(179j) Corrosion Modeling of Magnesium Alloys Using Internal State Variable

Magnesium, a lightweight metallic alloy, is currently being investigated for use within the automotive industry because of its low density, excellent castability, easy machinability, and high mechanical stiffness [1,2].  When exposed to saltwater, magnesium has a high corrosion rate, relegating its current use to unexposed locations within the vehicle [3-5].  By altering the microstructure with the addition of various elements, including aluminum, zinc, manganese, and rare earth elements, the corrosion rate of magnesium can be reduced [6-8].  In addition to the alteration of the microstructure, the method of magnesium formation can also affect the corrosion rate.  The presence of an as-cast skin, which consists of very small grains formed during alloy cooling, improves the corrosion resistance of the magnesium alloy ten-fold [7,9].  While casting is commonly used to form parts of a vehicle, such as control arms or engine cradles, extruded metal is also used.  Therefore, in addition to understanding the additive effect of elements to corrosion resistance, the method of magnesium alloy formation must be included.  A detailed understanding of magnesium’s response to corrosion condition, through the modeling of the pit nucleation and growth, is needed.

     In the Center for Advanced Vehicular Systems (CAVS) at Mississippi State University, a complete corrosion model based upon internal variable theory is introduced that captures the effects of general, pitting, and intergranular corrosion in a multiplicative decomposition manner.  General corrosion is signified by volume loss of the material. Pitting corrosion is used as localized corrosion for the sake of our theoretical model’s paradigm and implementation. Lastly, the definition of intergranular corrosion in our context is localized corrosion that occurs at the grain boundaries caused by precipitates and segregation leading to the formation of microgalvanic cells. The total corrosion damage is the addition of the general, pitting, and intergranular corrosion.  A series of magnesium alloys are used to demonstrate the relative importance of the different mechanisms developed for this model.

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