The interaction of moisture and chemicals on metals can cause breakdown of wood fibres around metal fasteners. This breakdown can result in additional moisture traps and loosening of joints with a propensity for decay.
To prevent deterioration of timber around metal (particularly fasteners) where moisture is present, the following can be employed:
- metal fasteners should be detailed using material with the required resistance to corrosion. Table 1 provides a general guide to the selection of metals with appropriate levels of corrosion resistance
- countersink and plug or stop fasteners (prevents moisture traps)
- avoid the use of dissimilar metal for fasteners
- grease, coat or sheath fasteners in contact with CCA treated timber.
Reference: Timber Datafile P4 – Timber – Design for Durability of the NAFI Timber Manual.
Note: 1. The metals/coatings described are in descending order of resistance to corrosion.
2. Life expectancy of zinc coatings is determined primarily by their weight or thickness. As a minimum, hot dipped galvanised fasteners should have a coating thickness of 42 microns. Refer to AS1214.
|
Material |
Applications |
Remarks |
|
Stainless steel 304 |
Chemical, industrial, marine |
Grade 316 is preferred for marine environments. Additional protection via coatings should be applied to 304. |
|
Monel |
Marine |
Usually used in boat building, nails & screws available. |
|
Silicon bronze, copper and brass |
Marine |
Usually used in boat building, nails and screws available. Do not bring in contact with aluminium. Silicon bronze nails are also available for acidic species, e.g. western red cedar. |
|
Hot dipped galvanised and mechanically plated |
External exposed to weather and medium corrosivity. Industrial and marine environments. |
Where in contact with moist CCA treated timber, additional protection using plastic sheaths or bituminous epoxy coatings are suggested. |
|
Plated (zinc, cadmium) and gold passified. |
Internal exposed to view or protected from the weather and corrosive environments. |
Care required with handling and installation to avoid damage of the protective coating. |
|
Mild steel |
Fully protected from the weather, moisture or corrosive gases. |
Use zinc dust paint systems to provide a base for conventional paints. |
Table 1 - Selecting Corrosive Resistant Fasteners
