COATING SYSTEMS FOR JOINING ELEMENTS IN C ONTACT WITH LIGHT-METALS

Galvanic corrosion of light metal alloys can only be avoided, if the s teel fasteners are given an electrochemically compatible protection or if the corrosion circuit is interrupted by electric insulating layers . Current density vs. potential curves of chromated zinc alloy coating s show, that by means of zinc nickel coatings a largely electrochemica l adaption to the equilibrium potential of quenchaged aluminium alloys of the type AlMgSi is possible. On the other hand, the equilibrium po tentials of chromated systems based on ZnFe, ZnCo respectively, are to o low to avoid galvanic corrosion on aluminium. In practice-related co rrosion tests on joining elements, this result was confirmed. Besides this, zinc- and aluminium-bearing sinter systems, galvanic tin deposit ions as well as ternary mechanical coatings based on ZnSnAl have been proved to be compatible with AlMgSi-type alloys. For coating systems i n contact with magnesium a satisfactory protection against galvanic co rrosion can be achieved effectively with insulating top coats. The res ults with suitable duplex systems are presented and recommendations fo r practical applications are derived.