SUMMARY
Results concerning corrosion behaviour of some commercially used magnesium alloys (AZ31, AZ91, AM20, AE42) with zinc coatings deposited by electroplating are described. It was established that a relatively dense and uniform coating is formed on AZ31 alloy from non-cyanide alkaline bath. On AE42 substrate a coating delaminates. On other substrates an appearance and adhesion are insufficient to accept these coatings for further research. The reason of such behaviour is located at the substrate coating interface, especially due to flaws and imperfections in thin zinc layer deposited chemically in surface pre-treatment stage. Evaluation of corrosion processes in chlorides containing solutions was performed by electrochemical methods, such as potentiometric and voltametric measurements. The coating on AZ31 alloy was stable in aggressive solution for a limited time. Approximately 18th day of immersion resulted in significant shift of corrosion potential from the value of zinc to the value for bulk magnesium alloy. At the same time an onset of bimetallic corrosion shifted overall corrosion current density to higher values. In order to protect longer a two-step process is elaborated. Using the same standard surface pre-treatment the first coating is electrodeposited from alkaline bath followed by the second step from acidic bath. A special two-layer microstructure increased corrosion resistance at least three times.
Keywords
Magnesium alloys, zinc coating, electrodeposition, corrosion, polarisation curves.
1 Instytut Mechaniki Precyzyjnej, Warszawa
