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How to prevent rust and corrosion
A coated product (such as a galvanized lintel, for example) may fail over time because of its ‘thin’ anti-corrosion coating and because it is not suited to the conditions in which it used. What can be done to prevent this potentially expensive problem?
Anti-corrosion coatings and exposure conditions
Anti-corrosion coatings used by the building industry generally take the following forms:
- Hot-dipped galvanized coating
- Continuous galvanizing – a thin coating, e.g. sheeting, wiring and tube
- Supa-alloy 5 (SA5) – the distinct gold look on cyclone rods, stirrups, etc.
Exposure conditions are categorised as mild (the outback), moderate (Melbourne), tropical (North Queensland), industrial (Newcastle), marine (coastal regions of Queensland) and severe marine (surf beach frontages). Other localised factors that should be considered when assessing corrosion protection include UV radiation, high humidity, seasonal ventilation and drainage.
Selecting an appropriate coating system
Selecting the appropriate anti-corrosion product to suit exposure conditions is extremely important. Class Z200 galvanized steel veranda joists were used on a house exposed to a surf coast. Within two years there were obvious corrosion issues in the exposed steelwork. Additionally, the fasteners on the fascias showed corrosion stress.
The Building Permit specified hot dip galvanizing of all exposed steel. Z200 is a continuously galvanized section that is one-third of the thickness of hot dip galvanized corrosion coating. Z200 was not fit for use in this exposure location, leading to a rectification job that cost $30,000.
Use of timber products
Timber generally possesses a critical value of moisture content below which corrosion of metal fasteners will not take place. Thus, moisture content is the main factor in controlling corrosion.
Fasteners in enclosed positions in dry timbers (moisture content 18% and below) generally perform satisfactorily.
Some problems have been experienced with the use of chromated copper arsenate (CCA) timbers, so it’s useful to consider the following:
- Manufacturers recommend that treated timber products be withheld from sale for a ‘curing’ period. Fixing or setting of the CCA usually takes about six weeks
- Treated products now use an oxide-based formulation rather than a salt-based one, thereby further reducing any corrosion risk that may have resulted from incorrect use
- Some nailplate manufacturers allow their hot dip galvanized products to be used with CCA products provided the treated timber is dry after treatment and remains dry during service.
LSOP (Light Organic Solvent Preservative) treatment
LSOP is a solvent-based treatment and has water-repellent properties. No corrosion problems have been reported with fasteners installed with this type of treated timber.
Design against corrosion
It is important to consider design when examining the many factors that can affect corrosion prevention. The exposure conditions to which the timber/metal system will be subjected in service should be established and appropriate design features set in place.
In general, if the installed fastener systems are kept dry, their service life will be prolonged – even if used in high-exposure conditions.
To find out more, contact HIA's Building Services team.
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