Solid state characterization of corrosion products: determining time since decomposition
Wogan, Megan Elizabeth
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Forensic analysts commonly examine ballistics evidence in forensic laboratories where gunshot residue, bullets and firearms can be examined to aid in criminal investigations. Despite extensive means and protocols to examine discharge residues from firearms, there is no current method available to determine time since submersion of a firearm in a body of water. When a firearm has been disposed of in a body of water and becomes corroded, its appearance is altered. However, characterizing the solid state of firearms and their corrosion products may provide information regarding time since deposition in aqueous environments. Previous studies which examined mass loss of four handgun slides and a stainless steel standard were conducted, and showed that without previous knowledge regarding metal composition, little can be said about the length of time a metal object has been exposed to a particular environment. Since a priori knowledge of gun composition is required, a detailed study into the solid-state characterization of the metals and their corrosion products via SEM/EDX and X-ray Diffraction (XRD) was performed. During the characterization, special attention was given to analysis of particle size, morphology and resultant crystalline phases. The x-ray diffractograms were analyzed against the NIST Powder Diffraction Database to determine number of phases and phase composition. Crystallite sizes were determined using the relationship between the full width at half maximum via the Scherrer equation. Metal filings from the SS416 standard, Llama handgun slide, and Ruger handgun slide were determined to predominantly consist of iron alloys. After 180-days in solution, XRD analysis indicated that the adherent corrosion products consisted of 1) y-FeOOH and 2) iron oxide (Fe3O4 or Fe2O3). XRD analysis indicated that the adherent corrosion products of the SS416 standard also consisted of: 3) CrO3. Metal filings from the Raven and Jennings handgun slides were consistent with iron nickel zinc alloys and pXRD analysis of the corrosion products from these alloys, when submersed in deionized water, indicated the products consisted of: 1) γ-FeOOH, 2) iron oxide (Fe3O4 or Fe2O3), and 3) ZnFe2O4 or ZnO; where the Jennings adherent rust contained ZnFe2O4 and the Raven adherent rust contained ZnO. Further, pXRD analysis of the corrosion products from these alloys, when submersed in 25 PSU solution, indicated the products consisted of: 1) ZnO, 2) Zn(OH)2,3) Zn5(OH)8Cl2·H2O, and 4) NaCl. Morphology and crystallite data thus indicated that both metal composition and the presence of chloride ions had significant impacts on rates and products of corrosion. Compositional differences offer an explanation as to the differences in mass loss rates and products observed between handguns. However, while mechanisms and rates of the chloride driven corrosion processes offer explanations as to the different oxides and hydroxides observed between immersion conditions, they do not offer an explanation for the differences observed in the Jennings and Raven handguns. That is, the rates do not offer an explanation as to the change in species undergoing oxidation between aqueous environments. As such, this research demonstrates that determining mass loss rates and characterizing surface area coverage of corrosion products on metallic items are not sufficient to determine time since deposition.
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