Galvanizing nuts and bolts
A coating applied to fasteners must of necessity be tightly adhering and resistant to damage during and after assembly. For this reason, metal coatings applied prior to assembly, are generally preferred. Zinc is the most commonly used metal for protecting fasteners. It is applied either by the electroplating process or by hot dip galvanizing. For extended corrosion free life, zinc coating / thickness is of paramount importance. Unfortunately, zinc electroplating provides a thin protective zinc film (typically < 10 micrometers) which is inadequate for long term corrosion protection in most exposed applications where fasteners are exposed to the environment. Due to the terminology used when zinc protective coatings are specified, zinc electroplated coatings are frequently supplied when in fact hot dip galvanizing is required. This is due to the use of the term galvanizing to describe both electroplating (electro galvanizing) and hot dip galvanizing.
Hot dip galvanizing is a substantially thicker coating and it should always be specified in preference to zinc electroplating for external applications. Because of the thicker coating applied by hot dip galvanizing, the nut threads are tapped 0.38mm oversize to accommodate the zinc coating on the bolt threads. Nuts are normally galvanized in blank form and tapped subsequent to galvanizing.
The fact that no zinc is present on nut threads does not in anyway reduce the corrosion protective properties of galvanized fastener assemblies.
High strength fasteners
Both class 8.8 and class 10.9 high strength fasteners can be hot dip galvanized satisfactorily. The oversizing of threads does not reduce the required tension to below the minimum specified in the relevant specifications for uncoated fastener assemblies. Galvanized fasteners are used effectively for friction grip connections.
Tensioning of Fasteners: In the case of galvanized high strength assemblies, it is advisable to lubricate bolt threads before tensioning. This ensures that the outer pure zinc layer of the coating on bolt threads does not result in gauling between the bolt and nut threads. Contrary to the common misconception, the galvanized coating is not smeared away during tensioning. This is because the coating structure consists mainly of hard and abrasion resistant iron / zinc alloys.
Although not specified in SANS121 (ISO1461) it is recommended that coating thickness on bolts be a maximun 65µM thick. This is necessary to ensure easy turning on the oversized nut.
Torque / tension relationships can provide a considerable degree of scatter in achieved tension. While this does apply even in the case of uncoated bolts and nuts, the variation is more pronounced in the case of galvanized assemblies, particularly if thread lubrication is implemented.
The turn of the nut method of tensioning is without doubt the most reliable procedure.
Figure 1 shows clearly the reliability of this procedure while figure 2 provides the degree of nut rotation necessary to achieve the minimum tension for fasteners of various dimensions.
Hot dip galvanizing of fasteners is a specialised process. Products should therefore be purchased from an SABS approved bolt manufacturer who will ensure that the required manufacturing and galvanizing procedures are adhered to. In this way, problems such as hydrogen embrittlement and fatigue failure, will be avoided.
