BRANZ Research Now: Geothermal corrosion #1

Which metals are more sensitive to geothermal corrosion?

BRANZ conducted field trials in to examine how different metals corrode in geothermal environments. Identical samples were also exposed to a marine environment for comparison. Copper and mild steel suffered serious corrosion in the Rotorua exposure sites, and zinc did too where concentrations of sulphur- containing gases were high. Aluminium and stainless steel corroded more slowly in a geothermal environment than a marine environment.

Thermal vents, hot springs and The material deterioration frequently seen measured over three exposure periods: other features in geothermal in buildings and infrastructure in geothermal ● December 2014–December 2015 areas can release sulphur- regions can result in large repair and main- ● June 2015–June 2016 tenance costs. In addition, NZS 3604:2011 ● December 2015–December 2016. containing gases such as Timber-framed buildings requires specific The samples were regularly retrieved and hydrogen sulphide (H2S) and engineering design for buildings constructed their corrosion rates measured. The surfaces within 50 metres of a geothermal hot spot. and cross-sections of samples were also sulphur dioxide (SO2) into the environment. These gases can To assess the performance of metals in examined with microscopes to see what was a geothermal environment, field exposure happening. be aggressive towards susceptible sites were established at different locations in building and construction Rotorua, roughly in a line from west to east H2S concentration in air across the city (Figure 1). Two contrasting The concentration of airborne H S was materials. 2 exposure sites were also set up – one in the measured with passive tube sensors in the semi-rural BRANZ campus at Judgeford, first 3 weeks of each exposure period. A huge Porirua, and the other close to breaking surf range was found between exposure sites, with at Wellington’s Oteranga Bay. the bottom graph in Figure 2 providing finer The metal samples had a typical dimen- detail. The highest concentration of H2S in sion of 150 × 100 × 1–3 mm. They were set Rotorua – approximately 31.0 parts per billion into exposure racks with nylon fixings. To (ppb) at the wastewater treatment plant site minimise the influence of changes in weather (WWTP) – was around 500 times higher than and geothermal activity, corrosion rates were the lowest – 0.06 ppb in the far west.

BRANZ Research Now: Geothermal corrosion #1 | v2 April 2021 | www.branz.nz 1 BRANZ Research Now: Geothermal corrosion #1

and east city sites were relatively compact, Ohinemutu with fewer physical defects such as spallation or detachment. The corrosion products on the zinc samples at the west and east city sites developed into

Sulphur Bay relatively uniform, dense and thin layers. This type of corrosion product is expected to provide protection to the underlying zinc substrate and to grow slowly due to the WWTP Ngapuna absence of a lot of physical defects. Corrosion rates measured for zinc in the west and east of Rotorua were even lower than that measured at

the semi-rural Judgeford site, where H2S levels were exceedingly low. By contrast, high corrosion rates in the zinc samples were seen at sites with high

H2S concentrations. The corroded surface of Exposure site Indicative geothermally influenced area the zinc sample exposed at the wastewater treatment plant site was rough. It had a large Figure 1. Exposure sites established in Rotorua. number of clusters growing from underlying pit-like features. Mild steel, zinc and copper corrosion At the wastewater treatment plant site, The copper samples exposed at the waste- The corrosion rates of mild steel, zinc and a thick corrosion product layer formed on water treatment plant site had very high copper varied greatly based on exposure the mild steel sample (Figure 3). The sample mass losses of 443–495 g/m2 after 1 year of site location. Mild steel corrosion rates thickness had increased from 3 mm to around exposure. This extremely high corrosion rate lower than 200 g/m2/year were measured 5–6 mm after a 1-year exposure, leading to is a direct result of the fast interaction of at only two sites, both in the western area. a huge volume expansion. This thick corrosion copper and sulphur-containing gases and poor The highest corrosion rates were observed at product layer of around 1–2 mm thickness protection offered by the corrosion products the wastewater treatment plant site close to on each side could easily break off from the remaining on the surface. Corrosion products Sulphur Bay – 3,302 g/m2/year for mild steel, sample surface. started to crack and detach partially as flakes 84.2 g/m2/year for zinc and 495 g/m2/year By comparison, the corrosion product layers after approximately 1 month of exposure. for copper. on the mild steel samples exposed at the west Consequently, fresh copper substrate would continue to be exposed to the corrosive environment. Examination under a microscope 35 showed a very rough surface with large nodules Dec 2014 - Jan 2015 30 Jun - July 2015 and deep pit features. This type of corrosion product is unlikely to provide good protection 25 Dec 2015 - Jan 2016 to the underlying metal. 20 Corrosion products on copper samples 15 exposed in west and east areas showed no 10 obvious spallation or detachment, contributing S concentration (ppb) concentration S 2

H to lower corrosion rates when compared with 5 that exposed at the wastewater treatment 0 5 plant. However, the corrosion product layer on the copper sample in the eastern region 4 was thicker and more porous than that on the copper in the western area. The sizes of 3 clusters and particles were also larger. This 2 indicates that a small difference in concen- tration of airborne sulphur-containing gases S concentration (ppb) concentration S 2

