Besides aluminium of the above mentioned purities, the aluminium layer may also contain 0.25 to 5 wt. %, especially 0.5 to 2 wt. % magnesium, or 0.2 to 2 wt. % manganese, or 0.5 to 5 wt. % magnesium and 0.2 to 2 wt. % manganese, especially e.g. 1 wt. % magnesium and 0.5 wt. % manganese, or 0.1 to 12 wt. %, preferably 0.1 to 5 wt. % copper, or 0.5 to 5 wt. % zinc and 0.5 to 5 wt. % magnesium, or 0.