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Structural Characterization of Films Formed by Chemical Phosphating
Chemical phosphating is a chemical surface treatment used to improve corrosion resistance and paint adhesion on metals such as steel and aluminum. The process involves the application of a phosphoric acid solution to the substrate, forming a thin film of a phosphate/metal salt mixture which is then treated with a zinc or manganese salt solution to further harden the film.
The resulting phosphate film is composed of various compounds including ferrous phosphate, zinc phosphate, and dihydrogen phosphate. Due to the complexity of the film composition, determining the exact composition of the film is difficult. As such, a variety of analytical techniques are used to characterize to the phosphating films.
X-ray Photoelectron Spectrometry (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA), is often used to characterize the film composition. XPS is a surface analysis technique which is used to determine the elemental composition, chemical state, and chemical bonding environment of the film. Additionally, XPS can be useful in determining the thickness of the phosphating film, as differences in the intensity of the spectral peaks can indicate the depth of the elements in the film.
Scanning Electron Microscopy (SEM) is a technique used to analyze the morphological characteristics of the film, including size and shape of phosphate crystals and lightly bound particles. Additionally, Energy Dispersive Spectroscopy (EDS) can also be used in conjunction with SEM, enabling the concentration of elements in the film to be determined.
Atomic Force Microscopy (AFM) is another technique used to analyze the structural characteristics of the film. In this technique, a tiny probe attached to a cantilever is used to measure the surface properties of the film. These measurements can be used to determine the surface topography of the film and the roughness of the surface.
Raman spectroscopy is a powerful technique used to determine the structure of the phosphating film. It measures subtle shifts in the frequency of the monochromatic light scattered by the film, allowing the identification of the elements, as well as their chemical bonding characteristics.
To gain further insight into the structure of the film, Fourier Transform Infra-Red Spectroscopy (FTIR) can also be used. FTIR is an analytical technique used to identify the functional groups present in the film, as well as determining their relative concentrations.
In summary, chemical phosphating is a widely used process to improve the corrosion resistance and paint adhesion of metals. The resulting film is composed of various compounds, the exact composition of which is difficult to determine. For this reason, various analytical techniques are used to characterize the phosphating film, including XPS, SEM/EDS, AFM, Raman spectroscopy, and FTIR. Each of these techniques provides unique information about the film, resulting in a more complete characterization.
