The Use of 3D-Svet for the Examination of Plasticised PVC Coatings: The Effect of Deformation and UV Irradiation on Barrier Properties

The 3D-SVET is a valuable technique for the examination of corrosion phenomena on non-planar samples. A bi-functional probe is first used to record topographical data to obtain a surface height map; subsequently this height map is used to retrace the surface, obtaining current density data at a known fixed height. In this work the apparatus was utilized to investigate the effects of forming and irradiation on model plasticized PVC coatings and specifically the influence on the barrier properties of the coating. Samples were prepared via bar casting onto galvanneal steel substrate and then either examined in a planar state or after a bending process that created a 30° deflection from flat. Samples were examined via 3D-SVET either as cast or after irradiation with UV in order to investigate the effects of weathering. In addition, an industrially produced coating that had undergone natural weathering for a period of 17 years in a coastal industry environment was also analyzed as a comparison.

The model study demonstrated that both the planar non-irradiated and the planar irradiated samples retained their coating integrity, with the 3D SVET detecting no corrosion activity during a 24hr immersion study. In contrast, the samples that underwent a forming process showed evidence of a cathodic band of activity that coincided with the peak of the bend. This band increased in activity during the period of immersion, most likely due to the improved oxygen availability at the thinnest part of the coating. Findings from the industrial coating (See Fig 1A-C) showed the presence of numerous penetrative defects and it was found that the majority of the longer lived defects existed within the emboss pattern, coinciding with thinnest part of the coating. Subsequent removal of the industrial coating evidenced the extent of the zinc dissolution that occurred during the samples lifetime.

Figure 1: A) The surface profile of the industrial coating, illustrating the leathergrain emboss pattern within the paint. B) A Surface image of the coating overlaid with the most active anodic areas as measured over 24hrs immersion in 5% NaCl. C) The resulting galvanneal surface post removal of the plastisol coating, illustrating the metal dissolution during the samples life.

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