Corrosion handbook

36 06/21
Selection tables for fasteners and installation systems
Fasteners such as anchors, nails and screws are often used for single-
point and safety-relevant fixtures. Poor or limited visibility of the fasteners after
installation, the inability to repair or replace them, the part to be fastened, and (in
the case of expansion anchors) the friction conditions in the drilled hole all mean it
is necessary to take a conservative approach to minimize the risk of corrosion to
the fastener material. With zinc-coated fasteners, the end of the fastener’s lifetime
coincides with the start of the loss of the underlying steel. The selection table
for fasteners therefore does not state a lifetime range, but gives a general yes/no
recommendation based on the building service defined in the approval guidelines,
e.g. 25 or 50 years – see selection tables footnotes for details.
Fasteners often form part of a building’s structure, which means that the lifetime
of the fastener is required to be equal to the design lifetime of the building.
Secondary structures such as installation supports are often not required to last
the entire lifetime of the primary structure as building services are subject to more
frequent exchanges and modifications.
Furthermore, many applications with installation channels allow repair or lifetime
extension measures, for example, through application of zinc spray. With proper
assessment of the corrosivity of the environment including the planned lifetime
and also the implementation of periodic inspection and repair intervals, coated
carbon steel products can be used suitably and economically for a wide range of
applications in a wide range of surroundings.
In the case of zinc-coated installation systems the onset of corrosion of the steel
substrate does not cause immediate problems in terms of mechanical integrity.
The occurrence of around 5 % of red rust on the surface is considered to indicate
the end of the period of protection provided by the zinc coating. It is therefore
possible to state a guideline range of expected lifetime for a specific environment.
European Technical Approval – metal anchors for use in concrete
For anchors, the current European Technical Approval (ETA) requires use of
stainless steel in the corrosion resistance class III (“A4 class”) for outdoor
applications in general. Special applications such as road tunnels and indoor
swimming pools require an even higher grade (class IV or V). The reason for this
is, of course, to provide the user with a conservative and practical guideline for
selection of a proper material for outdoor applications. The intent of the ETA
approach is to eliminate the need for a detailed assessment of the environment
and the application when strictly following the ETA (assuming non-atmospheric
corrosion issues are not present). The user, for instance, does not have to
evaluate the influence of chlorides on the jobsite resulting from proximity to sea
water or the use of de- icing salts.
However, in cases where chlorides can be neglected it is possible to go beyond
the ETA specifications. From a technical point of view, hot-dip galvanized or
stainless steel anchors in the CRC II (“A2 class”) may be suitable for outdoor
environments with certain lifetime and application restrictions. This is based
on long-term experience with these materials. Please note that this requires
a specific assessment and deeper understanding of the application and the
environment (see following tables).