Metal connectors, fasteners and anchors can corrode when installed in corrosive environments or when installed in contact with corrosive materials, and potentially lose load carrying capacity. The many variables present in a building environment make it impossible to predict accurately whether, or when, corrosion will begin to reach a critical level. This relative uncertainty makes it crucial that specifiers and users be knowledgeable about the potential risks and select a product with suitable corrosion protection for the intended use.

Commonly outdoor applications and uses where the product might intermittently be exposed to elevated moisture levels would result in corrosion of steel products without corrosion protection. In low risk (‘dry’) environments unprotected ‘black’ steel may be acceptable. At medium risk levels protection is added to the parent steel by way of a sacrificial zinc layer, that essentially ‘corrodes’ or slowly diminishes over time sacrificing itself to prevent the steel corroding. This method obviously has limitations in that if left in too high a corrosive environment for prolonged periods the zinc initially present will eventually be completely dissipated and corrosion protection lost. The zinc can be applied to the parent steel material by different methods, including electroplating or hot dip galvanizing. It is worth noting that the thickness of steel is a critical aspect to the amount/weight of zinc that will bind itself to the steel; for thin steel products it is not feasible to achieve a high density/ weight of zinc. That’s because the zinc will no longer be properly bound to the steel but only bind to itself. Therefore, higher weights of zinc coating can only be achieved with thicker steel products. An alternative method to sacrificial surface protection is changing the composition of the steel itself – this is what defines stainless steel for example. While the entire material composition changes, rather than relying on a sacrificial layer, even stainless steel depending on the grade can be affected by moisture. Stainless grade 304 for example can exhibit what is often referred to as ‘tea staining’ on the surface, whereas this is far less likely to occur with the higher 316 grade. The presence of some surface corrosion does not mean that load capacity has been affected or that failure is imminent. Factors other than moisture can also affect corrosion, including exposure to chemicals. So if significant corrosion is apparent then the environment should be considered also, and the fastener, anchor, or connector and the timber it is fitted against should be inspected and replacement of affected components be undertaken.

Article written by Daniel Scheibmair from Simpson Strong-Tie New Zealand Limited dscheibmair@strongtie.com