(647f) Modeling the Corrosive Effects of Various Magnesium Alloys Exposed to Two Saltwater Environments

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 10 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, the pitting characteristics involved in the corrosion of magnesium exposed to a saltwater environment are being examined. The total corrosion of multiple Mg alloy coupons, including extruded AZ61 and AZ31 as well as as-cast AE44 and AM60, in two saltwater environments, either immersion or cyclical salt spray, was determined over a 60 hr total exposure. The total corrosion is determined based on the following model [10]:

f_T = f_GC + f_IC

where

f_T is the total damage from corrosion arising from any type of corrosion mechanism

f_GC is the damage from general corrosion (loss of thickness)

f_IC is the damage from pitting

To calculate the corrosion due to pitting, the following model was developed [10]:

f_IC = h_pn_pc

where

h_p is the pit number density related to nucleation of pits (number per unit area or volume)

n_p is the area of pit growth related to growth of the pits

and c is a function of the nearest neighbor diameter related to the coalescence of the pits

Initial exposure to either saltwater environment resulted in changes to the surface as pitting nucleation was initiated [11-14]. As exposure increased, the pits grew in number and size until general corrosion began to dominate, as illustrated by changes to the pit growth and nearest neighbor distance, for both surfaces [11-14]. The research presented here will cover the further development of the model, including the calibration and validation of the model using experimental data.

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