Zinc coatings on mild-steel fasteners

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Avoiding corrosion of fasteners requires careful detailing to ensure detrimental chemical reactions do not occur, which can reduce the life of the fastener.

Galvanising is one way of adding protection to metal fasteners, otherwise they may end up like this.

It’s a fact of life that most metals corrode, particularly in the presence of moisture. If fixings aren’t protected they can corrode away with unseen, and often unwanted, consequences. The answer is to apply a protective coating, such as galvanising, paint or even grease. In past times, concealment of the fixing helped play a part in its protection. For instance, cladding nails were not galvanised but punched well home and an oil-based paint system applied with an oil-based putty filling the nail holes. This was a standard way of doing things for many years. Roofing nails were lead-headed but had bright steel shanks. They didn’t give trouble because the bright steel was protected from the elements. What about fixings exposed to the elements?

Hot-dip galvanising

Galvanising is a most effective coating but it is not always well understood. It was originally discovered and patented by the French chemist Sorel in 1837. His process of dipping steel in molten zinc is the basis of hot-dip galvanising we use today. After suitable preparation and cleaning, an item to be galvanised is put in a ‘kettle’ containing molten zinc at a temperature over 400°C. The zinc adheres metallurgically to the steel item being treated — it is not a ‘skin’ that can peel off. When the metallurgical reaction is complete the zinc in the kettle stops bubbling and the item is removed. These coatings tend to build up more on sharp edges and corners and this needs to be kept in mind when designing items for galvanising.

The zinc also provides a self-healing service. When a section of the coating is damaged the neighbouring zinc sacrifices itself by galvanic action, corroding in preference to the underlying steel. It will keep doing this until there is no zinc left on the steel.

From the time the item is taken from the kettle and put in service the zinc coating is slowly corroding away. Calculations can be done to estimate how much zinc coating is needed to provide protection for the life required. Tables 1 and 2 in AS/NZS 4680: 1999 Hot-dip galvanized (zinc) coatings on fabricated ferrous articles give the coating thicknesses that must be achieved. Small items are often centrifuged immediately after hot-dip galvanising and the coating can be reduced on these items because the centrifuging provides a better finish and more even zinc deposits.

Remember that Table 4.1 of NZS 3604: 1999 requires an additional protection, such as epoxy powder coating, to hot-dip galvanised fixings and fastenings (excluding nails) in all zones where the article is exposed to air-borne salts and rain washing.

Zinc plating

Zinc can also be applied by electroplating but the coating is much thinner and does not protect the metal for as long. However, the thinner coat may be appropriate for finer, threaded items. The process also gives a smoother finished surface more suited to additional coatings such as paint. Items that have been electroplated often have a conversion coating (passivation) to give an attractive finish. The passivation provides some additional longevity but not significantly.

Galvanising is one way of adding protection to metal fasteners, otherwise they may end up like this.

Zinc plated coatings are not suitable for exterior exposure applications. Zinc plated bolts and hardware fittings such as gate hinges will not provide adequate protection from corrosion, and will rarely last more than 12 months in exterior exposures in most urban coastal environments.

Nail-gun nails

The nails for nail guns are zinc electroplated, which improves their performance over unplated nails, but the plating is only about 10–12 microns thick. This compares unfavourably with AS/NZS 4680, which requires an average of 35 microns for centrifuged articles less than 8 mm thick.

Nail guns should not be used for nailing where hot-dip galvanised fixings are required. This is because the nails don’t comply in either the method of manufacture or in corrosion-resistant performance. In these cases hand-driven, hot-dip galvanised nails are still the answer. Stainless steel nails are available for nail guns and are a viable alternative if they meet the pull-out requirements.

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Galvanising is one way of adding protection to metal fasteners, otherwise they may end up like this.

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