Holly J. Martin

Effects of Variations in Salt-Spray Conditions on the Corrosion Mechanisms of an AE44 Magnesium Alloy

The understanding of how corrosion affects magnesium alloys is of utmost importance as the automotive and aerospace industries have become interested in the use of these lightweight alloys. However, the standardized salt-spray test does not produce adequate corrosion results when compared with field data, due to the lack of multiple exposure environments. This research explored four test combinations through three sets of cycles to determine how the corrosion mechanisms of pitting, intergranular corrosion, and general corrosion were affected by the environment. Of the four test combinations, Humidity-Drying was the least corrosive, while the most corrosive test condition was Salt Spray-Humidity-Drying. The differences in corrosivity of the test conditions are due to the various reactions needed to cause corrosion, including the presence of chloride ions to cause pit nucleation, the presence of humidity to cause galvanic corrosion, and the drying phase which trapped chloride ions beneath the corrosion by-products.

Piranha Treated Titanium Compared to Passivated Titanium as Characterized by XPS

Titanium is commonly used as an implant material because of its strength, durability, density, and ability to passivated. Two titanium surfaces were compared, one produced by passivation following the ASTM F87 standard and one produced by treating coupons with piranha. The XPS spectra of the two treated titanium surfaces were collected with a Mg Kα (1253.6 eV) x-ray source operated at 300 W and 15 kV. This report includes XPS spectra of the elements associated with the treated titanium, which consisted of carbon, oxygen, and titanium. Significantly more titanium and oxygen were seen when comparing the piranha treated titanium to the passivated titanium, while significantly more carbon was present on the passivated titanium as compared to the piranha treated titanium.

Corrosion‐Stress Relaxation Effects on Tensile Properties of an AZ61 Magnesium Alloy

One way to increase gas mileage 50% by 2050, a goal of the U.S. government, is to reduce the weight of the vehicle, using lightweight alloys such as magnesium. AZXX Mg alloys have been investigated for this purpose, but are still highly susceptible to stress corrosion cracking. The presence of pits and hydrogen embrittled sections concentrate the stress, leading to cracking and failure. In order to determine the interaction between tensile properties and corrosion behavior, two saltwater environments were used to examine the effects of chloride ion exposure on the corrosion of an extruded AZ61 alloy held under constant strain (approximately 80% of tensile yield strength) over 60 hours. The effects of constant strain on the surface corrosive behavior and the tensile strength were determined at various intervals. The stress-strain relationship minimally decreased over time for the salt spray environment, while large changes were seen in the stress-strain relationship for the immersion environment. In addition, there was a minimal decrease in stress over 60 hours of the AZ61 alloy in the salt spray environment but a 20% decrease in stress over 60 hours in the immersion environment. The differences between stress-strain relationships were attributed to a decrease in surface area of the samples due to the continuous presence of water for the immersion environment, which resulted in a decrease in the ability to withstand applied stress. The formation of pits due to chloride ions for both environments concentrated the applied stress, leading to a decrease in the elongation to failure for both environments.

Modeling the Corrosive Effects of Various Magnesium Alloys Exposed to Two Saltwater Environments

The use of magnesium within the automotive industry is limited by its corrosion rate in the presence of saltwater. By adding various elements, the magnesium micro structure and corrosion rate can be altered. In the Center for Advanced Vehicular Systems at Mississippi State University, a model is being developed to elucidate the total corrosion of magnesium alloys and is comprised of general corrosion and pitting corrosion, respectively, as shown below: Open image in new window where pitting corrosion is based on the pit number density, pit surface area, and a nearest neighbor distance function, respectively, as shown below: Open image in new window

Comparing the Corrosion Effects of Two Environments on As‐Cast and Extruded Magnesium Alloys

Magnesium is easily corroded in the presence of saltwater, limiting its use in the automotive industry. The magnesium microstructure greatly affects the corrosion rate, due to various additional elements. In the Center for Advanced Vehicular Systems at Mississippi State University, the effects of immersion and cyclical salt spray testing on various as-cast and extruded magnesium alloys is currently being examined. Previous work on an as-cast AE44 magnesium alloy has demonstrated that individual pit characteristics, such as pit depth, pit area, and pit volume, were deeper and larger following exposure to the immersion environment. However, the data elucidating the corrosion effects on individual pit characteristics has only been seen on as-cast magnesium containing rare earth elements, not on extruded magnesium alloys or zinc-containing magnesium alloys, both common magnesium forms. The research presented here will cover the effects of individual pit characteristics formed on various magnesium alloys due to the different environments.