Building materials in geothermal areas

By - , Build 149

BRANZ research is filling the knowledge gap in how geothermal corrosion affects typical building materials. This will lead to guidance for the design, construction and maintenance of buildings in geothermal areas.

Figure 2: Cracks in treated timber from contact with geothermally contaminated soil.
Figure 2: Cracks in treated timber from contact with geothermally contaminated soil.
Figure 1: Discoloured treated timber.
Figure 3: Zinc-protected fastener and stainless steel square washer.
Figure 4: Copper at varying distances from a geothermal vent.

NEW ZEALAND is right on the boundary between the Australian and Pacific tectonic plates. Plate movements provide significant energy and heat for New Zealand’s volcanoes and geothermal systems.

These systems discharge large amounts of water, steam and gas through surface features such as fumaroles, geysers, springs and mud pools. They also have significant cultural, economic, heritage, scenic, scientific and therapeutic values.

Sulphuric gases cause damage

Most of New Zealand’s geothermal systems have population centres nearby. For example, about 68,000 people live in Rotorua, in the Taupo Volcanic Zone. This approximately 350 km long by 50 km wide zone has numerous high-temperature, volcano-related geothermal systems.

Up to a third of geothermal emissions are sulphur-containing gases, dominated by sulphur dioxide (SO2) and hydrogen sulphide (H2S). At high concentration levels, these can cause persistent health problems and create environments highly corrosive to materials, buildings and infrastructure.

Discolouration of treated timbers is common in geothermal environments (see Figure 1). When exposed to high concentrations of geothermal emissions or in direct contact with geothermally contaminated soil or water, timbers turn dark yellow and exhibit deep cracks (see Figure 2).

Severe corrosion has frequently been observed with copper components and zinc-coated nails, bolts and plates.

Stainless steel, such as type 304 and 316, is more corrosion resistant within geothermal environments (see Figure 3). Stainless steel nails, however, can detach themselves from the cracked or deteriorated timbers. This reduces the strength and integrity of timber joints and leads to safety issues.

Paints, particularly those with metal-based pigments or drying agents such as lead, may blacken when used in geothermal environments.

Figure 1: Discoloured treated timber.
Figure 2: Cracks in treated timber from contact with geothermally contaminated soil.
Figure 3: Zinc-protected fastener and stainless steel square washer.

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Microclimate with accelerated corrosion

A geothermal zone, Zone 4, defined by the atmospheric corrosivity map in NZS 3604:1999 Timber framed buildings was removed when the standard was revised in 2011.

Areas within 50 m of bores, mud pools or steam vents are considered as a microclimate in which significant acceleration of corrosion can occur.

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Limited practical knowledge to date

There have been limited studies looking at materials performance in our geothermal environments:

• BRANZ measured atmospheric corrosion rates of mild steel and hot-dip galvanised steel at some sites within the Taupo Volcanic Zone.

• The University of Auckland investigated corrosion behaviour of zincalume and galvanised coatings in a sulphur-containing atmosphere in Rotorua.

• IRL, GNS and AIST of Japan evaluated the performance of low-alloy steels, stainless steels and nickel-based alloys in fumaroles, soils and acid pools on White Island.

These studies, with different objectives, produced little guidance to the building industry. International knowledge is also limited since most geothermal areas in other countries are sparsely populated.

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Better understanding crucial

Recently, several cases of premature failure of building components and protective coatings on infrastructure within the Taupo Volcanic Zone have been reported.

New knowledge balancing performance and cost is crucial for designing, constructing and maintaining buildings in New Zealand’s geothermal environments.

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Systematic study at BRANZ

BRANZ’s materials within geothermal environments project is systematically studying deterioration of typical building materials in environments influenced by geothermal emissions. Its objectives are to:

• survey sulphur-containing species influenced atmospheric corrosivity in Rotorua

• investigate the influence of distance from a geothermal source on species concentration and material deterioration

• study the influence of geothermal surface feature type on material deterioration.

Materials tested include metals, coatings, timbers, composites and sealants. Passive diffusion tube sensors are used for shortterm monitoring of H2S and SO2.

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Preliminary test findings

Environmental monitoring confirmed past conclusions that H2S concentrations are highest in central Rotorua, moderately high in the east of the city and lowest in the west.

Metal corrosion showed a similar trend. After 1 month, copper at the site with the highest H2S concentration was covered with non-adherent, flake-type, black corrosion. In comparison, samples exposed at western locations were covered with adherent, thin corrosion with a reddish-purple colour.

H2S concentration measured at locations 50 m from the geothermal vent monitored by this research can be six times lower than that at the source. Copper corrosion is significantly decreased at this distance (see Figure 4).

However, the surface morphology of copper exposed at locations with low H2S concentrations appears to be somewhat different from that of copper exposed at locations without geothermal influences.

Figure 4: Copper at varying distances from a geothermal vent.

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Risk indicators, specs and maintenance

The exposure testing is continuing and will include another 1-year field test using more materials. This will produce representative data for the investigation of degradation kinetics and mechanisms of building materials within geothermal environments.

Results should allow the development of environmental risk indicators as well as construction materials specification and schemes for maintenance.

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Figure 2: Cracks in treated timber from contact with geothermally contaminated soil.
Figure 2: Cracks in treated timber from contact with geothermally contaminated soil.
Figure 1: Discoloured treated timber.
Figure 3: Zinc-protected fastener and stainless steel square washer.
Figure 4: Copper at varying distances from a geothermal vent.

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