BUILD August/September 2009 47

CORROSION Of faSTeNeRSalthough 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

CORROSION

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 availableBRANZ 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 similarCorrosion 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 concernsGalvanised 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 nailsIn 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 excellentAll 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.Field exposure test site at Judgeford where the corrosion performance of typical building

hardware in treated timbers is being investigated.

48 BUILD August/September 2009

Less corrosion in CCA, more in CuAz and ACQThe 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.

Identification of CuAz and ACQ treated timberTimber 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 1: Galvanised steel nails removed from H4 CCA and ACQ treated timbers.

H4 CCA treated timber H4 ACQ treated timber

Figure 2: Galvanised steel screws removed from H4 CCA and ACQ treated timbers.

H4 CCA treated timber H4 ACQ treated timber

BRANZ recommendsWhen 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.

Figure 3: Identification of treated timber.

XXX 58 H4

XXX 90 H4

or

Treatment plant number XXXPreservative code number 58 is the preservative number for Copper Azole (CuAz) 90 is the preservative number for ACQHazard class number H4