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51318-11378-Improving Al-Ga Sacrificial Anodes via Simulation and Verification of Alloy Segregation.

Picture for Improving Al-Ga Sacrificial Anodes via Simulation and Verification of Alloy Segregation.
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Simulations showed that by controlling the cooling rate, the segregation of Ga at the grain boundaries can be maintained, avoiding passivation. Al-Ga alloy plates were so produced and subjected to galvanostatic testing, confirmed the simulations.

Product Number: 51318-11378-SG
Author: Michael Kidd / Alan Druschitz
Publication Date: 2018
Industry: Cathodic Protection
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Marine structures must deal with the corrosive effects of salt water in a way that is low cost, reliable, and environmentally friendly. Aluminum would be a good choice for a sacrificial anode to protect steel structures if it did not passivate. However, various elements can be added to aluminum to prevent this passivation. Currently, Al-Ga alloys are used commercially as sacrificial anodes but their performance is not consistent. In this research, Thermo-Calc software was used to simulate various aspects of the Al-Ga system in an attempt to understand and potentially correct this reliability issue. Simulations showed that Ga segregates to the grain boundaries during solidification and then diffuses back into the grains during cooling to room temperature. Simulations also showed that by controlling the cooling rate, the segregation of Ga at the grain boundaries can be maintained. Al-Ga alloy plates were produced with varying cooling rates, using steel and copper chills, along with a control set (cooled slowly in a sand mold). Samples were subjected to 168 hour galvanostatic testing. The performance of these plates was found to be strongly affected by cooling rate, indicating that the simulations were correct. Additional testing and verification that Ga is indeed segregated at the grain boundaries is currently in progress.

 

Key words: corrosion, cathodic protection, sacrificial anode, Al-Ga, galvanostatic, diffusion simulation

 

Marine structures must deal with the corrosive effects of salt water in a way that is low cost, reliable, and environmentally friendly. Aluminum would be a good choice for a sacrificial anode to protect steel structures if it did not passivate. However, various elements can be added to aluminum to prevent this passivation. Currently, Al-Ga alloys are used commercially as sacrificial anodes but their performance is not consistent. In this research, Thermo-Calc software was used to simulate various aspects of the Al-Ga system in an attempt to understand and potentially correct this reliability issue. Simulations showed that Ga segregates to the grain boundaries during solidification and then diffuses back into the grains during cooling to room temperature. Simulations also showed that by controlling the cooling rate, the segregation of Ga at the grain boundaries can be maintained. Al-Ga alloy plates were produced with varying cooling rates, using steel and copper chills, along with a control set (cooled slowly in a sand mold). Samples were subjected to 168 hour galvanostatic testing. The performance of these plates was found to be strongly affected by cooling rate, indicating that the simulations were correct. Additional testing and verification that Ga is indeed segregated at the grain boundaries is currently in progress.

 

Key words: corrosion, cathodic protection, sacrificial anode, Al-Ga, galvanostatic, diffusion simulation

 

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