Zinc flake coatings are non-electrolytically applied coatings, which provide good protection against corrosion. These coatings consist of a mixture of zinc and aluminium flakes, which are bonded together by an inorganic matrix. There are a number of components which are combined to produce a high performance protective coating. The wide variety of components gives a wide spectrum of possibilities to solve any coating requirement need. For specific applications, Shiva Co. is able to formulate a specific product that will achieve optimum technical performance within the cost/price economic window. Each coating formulation will have a number of components combined with Zinc Flakes, such as
• Binders are the materials which hold the other protective coating components to one another and to the surface of the substrate.
• Metallic Pigments provide sacrificial protection to the substrate; these are metals that are more reactive than the substrate. While many metallic pigments are used. The flake pigments give a brighter appearance than the dusts, which give a flat matt appearance. Corrosion protection is generally similar, although flake pigments offer better corrosion protection.
• Lubricants reduce the coefficient of friction of the coating, resulting in improved torque-tension relationships for threaded fastener assemblies and improved bulk processing of parts. The lubricating pigments are selected according to price performance parameters.
• Inhibitive Additives are added to protective coating formulations to enhance the performance of the metallic pigments by reducing their electrochemical activity. Typical inhibitive pigments which can be incorporated in protective coating formulations
Zinc flake coatings have thicknesses in the range of 5-12 microns and in contrast to electroplated coatings the coating is thicker at thread roots and in recesses, and thinner at crests and edges. Tightness to thread inspection gauges will be experienced with fine threads at M4 and below although in most situations this will not inhibit assembly into tapped holes and standard nuts. This phenomenon of dip-spin applied zinc flake coatings being thicker in recesses can ensure that recessed parts or complicated pressing with folds, etc. will have far better corrosion resistance than when electroplated due to the electroplated coatings being thin in “shielded” areas. In some cases, however, this recess build up with dip spin applied zinc flake coatings can be a major problem.
Performance Advantages of Zinc Flake Coatings :
• Excellent resistance to atmospheric corrosion.
•Limited “white” rust (zinc corrosion products) or other corrosion products in service.
•Neutral salt spray corrosion resistance exceeds that of many other common surface finishes, e.g. electro and mechanically plated zinc and sheradising.
•Resistance to many “mild” chemicals and solvents including petrol and brake fluids
•No hydrogen embrittlement. It is a non-electrolytic process.
•Can be electrically conductive.
•Galvanic protection by the zinc-rich coating ensures satisfactory performance at bimetallic contacts with steel, aluminium, zinc and cadmium in most situations.
•Complex shapes, recesses and holes are coated with suitable equipment.
•Low thicknesses can give corrosion resistance equivalent to much higher thicknesses of conventional coatings.
Typical parts which can be zinc flake coated are:
• Threaded fasteners particularly strength grades 10.9 and 12.9.
• Pressings, springs, clips as required for vehicles, domestic appliances and on buildings.
• High tensile steel (particularly above 1000 N/mm2) and case hardened parts requiring surface protection without possible hydrogen embrittlement.
• Sintered and cast steel and iron components.
• Parts of complex shape and those with holes and recesses.
• Compound assemblies e.g. lock parts and hose connectors.
•ASTM F1136: Standard Specification for Zinc/Al. Corrosion Protective Coatings for Fasteners
•ISO 9227: Corrosion tests in artificial atmospheres
•ASTM B117: Standard Practice for Operating Salt Spray (Fog) Apparatus
•DIN EN ISO 10683 sets out the requirements for zinc flake coatings for threaded fasteners
•DIN EN 13858 describes the requirements for zinc flake coatings for fasteners with no thread and for other parts as well (iron or steel components)
•DIN EN 15205: Determination of Hexavalent chromium in corrosion protection layer- Qualitative analysis
•DIN EN 62321; VDE 0042-1:2006 procedures for determination of levels of six regulated substances (Lead, Mercury, Cadmium, Hexavalent Chromium, Polybrominated Biphenyls, Polybrominated Diphenyl Ethers) in electro technical products (IEC 111/54/CDV:2006)
No Declarable Substances are present in Zinc flake coating, for instance Chrome VI, Chrome IV, Cadmium, Lead, MoS2, Cyanide, Cobalt, Nickel, Barium, Mercury, PBDE (Polybrominateddiphenyl Ether) and PBB (Polybrominatedbiphenyl). These hazardous materials in the production of various types of electronic and electrical equipment are strictly restricted.