Zinc Flake
The Zinc Flake Coating was developed to give an improved corrosion resistant coating compared to electro plated zinc and also give other advantages especially when applied to small parts and in particular fasteners. The coatings consist predominantly of zinc flakes, in some cases mixed with a small proportion of aluminium flakes. It is important in the formulation of these materials to use flakes and not zinc powder or dust, as the presence of the flakes with the correct drying and curing procedure allow a dense coating to be formed with the flakes lying parallel to the substrate surface which greatly improves the protective performance of the coating. The flakes are bound together using a matrix that can be organic or inorganic, this varies with the particular materials used. The coatings are conductive so are sacrificial to steel. They are often used with a top coat that can improve corrosion and give barrier protection, but in this case the total coating may not be conductive.
The performance advantages of zinc flake coatings are:
• 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.
• Usually 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.
The coating process for zinc flake coating in bulk is as follows:
(a) Cleaning to remove dirt, lubricants and oxide scale.
(b) Dip-spin procedure - components in baskets are immersed in the zinc flake liquid material. After wetting the work, basket is drained and spun at high speed to remove excess liquid. Maximum component size approx. 150mm x 20mm dia. or 0.5 kg. weight. Dip-drain procedure - components are loaded on to jigs. The jigs are lowered into the zinc flake liquid. Subsequently the coated parts are drained and spun to remove excess liquid. Maximum component size 1100 x 500 mm or 30 kg. weight. Spray - components such as brake discs are placed on jigs and are spray coated to provide corrosion protection and a pleasing cosmetic appearance.
(c) The parts are then dried and cured by passing through an oven at a time and temperature commensurate with the coating being applied.
(d) Stages (b) and (c) are then repeated for dip-spin coatings.
Typical parts 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 with holes and recesses.
• Compound assemblies e.g. lock parts and hose connectors.
Note:
Items which have predominantly flat or nesting surfaces may not always be suitable for zinc flake coating and initial trials should be arranged. In some instances a small change in design can make items suitable. Washers are most effectively treated as captive assemblies with screws (i.e. sems).
Zinc Flake Coating Suppliers
The use of zinc flake coatings by major OEM's worldwide has increased substantially due to the requirements for prolonged corrosion resistance and for fasteners, reproducible tightening performance. Anochrome Group can supply coating materials sourced from the four major suppliers in this area, who are:
• Dacral, suppliers of Geomet® 321, 500, 360 and 720 and Dacromet®
• Doerken, suppliers of Delta Tone®, Delta Protekt® and Delta Seal®.
• Magni, suppliers of Magni 560, Magni 565 and Magni 594.
• Atotech, suppliers of Zintek® and Techseal®.
Please see further sections for more specific details of these products.
* If a component is unsuitable for mechanical cleaning due to shot entrapment or damage (thin gauge clips, etc) then cleaning by chemical methods is an option, but carries the risk of hydrogen embittlement, especially with higher grade carbon steels.
