Mariners' Museum

ABSTRACT The permeability of biaxially oriented polyethylene terephthalate, PET, to vapors from 5%, 30%, and 100% formic acid solutions as well as the vapor from a 30% acetic acid solution was measured by gravimetric analysis in order to determine the transmission rate of the vapors through PET folders and enclosures made from Mylar™ and similar materials, and ultimately understand the microenvironment that is created inside an encapsulate. The data shows that the permeation of formic acid vapor is at least two orders of magnitude slower than the permeation of water vapor. Measurements of the permeation of 30% acetic acid vapor through PET seemed to be due to the permeation of the water only, and the permeation of the acetic acid, if it occurred, occurred too slowly to be measured. The size of the diffusing molecule determines the permeation. Since the permeability decreases as the size of the diffusing vapor molecule increases, a plot of permeability versus vapor size allows the permeability of various other vapors through PET to be predicted. Colorimetry showed that porous PET allowed ambient vapors to diffuse into the encapsulate, and the entire paper within the encapsulate was affected, not only the paper near the holes in the PET.

Artifacts from The USS Monitor have been recovered from the site where the Monitor sank and are presently undergoing archeological study and preservation at The Mariners’ Museum. The present condition, corrosion rate, and other characteristics of the artifacts have been assessed. Impressed current cathodic protection has been designed and installed to mitigate corrosion of the gun turret.

Abstract Spectroscopic analysis of artifacts from the American Civil War ironclad, USS Monitor, has been undertaken in order to determine the present state of degradation of the objects, and to identify foreign compounds that will require removal during the stages of conservation. Metallic artifacts consisting of wrought iron from the rotating gun turret have been studied by a variety of analytical techniques to determine the effect of long-term exposure to salt, and the anaerobic environment of the deep ocean. Mössbauer spectroscopy, and X-ray diffraction have been used to identify the corrosion products, concretion, and marine sediments attached to the turret when it was recovered in 2002, and subsequently during storage. Optical microscopy and Electron-Probe Micro-Analysis have been used to characterize the wrought iron morphology, and to locate and map the chlorine and other potentially detrimental elements in the iron. While submerged in the ocean, the corrosion of the turret has resulted in a thin coating of the reduced iron oxide, Corrosion Magnetite, which, when covered or incorporated with marine concretions, appears stable, and offers the wrought iron protection from accelerated corrosion often observed following recovery of metal artifacts from the ocean. However, if the concretion is breached and the rust or metal is exposed to air, further and immediate oxidation of the metal and existing rust occurs, and is believed to be detrimental to conservation processes. Elemental X-ray mapping by Wavelength Dispersive Spectroscopy shows that during ocean submersion, chloride ions diffuse deep into the inclusions in the wrought iron, where they are trapped and become responsible for significant and continued corrosion. Once exposed to air and dried, the trapped chlorides have a detrimental effect on the longevity of the artifact.