Corrosion characteristics of seven metals in three aqueous environments for forensic applications
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Corrosion characteristics of seven varieties of metals—zinc, brass C260, stainless steel 302, stainless steel 316, stainless steel 420, stainless steel 430, and stainless steel 440—in three aqueous media—Atlantic Ocean, Charles River, and deionized waters—were assessed via mass loss methods over 32 weeks, with supplemental data in the form of photomicrographic records. Concurrently, tests were conducted to determine the degree of measurement error resulting from the analytical scale used during corrosion assessment. This was accomplished by using reference samples of each type of metal and a glass vial as the container that held the metal and water samples. These error tests indicated that while the mass error associated with the metal samples was low, the error in mass associated with the vial displayed error margins two orders of magnitude larger than the error margins for the smaller metal samples. Further, control tests and statistical analysis indicated that this variation was the result of some quality inherent to the vial. The metal samples involved in the corrosion assessment experiment generally displayed corrosion characteristics in agreement with trends reported in the literature. Zinc produced the greatest quantity of corrosion residues out of all the metals studied. Brass C260 also developed visible corrosion. For example, brass C260 developed dark green/brown adherent residue and whitish blue-tinted nonadherent residue in Atlantic Ocean water, faint greenish tarnishing and some dark green spots and dots over time in Charles River water, and only faint greenish tarnish in deionized water. In contrast with zinc and brass C260, the stainless steels did not exhibit signs of significant corrosion rates excepting stainless steel 420 (SS420), which displayed pitted features surrounded by multi-colored rings on all of its Atlantic Ocean immersion samples and 25% of its Charles River immersion samples. Atlantic Ocean water generally caused the greatest degree of corrosion for all metals, followed by Charles River water, then deionized water, except in the case of zinc. Residues found on zinc samples immersed in the three different water types were similar to each other in coloration. SS420 samples immersed in different waters also displayed similar-colored residues. Comparisons between the corrosion features of SS420 and the single stainless steel 430 and 440 samples that did show visible corrosion in Atlantic Ocean water suggested that minor compositional variations between stainless steels have little effect on the visual characteristics of the corrosion residues they form. Corrosion rates were calculated using linear regressions of the mass loss data for all metal sample sets. While some of these corrosion rates approached literature-reported values for the metal in question, the distributions of the mass loss data sets indicated that any mass changes that resulted from corrosion were likely too small for the electronic scale to detect. Consequently, it is recommended that future corrosion studies using the mass loss method utilize metal samples similar in size to objects typically found at crime scenes.