COMMON FASTENER COATINGS IN NEW ZEALAND
Other processes such as spraying molten metal, vacuum metalizing, chemical vapour deposition, ion plating, enamelling and dip and bake are special purpose and economically impractical for stock commercial fasteners.
ZINC Zinc is favoured as a coating metal because it lasts much longer than uncoated steel. In the galvanic series it is less noble than carbon steel, stainless steel and most other nonferrous metals used in fastener applications. Therefore, in an electrochemical reaction, the Zinc corrodes, and through its sacrifice, the base metal remains protected. Only after the Zinc has been significantly lost to corrosion does corrosion of the base metal begin. Other plating metals are more noble than carbon steel. If these other coatings are breached, the base metal comes under immediate and often accelerated attack. Zinc is the most popular fastener coating also because it is the least expensive, has good appearance, can be applied in a broad range of thickness, by self-passivation has good-to-excellent corrosion resistance, and is relatively nontoxic. Zinc plated fasteners may require more tightening torque to develop equivalent preloads in threaded fasteners. Also zinc coatings without some supplementary protection may develop a dull white corrosion product on their surface which is nick-named “white rust”. Because of its unsightly appearance, most zinc plated fasteners are given chromate treatment, which is a chemical conversion process to cover the zinc surface with a hard non-porous film. This added coating effectively seals the surface, protects it against early tarnishing, and reinforces the fastener’s resistance to corrosion attack. Available coatings are clear (silver) or in a variety of colours (yellow or black most common) and are normally between 3-12 microns (μm) thick depending on fastener size.
In mechanical galvanizing a coating of zinc provides sacrificial or cathodic protection to the steel fastener. Because zinc is more reactive than iron, the zinc coating “sacrifices” or corrodes first, protecting the steel substrate. Mechanical Galvanizing provides excellent coating uniformity, but the process is inconsistent from batch to batch and within each batch, as thickness depends only on the amount of zinc powder. Fasteners are placed in a large rotary barrel along with zinc powder, special promoters and glass impact beads. The mechanical energy generated from the barrel’s rotation is transmitted through the glass impact media that blast the fastener surface with zinc particles. This causes the zinc powder to be mechanically welded on to the surface of the fasteners. CLASS 3 Class 3 screws are manufactured with a mechanically plated zinc-tin 25 micron (μm) coating for external use in a mild, moderate industrial or moderate marine environments corresponding to corrosivity categories C1 – C3 in accordance with ISO 9223. They should be able to withstand 1000 hours of salt spray/fog testing, without showing more than 5% red rust on the surface of the fastener head. Class 3 offers better protection than your typical zinc plated fastener, however over time they are typically being phased out and replaced with Class 4. However due to the slightly thinner coating there is less likelihood to have trouble with screw recesses like Square, Pozi and Phillips drive which may see the coating continue in popularity. CLASS 4 Class 4 screws are manufactured with a mechanically plated zinc-tin 45 micron (μm) coating for external use in a severe industrial or marine environments corrosivity categories C1 – C5 in accordance with ISO 9223. They should be able to withstand 1500 hours of salt spray/fog testing, without showing more than 5% red rust on the surface of the fastener head. If a long life in a severe environment is required, class 5 screws are more appropriate.
0800 425 262 steelandtube.co.nz fortress.kiwi Disclaimer: this data should be used as a guide only, for exact applications you should engage the services of a qualified engineer.
SPECIFICATION SHEET
Coatings are adherent layers applied to the surface of a base metal. For commercial fasteners, practically all deposition is accomplished by electroplating, hot-dipping or mechanically.
MECHANICAL GALV
APR 2024
Approximately 90% of all carbon steel fasteners are plated, coated, or furnished with some other type of supplementary finish. Although the principal reason is to protect against corrosion, such treatments also enhance appearance, control installation torque-tension relationships, minimise threadseizing, and assist product identification.