In reply to Chris Craggs:
> I have thought about this, my mate Colin mentioned the aluminium oxide being deposited on the rope and making it dirty. But two things occurred to me - the volume of metal eroded is tiny, surely not enough to coat a rope. And the metal in question is some shiny aluminium alloy, why does the rope go black?
> I think something else must be at work, though not sure what?
> Chris
Aluminium oxide in the thickness on piece of aluminium is transparent (or nearly so) as it is around 5nm thick an a nano-film, because the substrate is shiny you don´t really see it. A thicker layer on a non-reflective substrate has a different refractive index and shows up as black. The oxide itself is white in powder from which is why it is used to colour white paint (and toothpaste) but in other crystalline forms black (carborundum) or with the right trace minerals it becomes rubies or sapphires.
The same effect occurs with stainless steel where the oxide appears to be clear but work with it all day and clearly it isn´t! You can usefully use the change in colour with oxide thickness to heat-tint stainless, if you want a brown bolt (as the Americans sometimes prefer) you merely heat it in air and the oxide layer becomes thicker and the colour changes through pale straw, gold, brown and to dark magenta depending on the temperature. More commonly we see this effect on motorcycle exhausts and camping pans which have been overheated.
Returning to the Click-Up and the karabiner, apart from the discolouring of the rope once the hard-anodising on the karabiner is worn through it leaves a sharp step which begins to "shave" the rope and lowering with an older rope you notice rope-coloured dust on your hands, time for a new karabiner!
