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51315-5570-Corrosion of Thermally Sprayed Aluminum in Flowing Seawater

Picture for Corrosion of Thermally Sprayed Aluminum in Flowing Seawater
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Product Number: 51315-5570-SG
ISBN: 5570 2015 CP
Author: Kemal Nisancioglu
Publication Date: 2015
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Thermally-sprayed aluminum (TSA) is known to provide satisfactory and long-life corrosion protection to steel structures in seawater. Information about its performance under high velocity flow conditions is limited. The purpose of the present work was to investigate the self-corrosion rate of TSA on steel and whether TSA could provide the necessary cathodic protection to the steel substrate under high flow rates. The flow channel was designed for exposing several specimens along its length and each sample was equipped with own counter and reference electrodes for electrochemical measurements. The samples consisted of TSA coated steel and dense Al metal of identical geometry the latter as control. Both TSA and the dense metal were an aluminum alloy containing 5% Mg (AlMg5). Corrosion rate was determined by weight loss and change of TSA thickness. Corrosion products and morphology were characterized by SEM. Dense AlMg5 samples corroded under convective mass transfer control. Corrosion rates measured could be predicted by use of classical Sherwood number correlations for the hydrodynamic system. However the corrosion rate of the TSA samples was about an order of magnitude larger than the dense samples. The difference could not be rationalized by surface-roughness considerations or galvanic corrosion. It was attributed to cavitation erosion of the rough TSA samples. The corrosion potential of the TSA was about 100 mV more negative than that of the dense alloy both under stagnant and flow conditions. Furthermore the potential of TSA decreased to a level (about -1.06 VSCE) negative enough for successful cathodic protection of underlying steel. However high self-corrosion of TSA is a challenge in practical application as a sacrificial coating under high seawater flow conditions.
Thermally-sprayed aluminum (TSA) is known to provide satisfactory and long-life corrosion protection to steel structures in seawater. Information about its performance under high velocity flow conditions is limited. The purpose of the present work was to investigate the self-corrosion rate of TSA on steel and whether TSA could provide the necessary cathodic protection to the steel substrate under high flow rates. The flow channel was designed for exposing several specimens along its length and each sample was equipped with own counter and reference electrodes for electrochemical measurements. The samples consisted of TSA coated steel and dense Al metal of identical geometry the latter as control. Both TSA and the dense metal were an aluminum alloy containing 5% Mg (AlMg5). Corrosion rate was determined by weight loss and change of TSA thickness. Corrosion products and morphology were characterized by SEM. Dense AlMg5 samples corroded under convective mass transfer control. Corrosion rates measured could be predicted by use of classical Sherwood number correlations for the hydrodynamic system. However the corrosion rate of the TSA samples was about an order of magnitude larger than the dense samples. The difference could not be rationalized by surface-roughness considerations or galvanic corrosion. It was attributed to cavitation erosion of the rough TSA samples. The corrosion potential of the TSA was about 100 mV more negative than that of the dense alloy both under stagnant and flow conditions. Furthermore the potential of TSA decreased to a level (about -1.06 VSCE) negative enough for successful cathodic protection of underlying steel. However high self-corrosion of TSA is a challenge in practical application as a sacrificial coating under high seawater flow conditions.
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