1 H would be enough to make a large difference in atmospheric corrosion of copper. 0 The corrosion rate measurement and Malfroy 216 Malfroy 116 Malfroy 22 WWTP Ngapuna Lynmore Judgeford observation of the form and structure of the

Figure 2. Average H2S concentration during a 3-week exposure. corrosion products indicated that different

BRANZ Research Now: Geothermal corrosion #1 | v2 April 2021 | www.branz.nz 2 BRANZ Research Now: Geothermal corrosion #1

wind-blown sand and airborne chloride-con- taining salt particles. Severe corrosion and rusting was seen on the aluminium and stainless steel samples Corrosion product layer under and/or very close to the nylon fasteners. Metal substrate These areas could trap sea salt particles for Corrosion product layer long periods. Pitting was also seen on the ground-facing surface of AISI 304 stainless steel where sea salt particles had been deposited.

Figure 3. Corrosion products on the mild steel sample exposed at the wastewater treatment plant site for 1 Conclusion year. Measurements of corrosion in the metal samples indicate that different metals have 2 metals had quite different response to high corrosion rates (35.6–49.9 g/m /year) different resistance to H2S-induced attack in geothermal attack. Copper showed high even in areas with relatively weak geothermal geothermal environments. Copper is more corrosion rates in the west and east areas with influences (low concentrations of H2S and sensitive than mild steel and zinc. It appears

(very) low H2S concentrations and appeared to SO2). This was similar to the earlier studies to be sensitive to the presence of even very be more prone to geothermal attack than mild and again indicated that copper has a higher low concentrations of H2S in the atmosphere. steel and zinc. sensitivity to the attack of airborne sulphur- Zinc appears to be less prone to atmospheric To verify this, the corrosion rates measured containing gases compared to mild steel and zinc. corrosion from sulphur-containing gases of at Judgeford (where airborne H2S concen- relatively low concentrations compared with tration was extremely low) were used as a Aluminium and stainless steel mild steel and copper. Above these lower baseline to calculate corrosion rate ratios The field trial found that atmospheric corro- concentrations of gas, corrosion in zinc can (Table 1). In geothermal environments with sion of aluminium and stainless steel (AISI be accelerated. low H2S concentrations, there was no increase 304) is very limited when exposed in areas In contrast, aluminium and stainless steel of mild steel and zinc corrosion rates. However, with strong geothermal influences. Close were much more resistant to atmospheric there was an increase of copper corrosion examination of the metal surfaces after expo- geothermal attack. These materials suffered rate, and the ratio was quite similar to the H2S sure did not find any significant corrosion greater corrosion in a marine environment. concentration ratio. products. Only some small black spots were More recent BRANZ field research over seen, and these could be cleaned chemically. 2016–2018 (see Study Report 458) confirmed There was no visible severe attack to the metal that mild steel, zinc and copper all had severe substrate underneath these spots. More information corrosion when exposed in an area with strong Corrosion rate measurements indicated geothermal influence - , which had that these materials had higher mass losses BRANZ Study Report SR393 Materials the highest concentrations of H2S and SO2 when exposed to the severe marine environ- within geothermal environments among all exposure sites. Corrosion occurred ment, indicating that chloride-containing sea at lower rates at sites distant from a large-scale salt particles are more corrosive to them. BRANZ Study Report SR458 Atmospheric geothermal source. Degradation of aluminium also appeared corrosivity of the region However, copper still showed relatively to be a result of the mechanical erosion by BRANZ Research Now: Geothermal corro- sion #2 Distance effects of corrosion in Table 1. Metal corrosion sensitivity to airborne H2S. geothermal environments

H2S CONCENTRATION RATIO CORROSION RATE RATIO EXPOSURE SITE COMPARISON DEC 2014 – JAN 2015 MILD STEEL ZN CU BRANZ Research Now: Geothermal corro- sion #3 Discolouration and deterioriation of 216 Malfroy Rd (west) vs Judgeford 3 0.7 0.6 2.1 wood in geothermal environments

Lynmore (east) vs Judgeford 5.7 1.1 0.4 4.6 BRANZ Research Now: Geothermal corro- sion #4 The performance of aluminium-zinc

Disclaimer: The information contained within this publication is of a general nature only. BRANZ does not accept any responsibility alloy coating in geothermal environments or liability for any direct, indirect, incidental, consequential, special, exemplary or punitive damage, or for any loss of profit, income or any intangible losses, or any claims, costs, expenses, or damage, whether in contract, tort (including negligence), equality or otherwise, arising indirectly or indirectly from or connected with your use of this publication, or your reliance on information contained in this publication. Copyright © BRANZ 2021.

BRANZ Research Now: Geothermal corrosion #1 | v2 April 2021 | www.branz.nz 3