The Material Corrosion Test (MaCoTe)

Modules are removed using a specially designed capture tool and inserted into argon filled transport flasks. The top of the borehole is continually injected during the removal process to prevent air from entering the borehole whilst the double packer system is out of the borehole.

Careful dismantling, dissection, and removal of metal coupons in the glove box.

CHARACTERISATION OF METAL COUPONS

Postmortem characterisation of the coupons (Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) analysis) was carried out at the University of Oxford. Weight loss measurements were carried out at Jacobs Harwell laboratories. Profilometry was carried out at the University of Manchester.

AVERAGE CORROSION RATES

The corrosion rate of carbon steel ranged from 1.87 μm year−1 to 2.05 ± 0.05 μm/year, while the corrosion rates for the copper coupons were much lower and the corrosion rate of stainless steel was negligible. At a bentonite density of 1250 kg / m3, the cold‐sprayed copper had an average corrosion rate of 0.17 μm / year while the electrodeposited and wrought copper had an average corrosion rate of 0.24 μm / year. The corrosion rates did not significantly vary with bentonite density. So far there was no evidence of material loss due to microbially induced corrosion.

PROFILOMETRY MEASUREMENTS

Profilometry measurements for pristine coupons and coupons after one year exposure. For all metals, the roughness after exposure was similar to those of pristine samples, indicating that pitting or surface roughening did not occur or was minimal.

 BASE Federal Office for the Safety of Nuclear Waste Management (BASE), Germany

 BMUKN (FSU Jena, GRS, KIT) (Federal Ministry for the Environment, Nature Conservation, Nuclear Safety and Consumer Protection / Karlsruhe Institute of Technology), Germany

 KIGAM, South Korea

 NWS, United Kingdom

 NWMO, Canada

 NUMO, Japan

SURAO, Czech Republic

Nagra, Switzerland

The Material Corrosion Test (MaCoTe) consists of non-heated and heated experiments to study in-situ corrosion of candidate canister materials embedded in bentonite.

Two broad aims are as follows:

• Provide confirmation of the long-term anaerobic corrosion rate of carbon steel, stainless steel and copper in compacted bentonite under repository-relevant environmental conditions,
• Provide experimental evidence of the inhibiting effect of the bentonite buffer on microbial activity and microbially-influenced corrosion.

Experimental Layout (non-heated test)

The current ongoing in-situ experiment is made up of a series of specially designed modules (0.3 m long) that are inserted into a 10m long vertical borehole and sealed with a double packer system. Each module contains 12 specimens embedded in MX-80 bentonite with dry densities of either 1.25 or 1.5 Mg/m³ (Figure 1)

Figure 1: Experimental layout: Borehole with modules; cutaway of one module showing the distribution of metal coupons.

Schedule

The first eight modules were inserted into the borehole in September 2014. Retrieval will be made at increasingly longer intervals with the final two modules will be retrieved after 10 years.

MaCoTe non-heated test sampling plan (Phase 1: 2013 – 2018; Phase 2: 2019 – 2023). The vertical axis indicates the position of the module in the borehole. The bottom two modules (modules 7 and 8) will remain in the borehole for at least 9 years.

Planned analyses

Corrosion rates of samples are determined by weight loss method and the mineral alterations at the interfaces are analyzed with available analytical methods (e.g. SEM-EDX, XRD, etc.). Microbial populations both in the bentonite and the borehole water are analyzed using advanced cell counting and DNA mapping techniques.

In-situ heated corrosion test

The heated in-situ test consists of 5 specially designed heater probes with rings made up of the target material (copper coatings and carbon steel), surrounded by bentonite rings and inserted into 5 boreholes. The duration of each test is 1, 2, 3, 5 and 7 years.