NB We have found in use that aluminium screws tend to clog up more than steel screws, particularly in cold conditions, so while they are great for lightening your rack, it's probably still worth carrying steel screws for those strenuous one handed placements and save the alloy ones for situations where you are in balance. We suspect this is due to the nature of the metal rather than any fault in the design of the screw though we have recently received this explanation from Petzl: "Something important to remember with aluminium ice screws is that the tube is thicker causing higher resistance and this can in certain situations, such as wet, or very hard ice, lead to a less good screwing performance (sic). To avoid any problems, we do recommend to always take some steel ice screws as well."