ZINK FLAKE COATINGS
These coatings are deposited by a so called “dip-spin-bake” process; the steel products are submersed in a very thin inorganic liquid carrier filled with very small zinc particles (flakes), often also joined by aluminum particles. This paint-like substance covers the products, and spinning is needed to remove the excess liquid afterwards. Subsequently, the parts are baked in an oven, which forms a solid layer filled with zinc and aluminum flakes to protect the steel from corrosion. This layer provides very durable corrosion resistance for normal outdoor applications, and it visually resembles a hot dip galvanized layer in some respects, as both are matte grey. The layer thickness of the zinc flake coatings may be somewhat thicker compared to zinc electroplating (approximately 6 to 12 microns). Therefore, care must be taken for screw thread sizes below M8 and standard nuts with thread class 6H. Gauging may require some effort. Therefore, ISO 10683:2014 and the publication entitled, “Richtlinie Verschraubbarkeitsprüfung von Muttern mit Zinklamellenüberzügen” (Deutscher Schraubenverband, DSV, 2012) contain elaborate information about gauging, judging and measuring zinc flake coatings.
SCHEMATIC ZINC FLAKE LAYER
The layering effect of the zinc and aluminum particles when stacked upon each other acts like multiple protection layers, and together with the inorganic carrier, this barrier provides high corrosion resistance.
A significant advantage of this type of coating is that there is no risk of hydrogen embrittlement. The parts are not pickled, there is no electrolytic action involved and the plating process is completed at room temperature until it is baked.
There are many different developers and manufacturers of zinc flake coatings. NOF Metal Coatings and Dörken are important players, with brand names like Geomet© and DeltaProtekt©. The zinc flake coatings can be provided in topcoats that may improve corrosion resistance or provide a specific color, and lubricants can also be integrated into the zinc flake coating or applied over the basecoat when not integrated. The ISO 10683 standard, which concerns the properties and requirements of zinc flake coatings, created a division into four coating systems:
- Only basecoat
- Basecoat and lubricant
- Basecoat and topcoat
- Basecoat, topcoat and lubricant
A further distinction is made based on the content of hexavalent chromium, or Cr(VI): zinc flake coatings containing Cr(VI), which is designated as flZn/yc, and zinc flake coatings without Cr(VI), designated as flZn/nc. The latter are in compliance with ELV, RoHS and Reach regulations.
Zinc flake coating systems that contain a lubricant integrated in the basecoat or with a lubricant applied over the basecoat provide a controlled friction coefficient. As the friction coefficients reached with these coatings can significantly differ from traditional coatings, like zinc electroplating, it is advised to predetermine the assembly torque values of fasteners when changing to a zinc flake coating from other surface coatings.
SALT SPRAY TESTING
As it is virtually impossible to predict outdoor environments over long periods of time in all the different climates in the world, a comparative test has been developed. The coated steel parts are quickly corroded in a salt spray cabinet, and the time until white rust (zinc corrosion) or red rust (base metal corrosion) develops is recorded. This timeframe is an indication of the relative corrosion resistance of a protective layer.
This test is called a salt spray test. Standards for this test are EN ISO 9227 and ASTM B117, which are nearly identical. Corrosion resistance is expressed in ‘hours neutral salt spray’ (h NSS). The test is accepted worldwide as the standard test for metal surface coatings.
COMMON SURFACE COATINGS IN ORDER OF INCREASING CORROSION RESISTANCE
The NSS results, derived from various standards for coatings, are given here for guidance only. They are highly dependent on the (feasible) layer thickness and applied topcoats or sealants of the coatings.