Corrosion Aspects Regarding the Use of Martensitic Stainless Steels in Automotive Chassis Parts 2015-01-1347

In order to meet new environmental regulations (i.e. mass of CO₂ rejected in the atmosphere per km), car manufacturers are looking for new solutions to lighten chassis and structural parts in cars. High strength steels formed by hot stamping have proved to be good candidates for achieving better in-use performances together with a lighter structure. In particular, the martensitic stainless steel MaX fulfils the industrial targets for chassis parts in terms of mechanical and fatigue properties. For instance, from a cold formed baseline made of 600 MPa carbon steel, a 50 % mass reduction can be expected with a hot stamped suspension arm made of MaX and included a new clamshell design. However, those parts are often made of a complex assembly of different materials (high strength steels, aluminium and cast iron among others) which are subjected to aggressive environments in service. Therefore galvanic corrosion of those complex assemblies has to be evaluated. Galvanic coupling measurements have been made on MaX / Cast iron, MaX / Aluminium and Cast iron / Aluminium assemblies using the Zero Resistance Ammeter (ZRA) technique. Results reveal that the galvanic current of the Aluminium / Cast iron couple is higher than the galvanic current of the Aluminium / MaX couple and Cast iron / MaX couple. These results are discussed in terms of cathodic currents and polarisation resistances of metals showing that the anodic dissolution rate is higher in the case of the Aluminium / Cast iron couple compared to the Cast iron / MaX and Aluminium / MaX couples. In this regard, the MaX material shows better performances than Aluminium, a result which goes against predictions that are made based on the electro-chemical potential differences of MaX and Aluminium compared to Cast iron. The importance of these findings regarding the open circuit potential difference used for the choice of materials in the automotive industry is discussed and a new criterion for material assessment is proposed.