Edge alignment

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Edge migration is a physical effect that during the coating occurs from surfaces and which leads to that the coating edges fails thinner than on flat surfaces. The edge alignment is a serious problem in coating technology because parts of the workpiece are not sufficiently coated. This can prevent the desired protection , especially in the case of anti-corrosion coatings .

mechanism

A liquid, for example a lacquer or an oil , that runs over an edge on a workpiece experiences a curved surface on this edge. The surface tension minimizes the surface and leads to the fact that the liquid is displaced in the direction of the flat surfaces. This significantly reduces the layer thickness in the area of ​​the edge. This can lead to the layer tearing off and, in extreme cases, to dewetting (as opposed to wetting ) the edge. In the vicinity of the edge, the layer thickness increases until the bulge forms.

The causes of the edge alignment on the part of the coating material are too low a viscosity and too high a surface tension of the coating. The too low viscosity can be caused by the use of too much or wrong solvents . Excessive surface tension is usually caused by the use of surface-active additives such as wetting and dispersing agents or leveling agents. On the part of the workpiece to be coated, an insufficient edge radius of the workpiece can be the cause.

Zinc coatings produced by hot-dip galvanizing are no less thick on corners and edges than on smooth surfaces. Due to the process (hot-dip process), iron-zinc alloys grow parallel to the component surface during hot-dip galvanizing. This alloy layer fans out at the component edges and the existing gaps are filled with metallic zinc.

Measurement

The most precise method for determining the edge alignment is to make a cross-section. Alternatively, the coated test specimen can be tested in a corrosion test.

Countermeasures

The extent of the edge alignment can be minimized by increasing the viscosity of the coating material or reducing its surface tension. An additional improvement is possible through thixotropy or structural viscosity, i.e. an increase in viscosity at low shear rates . Another possibility is the use of electrostatic coating processes in which the paint is electrically charged.

A rounding of the edge also leads to a reduction in the edge alignment due to the increase in the edge radius.

Individual evidence

  1. ↑ a b c d e H. Römpp: Römpp Lexikon Lacke und Druckfarben . Thieme, Stuttgart 1998, ISBN 978-3-13-776001-6 , p. 316 .
  2. ↑ a b c d e Somborn, Roland: High edge flight . In: paint and varnish . Vincentz Network, August 2007, ISSN  0014-7699 , p. 47 f .
  3. ↑ Hot-dip galvanizing worksheets A.3 Properties of hot-dip galvanizing, high mechanical strength and optimum edge protection, Institut Feuerverzinken, Düsseldorf, 2020. https://www.feuerverzinken.com/wissen/arbeitsblaetter/a-korrosionsschutz-feuerverzinken/a3-elösungen-der- hot-dip galvanizing