OUCC Proceedings 11 (1983)

Andy Riley

During a caving trip down Swinsto Hole, Kingsdale, N Yorkshire, a heavily pitted karabiner was found at the bottom of the 20 foot pitch lying under 0. 5 m of water in the gravel bed (Figures la and b).

The whole of the body of the karabiner was covered in pits several millimetres wide and deep. At the bottoms of the pits was a white, insoluble deposit. Only the screw-up gate was unaffected by corrosion. The unattacked parts of the surface did not show any more abrasion damage than would be expected to be caused by normal caving use.

A section was made across the backbone of the karabiner which was hot-mounted in perspex and polished to a 1 micron finish. Under optical examination, the entire section was found to be full of cracks. This unexpected discovery led us to make further investigations into the nature and origin of the cracks.

Under low power observation, all the cracks were seen to run in circles, concentric with the outer circumference of the section. The cracks were extremely numerous, branched and finely divided (Figure 2). They seemed to be evenly distributed throughout the whole section and were not located primarily at the surface. The bases of some of the pits were examined. There was a definite orientation dependence of the cracks to the base of the pits. In many of the pits, the cracks were seen to emerge at the bottom (Figure 3), the pit growth direction lying parallel to the crack direction. In other pits, the crack direction was perpendicular to the direction of pit growth (Figure 4) and had not yet reached the surface. All the pits had a deposit at their base. The aluminium metal at the base of the pits showed signs of extensive corrosion attack and was very spongy and porous in nature.

The composition of the karabiner was unknown, but was determined by electron microprobe analysis to be an Al-Zn-Mg-Cu alloy (see Table 1). An analysis of the deposit in the base of the pits was attempted using energy dispersive analysis (see Table 2). Wavelength dispersive analysis would have been superior but was unavailable at the time of writing. The deposit was predominantly composed of aluminium oxide.

Discussion of observations

The chemical analysis suggests that the karabiner is a high-strength Al-Zn-Mg alloy. Shreir (1976) suggests that these alloys have a high risk of suffering stress-corrosion cracking which can be accelerated by incorrect heat treatment. It seems likely that the surface pitting was probably initiated by the emergence or interaction of the internal cracks with the surface. Once pitting has been initiated, the pits grow unhindered by external effects or microstructure. This is because pits generate an acid environment at their base which prevents reformation of the passive film on the exposed aluminium surface (Robinson, 1960) and therefore corrosion proceeds rapidly. Impurities in the water can assist in the initiation and propagation of pits - a combination of carbonates, chlorides and copper ions can be very damaging (Davies, 1959). In hard water, as little as 0.02 ppm of these ions can initiate pits (Porter and Hodder, 1953, Rowe and Walker, 1961).

The orientation of the cracks in the section strongly suggests that the microstructure is exerting a major influence on their growth direction. The very fine branched nature of the cracks suggests that they are intergranular. Unfortunately, I was not able to etch up the grain boundaries in order to demonstrate this. Exfoliation corrosion is a well-known phenomenon in high strength aluminium alloys. Robinson has examined the effect of elongated grain structure and heat treatment on the formation of surface blisters (Robinson, 1982). It seems likely that an elongated grain structure is formed in the alloy karabiner during manufacture as it is extruded and that this initiates surface and filiform attack. Grain boundary attack then causes the production of corrosion products, creating large stresses at the grain boundary which force up grains at the surface to create blisters. If these blisters reach a certain size, a pit will form and pitting corrosion will dominate.

Conclusion

Grain boundary attack has probably occurred because of precipitation and segregation of alloying elements at grain boundaries during heat treatment. Exfoliation corrosion produced blisters on the surface which in turn caused deep and severe pitting. The intergranular attack and pitting in this karabiner has become apparent due to its immersion for an unknown time in cave water which might be expected to contain the necessary impurities for this kind of corrosive attack. It would be interesting to know how long it would take for such attack to occur and whether such slight attack which may occur during normal caving use has any effect on the strength of the karabiner.

References

Davies, D.E., 1959. Pitting of aluminium in synthetic waters. J. appl. Chem. 9, 651-660.

Porter, F. C. and Hadden, S.E., 1953. Corrosion of aluminium alloys in supply waters. J. appl. Chem. 3, 385-409.

Robinson, F.P.A., 1960. Pitting corrosion - cause, effect, detection and prevention. Corros. Techno1. 7, 237-239, 266.

Robinson, M.J., 1982. Mathematical modelling of exfoliation corrosion in high strength aluminium alloys. Corros. Sci. 22, 775-790.

Rowe, L. C. and Walker, M.S., 1961. Effect of mineral impurities in water on the corrosion of aluminium and steel. Corrosion 17, 353t-356t.

Shreir, L.L., 1976. Corrosion. Butterworth, London, 2 vols., 2nd edn.

Table   1.   Electron microprobe  analysis  of karabiner  alloy  composition
Standardless EDS analysis  (ZAF  corrections  via magic V)

+ ca. 1% Mg

Table 2. Electron microprobe analysis of pit deposit composition
Standardless EDS analysis (ZAF corrections via magic V)

* - determined by stoichiometry