are often used to protect marine structures from corrosion in seawater due to their high electrochemical properties. However, there exist some cases such as in ship ballast tank, where the anodes are often immersed in seawater when the tank is ballasted and exposed in atmosphere with the tank empty. This research studies the electrochemical corrosion behavior and protection efficiency of the sacrificial anode on sacrificial anode cathodic protection under condition of cyclic immersion. In this experiment, cyclic immersion testing, open circuit potential and linear polarization were carried out to determine the performance of zinc alloy sacrificial anode with cyclic immersion in protecting the steel from corrosion. The samples were weighted before and after the experiment in order to determine the passivation of the protective layer on the surface of the protected samples. Open circuit potential was carried out to determine the effectiveness of zinc alloy sacrificial anode in protecting the cathode under cathodic protection by lower down the corrosion potential to the immunity region. For linear polarization measurement, the corrosion potential was measured every 2 cycles during the immersed conditions. The corrosion current (Icorr) was taken as an index of the corrosion rate. The samples were taken out at the end of the 5th cycles of exposure and were analyzed for the corrosion products using FESEM-EDS. The EDX shows that the corrosion product formed from protected zinc-steel coupled have less Fe ions compared to corrosion product of unprotected samples. The morphology of the samples was observed under the optical microscope. It shows that longer immersion time in seawater and shorter time in atmosphere will prolonger the lifespan of the zinc alloys sacrificial anode in seawater. Alternate cycling through immersion and exposure to air produces more pitting attack than continuous immersion.
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