Systematic Failure Analysis of failed Anchors from Hong Kong

The current practice of both the UIAA 123 and EN 959 standards has been to leave anchor corrosion/SCC resistance as the responsibility of the manufacturers. However, numerous incidents of fixed rock climbing anchors failures mostly in seaside locations show that this approach is not optimal.

The Safety Commission of Union Internationale des Associations d’Alpinisme (UIAA) is preparing a new standard, which will classify anchors into classes according to their resistance to SCC and / or corrosion. The UIAA SafeCom working group consists of technical experts supported by regional volunteers around the globe  who are providing failed bolt samples and also installing test cells of different bolts which are being monitored over time for corrosion behaviour.

Hong Kong is located within a region that has an elevated corrosivity potential and failed samples recovered during re-bolting work / catastrophic failures have been provided to the working group in recent years.slamova-fig-5

All samples of failed / corroded hardware were sent to the University of Chemical Technology in Prague where Tomas Prosek and his team conducted visual analyses at cross-sections using metallographic techniques and by scanning electron microscopy (SEM). Material chemical composition was verified by x-ray fluorescence (XRF) analyser, chemical composition of deposits and corrosion products determined by energy dispersive spectroscopy (EDX).

scc

Example of a submitted sample recovered from Hour Glass Crack, Technical Wall, Tung Lung Chau.

Samples submitted included hangers, bolts and the cracked anchor set from Green Slab in Sea Gully.

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Crevice corrosion occurred on all four analysed hangers, either under a washer or between a hanger and rock. There were branched cracks on two analysed hangers, one of them was completely torn after failure under 5 kN force (Hour Glass Crack). Cracking was observed also on both sleeves in a longitudinal direction of the bolts. Cross-section and SEM analysis after removal of corrosion products showed branched transgranular crack morphology.

The Green Slab anchor set featured a crack located 1–2 mm from the weld on the chain-link attached to one of the hangers that showed intergranular morphology, which suggests sensitization of heat affected zone during welding leading to intergranular corrosion or intergranular SCC.

SEM

SEM micrograph of transgranular fracture of cracked surface after corrosion product removal.

The conclusion from the preliminary report confirms that chloride presence was principal for the classical low-temperature chloride-induced SCC to occur and it is probable that concentration of chlorides in confined zones (e.g. behind hangers, washers and nuts) helped to initiate the SCC, at least in some cases. Crevice corrosion in this case occurs where salt solution (sea water) remains stagnant behind hangers.

Corrosion type sketch

Hong Kong geology can be very broadly defined as either volcanic tuff, or coarse grained granite, and neither rock type features magnesium in the concentrations required to form aggressive magnesium chloride based solutions that are found in limestone climbing areas like Tonsai. Super accelerated SCC is unlikely to be a problem for Hong Kong sport crags however the change in anchor type from multi component expansion bolt to a single piece glue-in bolt or adopting certified titanium glue-in bolts on sea side sport cliffs will reduce the rate of corrosion or in the case of titanium; eliminate it altogether.

Acknowledgement and thanks is extended to Alan Jarvis of the UIAA and Tomas Prosek and his team in Prague.

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