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National Capital Section


January 14, 2003

Novel Sacrificial Coatings by Electrodeposition of Mn Alloys Dr. Giovanni Zangari University of Virginia Electrodeposited coatings of manganese and its alloys potentially combine high corrosion protection performance, good tribological properties and suitable mechanical characteristics. Therefore, they have been studied as a replacement for cadmium in the sacrificial protection of steel. This presentation will focus on the development and characterization of a set of novel Mn alloys presenting both improved stability and better corrosion performance than pure Mn. Pure Mn coatings of good quality can in fact be obtained by electrodeposition, but they present two main drawbacks. Due to their low corrosion potential, their dissolution at open circuit is relatively fast. In addition, as-deposited ductile Mn transforms with time at room temperature to a brittle phase, with potential deterioration of its mechanical performance. To overcome the first drawback, Mn has been alloyed with Sn. Alloy composition can be controlled by varying the current density; the corresponding alloys are composed of a mixture of various phases, including MnSn2 and Sn-rich intermetallics. Sn additions slow down the dissolution process by varying the corrosion potential and current, but the corrosion performance is limited by the presence of several phases in the coating and the consequent potential for galvanically-enhanced corrosion. An alternative route to control corrosion potential, improve corrosion resistance, and at the same time increase the metallurgical stability of the as-deposited coatings, is to alloy Mn with Cu. It is found that a Cu content above 4 at% is sufficient to stabilize the as-deposited ductile Mn phase. In addition, the corrosion potential of Cu-Mn alloys increases monotonously with Cu content, and the corrosion current correspondingly decreases, indicating an improved corrosion resistance. Cu-Mn coatings exhibit in addition a lower friction coefficient than Cd and possess a lower hardness and modulus. Overall, Cu Mn alloys appear to be an outstanding candidate for the potential substitution of Cd coatings in sacrificial corrosion applications, with the attendant environmental and processing advantages.

Attendance: 16 total, approximately 8 student member

 

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