CORROSION
Corrosion of fasteners
Although timber treatments protect timber, they can corrode nails, screws and other
hardware used in building. A BRANZ research project is investigating the effects of
different treatments on some common types of metal fasteners.
By Zhengwei Li, BRANZ Corrosion Scientist
T
imber’s versatility, diversity and aesthetic properties make
it very important in the construction industry. However, it
deteriorates when exposed to the environment. Preservation
treatment can protect it from decay fungi, harmful insects
or marine borers but the degree of protection depends on the type,
penetration and retention of the chemicals used.
These treatments may present a potential corrosion hazard to
metallic components in the timber, since the metallic ions in waterborne
preservatives can act as an added oxidiser. The corrosiveness of timbers
treated with alkaline copper quaternary (ACQ) and copper azole (CuAz),
alternatives to copper chrome arsenate (CCA), has become a concern due
to their higher copper contents. These treatments are not widely used and
can be identified by a number 58 for ACQ or 90 for CuAz in the timber
treatment markings.
First year results available
BRANZ is currently conducting a 3-year field exposure test at out Judgeford
site to investigate the corrosion performance of typical building hardware
in treated timbers. This project follows on from previous BRANZ laboratory
studies (see Build 98 February/March 2007, pages 68–70 and BRANZ
study report SR153).
The first year of exposure has been completed, and the degradation of
nails and screws made from mild steel, galvanised steel and stainless steel
embedded in CCA, CuAz and ACQ treated timbers has been assessed.
Mild steel nails and screws corrosion similar
Corrosion surface morphologies of the mild steel nails and screws driven
into untreated and CCA (H3 and H4) treated timbers were found to be
similar after this initial period. These fasteners suffered from uniform and
gentle corrosive attack on their whole surfaces.
Nails and screws underwent more serious degradation in ACQ and CuAz
treated timbers. In these two timbers, an increase of preservation level
from H3 to H4 also increased the corrosion.
Galvanised nails perform better but still concerns
Galvanised nails showed better performance than mild steel nails in all
timbers. The coatings on the nails embedded in untreated and CCA treated
timbers were still in relatively good condition after 1 year of exposure. No
obvious failure was found.
CuAz and ACQ treated timbers, particularly the timbers treated to H4
level, showed a higher corrosivity (see Figure 1). Iron-rich rust could be
easily seen on most of the shaft area of the nails, indicating that the top
zinc-rich coating had been consumed.
Galvanised screws corroded more than nails
In comparison with the galvanised nails, the galvanised screws exhibited a
slightly lower resistance to corrosion, as iron-rich rust was more apparent
and covered more surface area. The thread area was the first to show
corrosion of the underlying steel substrate. When embedded into ACQ and
CuAz treated timbers, even with an H3 level, screws showed coating failure
in their shank areas (see Figure 2).
These results indicate that the driving-in process could damage the
coating in the thread area, leaving more structural defects here than in
the shank area.
Stainless steel excellent
Field exposure test site at Judgeford where the corrosion performance of typical building
hardware in treated timbers is being investigated.
All stainless steel nails and screws showed excellent corrosion resistance
in all timbers. No obvious sign of corrosion was observed on shaft and tip
areas. Occasionally, very limited iron-rich rust could be found on the nail
heads. This probably results from hammering of like fasteners into the
timber. Clearly, this is not related to the timber preservation treatment.
BUILD August/September 2009 47
Less corrosion in CCA, more in CuAz and ACQ
The trends for steel corrosion in treated timber are reasonably clear. In
general, nails and screws had a relatively low corrosion rate in CCA treated
timbers. It is interesting to note that the mass loss rate of the nails and
screws embedded in CCA (H3 and H4) treated timbers was similar to that
of the nails and screws embedded in untreated timbers.
By comparison, CuAz and ACQ treatments showed markedly increased
corrosivity. The galvanised coating may provide some protection to nails
and screws. However, the coating in the thread area of screws was prone
to damage during the driving-in process, thus a high protection capability
cannot always be relied on.
These observations also indicate that the copper retention has some
correlation with the corrosion rate of the metallic component, but the
increase of copper retention in treated timber might not be fully responsible
for the enhanced corrosion. The influences of different components in
CCA, ACQ and CuAz on corrosion of metal need to be considered. This will
be examined further as the work proceeds.
H4 CCA treated timber
H4 ACQ treated timber
Figure 1: Galvanised steel nails removed from H4 CCA and ACQ treated timbers.
H4 CCA treated timber
H4 ACQ treated timber
Identification of CuAz and ACQ treated timber
Timber treated with copper azole (CuAz) or alkaline copper quarternary
(ACQ) will be marked in accordance with the requirements of NZS 360.
This standard requires the timber to be identified, either on one end of
each piece or at 1,500 mm centres along the length, with the treatment
plant, preservative and hazard numbers (see Figure 3).
Figure 2: Galvanised steel screws removed from H4 CCA and ACQ treated timbers.
XXX 58 H4
or
XXX 90 H4
Treatment plant number
Preservative code number
Hazard class number XXX
58 is the preservative number for Copper Azole (CuAz)
90 is the preservative number for ACQ
H4
Figure 3: Identification of treated timber.
BRANZ recommends
When this problem was first identified, BRANZ recommended that the
fasteners, including nails and screws, to be used with CuAz and ACQ treated
timbers in situations requiring 15- or 50-year durability, should be either:
❚❚ 304/316 grades of stainless steel, or
❚❚ durable equivalents, such as silicon bronze.
Use of these materials for fasteners is still recommended.
48 BUILD August/September 2009