Corrosion of aluminum alloy 2024 belonging to the 1930s in seawater environment
Gujarathi, Kedar Kanayalal
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Wreckage of ?Carnauba?, a 1930s vintage Sikorsky S-38 aircraft, a beloved icon of SC Johnson's early history, was found on July 5, 2000, in seawater off of an Indonesian island of West Irian Jaya. The company decided to recover this aircraft from seawater, conserve it, and display it in its museum, as part of their rich heritage. The objective was to study the aluminum alloy used on the aircraft for its chemical and mechanical properties, suggest the corrosion mechanism of aluminum alloy 2024 in seawater, and recommend preservation methods for the same. Chemical analysis performed on the samples collected from the site revealed that copper was the primary alloying element. Copper is responsible for increasing the strength. However, copper is also the reason for pitting corrosion of the aluminum alloy, causing material loss and reducing the structural stability of the wreckage. Copper forms intermetallics with other elements, such as magnesium and aluminum, and is distributed in the aluminum matrix heterogeneously. In order to study the corrosion mechanism of aluminum alloy 2024, it was subjected to potentiodynamic tests in sodium chloride solution. In the presence of an electrolyte like seawater, the difference between the potentials of these intermetallics and the surrounding aluminum matrix creates a galvanic cell. The galvanic cells serve as sites for localized corrosion. Chloride ions are responsible for pitting of alloy 2024. A pitting potential of around -600mV was observed when sodium chloride was used as an electrolyte. The average corrosion rate measured for wrought aluminum alloys was around 0.05 mm/year. The thesis provides guidelines or recommendations for the procedure to be followed in recovering aircraft from seawater, and retain it in its as found condition. Recommendations about various measurements like pH, dissolved oxygen, salinity, pressure, temperature, and velocity need to be taken and the visual assessment needs to be done before the aircraft is hauled from the seawater were specified. After the aircraft has been recovered, recommendations for handling, cleaning, and prevention of corrosion using coatings such as carnauba wax and inhibitors such as chromates, have been stated.