Concentration of vapor phase corrosion inhibitor in anticorrosion polymer films : correlation of ion-beam analysis results and UV-VIS spectrophotometry

Anticorrosion polyolefinic films are being increasingly used in corrosion protection, from packaging of electronic products to protection of firearms of all sizes. Anticorrosion polymer films are produced by coextrusion of polymer and vapor phase corrosion inhibitor (VCI) that slowly diffuses out of the film and forms a protective layer on metal surface. Thereby the concentration of VCI in the film decreases with time what limits duration of anticorrosion protection. Earlier [1] we analyzed a number of such films of various ages, identified polymer carrier as low-density polyethylene and VCI as an organomolybdate salt. We also confirmed that VCI provides protection by migrating out of the film. We determined that the protection last up to 6 or 7 years, depending on initial concentration of VCI in the film. Particle Induced X-Ray Emission spectroscopy, an ion-beam analysis method was necessary both to confirm the nature of VCI and determine its concentration in anticorrosion films. The concentration of molybdenum in the films is very low so the error of classical analytical methods would be greater than the expected concentration change or even concentration itself. In PIXE the X-ray emission characteristic of a particular nuclei produced by, in this case proton impact on a material, is measured so it is possible to identify the nuclei present in the film if corresponding atomic number is higher than 12 and to determine its concentration in the sample if larger than 1 ppm. The drawback of PIXE method is its limited availability. Because of that, a method that provides fast and reliable determination of VCI concentration change with time to detect when the anticorrosion protection ends was needed. Since UV-VIS spectrophotometers are much more widespread, the goal of this investigation was to determine whether there is a correlation between the concentration of Mo determined by PIXE and UV-VIS absorptions in anticorrosion films. The assumption that the anticorrosion films are of same thickness was checked by FTIR. The UV-VIS spectra of selected anticorrosion films with various concentration of VCI were recorded showing two prominent absorptions at about 710 and 610 nm. In spite of the problems of determination of the molar absorptivities of the VCI, the intensities of both absorptions had excellent correlation to molybdenum concentrations determined by PIXE so it was concluded that UV-VIS spectroscopy could be used to control the decrease of VCI concentration. In that manner control of the properties of anti-corrosion films is possible provided initial concentration of VCI is determined by another method that gives absolute concentration of VCI in the sample. [1] Pucić, I. ; Madžar, T. ; Jakšić, M. Chem. Monthly. 137 (2006), 953-961.